PixInsight Forum (historical)
PixInsight => General => Off-topic => Topic started by: DanielF on 2013 November 14 05:06:09
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Hello.
My latest image (attempted image at least) was a shot at M45.
I only got 30 min on it and then collected bias, flats and darks.
The problem I'm having is that the flat seems to overcompensate the uneven illumination during calibration.
This image shows an uncalibrated light and flat frame and the resulting master light straight out of the batch preprocessing script.
All with AutoSTF.
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/flat%20problem.png)
Anybody has any idea what could be wrong and is there anything that I can do to fix it in PixInsight. If I have done something wrong acquiring the flats then what could done to correct it the next time?
I use a home built light-box to shoot my flats. Could it be that the lit surface is unevenly illuminated? (I have two diffusers inside the box). Is the exposure wrong?
(http://danielfranzen.files.wordpress.com/2011/07/img_2883.jpg?w=640&h=426)
(http://danielfranzen.files.wordpress.com/2011/07/img_2888.jpg?w=640&h=426)
Here are links to one light and one flat fit file if anybody wants to take a look.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_2m-1_010.fit
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Flat_L_001.fit
Thanks!
/Daniel
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This is normally because the vignetting is so strong that the flat cannot flatten the field and over-compensates. Check out the dark areas of the flat; if the pixels values in there are zero or very low then that is probably what is happening. I'll download the flat and take a look.
What camera and scope are you using and how EXACTLY is the camera attached to the scope? - ah OK it's an 8300 chip Starlight Xpress? - shouldn't really have this much vignetting unless you are using 1.25 inch filters or something like that?
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This is normally because the vignetting is so strong that the flat cannot flatten the field and over-compensates. Check out the dark areas of the flat; if the pixels values in there are zero or very low then that is probably what is happening. I'll download the flat and take a look.
What camera and scope are you using and how EXACTLY is the camera attached to the scope?
Hi.
The dark areas do not seem to be very low and are not zero.
The camera is a Starlight Xpress SXVR-H18 (KAF8300) and is connected to a filterwheel and a Baader MPCC and the scope is a SkyWatcher Explorer 200 8" f/5 Newton.
And it all looks like this:
(http://danielfranzen.files.wordpress.com/2013/11/2013-11-05-12-10-59.jpg?h=600)
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Sorry, got it the wrong way round... I downloaded your light and flat and did a linear fit of the flat to the light (so they have similar "exposures") and the flat worked MUCH better.
Your flat is too bright. You need to dial the brightness of your light box back a bit or use a shorter exposure for the flats. Try taking flats at various lower exposures and see what works.
Personally I'd still take a long hard look at that vignetting because in my opinion it is asking a lot of the flats to deals with all of it. I can see from your setup that the scope and focuser are not an issue - my guess is the filter wheel or the filters within it. Which wheel/filters are you using?
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the flat seems okay to me. maybe just a tad bright and the histogram looks a little weird but it's not grossly overexposed.
but what is wrong with it is that it was apparently created in nebulosity... calibration frames usually do not work well across programs.
usually when you get overcorrection from a flat it's because something went wrong in the flat calibration.
if the flat really came out of neb, i'd re-make the flat master (and dark and bias) in PI and try again.
rob
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Sorry, got it the wrong way round... I downloaded your light and flat and did a linear fit of the flat to the light (so they have similar "exposures") and the flat worked MUCH better.
Your flat is too bright. You need to dial the brightness of your light box back a bit or use a shorter exposure for the flats. Try taking flats at various lower exposures and see what works.
Personally I'd still take a long hard look at that vignetting because in my opinion it is asking a lot of the flats to deals with all of it. I can see from your setup that the scope and focuser are not an issue - my guess is the filter wheel or the filters within it. Which wheel/filters are you using?
Hi and thanks for helping me.
So do you think I could salvage the flats by running linear fit in this case?
The exposure times of the flats where 10 seconds, so I can definitely dial that down a bit. I never really know exactly for how long I should expose my flats…
The filter wheel is also from Starlight Xpress and the filters are unmounted 36mm LRGB.
/Daniel
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the flat seems okay to me. maybe just a tad bright and the histogram looks a little weird but it's not grossly overexposed.
but what is wrong with it is that it was apparently created in nebulosity... calibration frames usually do not work well across programs.
usually when you get overcorrection from a flat it's because something went wrong in the flat calibration.
if the flat really came out of neb, i'd re-make the flat master (and dark and bias) in PI and try again.
rob
Hi.
All raw frames are from Nebulosity but all the calibration frames was created in PixInsight with the Batch Preprocessing script.
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whoops, sorry, i thought that it was a master flat but i can see that it's a flat sub.
still nothing seems to be terribly wrong with that flat, the histogram is right in the middle of the display. i suspect that something has gone wrong in the calibration of the flat. that's usually the cause of overcorrection.
can you post a bias frame as well?
rob
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Sure thing, here's a bias sub.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Bias_001.fit
/Daniel
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just some observations.
the bias has about 4x the signal level of my camera, which is also based on a KAF-8300M. the bias also has some very interesting "moire" type patterns in it. i don't know if this is significant at all as i've only ever looked at bias frames from my camera.
the flat does not seem to match the light. the dust motes are in different places. there can be a lot of reasons for this, from as simple as the dust having moved to something as bad as the filter wheel not being able to return to a precise position. was the camera moved or rotated between the lights and the flats? did you take the flats before moving the filter wheel?
what's strange is the dust motes seem to have moved outward radially. that could have something to do with the focusing point. was focus changed between lights and flats?
i don't see vignetting as bad as yours in a single sub calibrated with one bias frame and one bias-subtracted flat. but that's probably because it's a single image.
rob
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That bias frame also has 4x the signal and 22x the noise of my cooled DSLR bias frames...
Those horizontal patterns look like electrical interference to me...
Were the bias frames taken with the camera on the scope? - what power supply was used?
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just some observations.
the bias has about 4x the signal level of my camera, which is also based on a KAF-8300M. the bias also has some very interesting "moire" type patterns in it. i don't know if this is significant at all as i've only ever looked at bias frames from my camera.
the flat does not seem to match the light. the dust motes are in different places. there can be a lot of reasons for this, from as simple as the dust having moved to something as bad as the filter wheel not being able to return to a precise position. was the camera moved or rotated between the lights and the flats? did you take the flats before moving the filter wheel?
what's strange is the dust motes seem to have moved outward radially. that could have something to do with the focusing point. was focus changed between lights and flats?
i don't see vignetting as bad as yours in a single sub calibrated with one bias frame and one bias-subtracted flat. but that's probably because it's a single image.
rob
Hi.
I have no idea why the bias has more signal than your camera. Maybe it's just how this camera performs? Same with the pattern you described.
It's strange what you say about the flat that it doesn't match the light, the really bad specs are actually on the ccd and not the filters so they shouldn't move during filter changes. The camera wasn't moved or rotated but I did a very small adjustment to the focus one time but can that really move the dust motes that much? How do people do that refocus after filter changes for example?
If I calibrate just one light with the posted bias and flat I get the same overcorrected vignetting as the master shown in the first post.
/Daniel
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That bias frame also has 4x the signal and 22x the noise of my cooled DSLR bias frames...
Those horizontal patterns look like electrical interference to me...
Were the bias frames taken with the camera on the scope? - what power supply was used?
I had removed the camera from the scope when I took the bias and dark frames.
I used the original 220V power supply that came with the camera.
/Daniel
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OK - not sure about the Bias frame - personally I would discuss with the manufacturer and/or other users of that camera.
I tend to agree with Rob - if you subtract your bias frame from your flat (to get a calibrated flat) the flat ends up with the Bias pattern noise superimposed on it...
I'm not seeing the same thing as Rob regarding the flats moving vs the lights... they seem to line up pretty well to me.
I would try running the script again without supplying any bias frames and see what happens.
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here's the dust in the lower left corner. to me it looks like it just does not match right. obviously part of it is overcorrected but i don't see how part can be overcorrected and part of it not corrected at all…
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I was looking at the large spots on the right hand side... they haven't moved which rules out focus/rotation.
I still think that the solution is some combination of the flat being too bright and the bias frames being too noisy.
A master light created without bias frames would be helpful...
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Hi.
I have no idea why the bias has more signal than your camera. Maybe it's just how this camera performs? Same with the pattern you described.
It's strange what you say about the flat that it doesn't match the light, the really bad specs are actually on the ccd and not the filters so they shouldn't move during filter changes. The camera wasn't moved or rotated but I did a very small adjustment to the focus one time but can that really move the dust motes that much? How do people do that refocus after filter changes for example?
If I calibrate just one light with the posted bias and flat I get the same overcorrected vignetting as the master shown in the first post.
/Daniel
well the entire camera vibrates a little bit when the filter wheel is moved, so in theory it could cause dust on the CCD to move a little bit as well.
focus is not super critical for vignetting. i don't have any experience with newts but given the optical distortions inherent in newts i wonder if focus could affect dust shadows at the corners of the image more than at the center. this is certainly something you could try to characterize.
anyway we have had recent threads here with bad flats where some attempt was made to modify the flat to fix it up. strictly speaking this is the wrong thing to do, and to be honest i don't think it has anything to do with pixinsight's calibration - the methods for calibrating a frame are very much fixed and every program is going to do pretty much the same thing. the only real "freedom" is whether or not and how much to scale a dark frame. of course the flat scaling factor has to be computed but i think this should be the same no matter the program. but the bias signal (and dark signal) in a flat can throw that scaling computation off, which is why i'm thinking about the bias frame so much.
i still think there is something funny about the bias frame, but i guess you'd have to ask other SX users (or SX themselves) about it. although my camera is also based on the KAF-8300M all of the surrounding electronics are going to be different, so it's hard to say if your camera is performing normally.
anyway, outside of the bias, phil's suggestion to experiment with flat exposure is certainly something you can try.
by any chance are you using a fast readout mode? for me fast readout is not supported by my imaging capture program so i never use it. takes about 1.5 or 2 sec to download a bin 1x1 frame over USB2.0.
rob
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OK - not sure about the Bias frame - personally I would discuss with the manufacturer and/or other users of that camera.
I tend to agree with Rob - if you subtract your bias frame from your flat (to get a calibrated flat) the flat ends up with the Bias pattern noise superimposed on it...
I'm not seeing the same thing as Rob regarding the flats moving vs the lights... they seem to line up pretty well to me.
I would try running the script again without supplying any bias frames and see what happens.
This is the master light created with the PP-script but without a bias frame (and then AutoSTF)
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_L_master_no_bias.jpg)
As you can see overcorrection is still occurring.
I was looking at the large spots on the right hand side... they haven't moved which rules out focus/rotation.
I still think that the solution is some combination of the flat being too bright and the bias frames being too noisy.
A master light created without bias frames would be helpful...
Here's a master light without bias. But still dark calibrated.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Flat_master_no_bias.fit
/Daniel
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well the entire camera vibrates a little bit when the filter wheel is moved, so in theory it could cause dust on the CCD to move a little bit as well.
focus is not super critical for vignetting. i don't have any experience with newts but given the optical distortions inherent in newts i wonder if focus could affect dust shadows at the corners of the image more than at the center. this is certainly something you could try to characterize.
anyway we have had recent threads here with bad flats where some attempt was made to modify the flat to fix it up. strictly speaking this is the wrong thing to do, and to be honest i don't think it has anything to do with pixinsight's calibration - the methods for calibrating a frame are very much fixed and every program is going to do pretty much the same thing. the only real "freedom" is whether or not and how much to scale a dark frame. of course the flat scaling factor has to be computed but i think this should be the same no matter the program. but the bias signal (and dark signal) in a flat can throw that scaling computation off, which is why i'm thinking about the bias frame so much.
i still think there is something funny about the bias frame, but i guess you'd have to ask other SX users (or SX themselves) about it. although my camera is also based on the KAF-8300M all of the surrounding electronics are going to be different, so it's hard to say if your camera is performing normally.
anyway, outside of the bias, phil's suggestion to experiment with flat exposure is certainly something you can try.
by any chance are you using a fast readout mode? for me fast readout is not supported by my imaging capture program so i never use it. takes about 1.5 or 2 sec to download a bin 1x1 frame over USB2.0.
rob
I don't think there's something wrong with the calibration in PI either. I'm gonna try to take new flats with a bit shorter exposure and see if that helps with the vignetting (the dust motes will be hard to align up perfectly again but I'll give it a try.)
I'll post a message on the SX Yahoo group and see if I can get an example of a bias from another identical camera to compare to mine and see if there are any differences.
I'm not using the fast readout mode.
/Daniel
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I wouldn't worry about the dust motes...the 1st objective has to be to get flats that correct the vignetting. Once you have that cracked, the dust will vanish from your subsequent images (you may have to write this image off to experience, frustrating though it may be).
I'd still like to get to the bottom of the root cause of the serious vignetting you have. Are you using 1.25 inch filters?
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I wouldn't worry about the dust motes...the 1st objective has to be to get flats that correct the vignetting. Once you have that cracked, the dust will vanish from your subsequent images (you may have to write this image off to experience, frustrating though it may be).
I'd still like to get to the bottom of the root cause of the serious vignetting you have. Are you using 1.25 inch filters?
Hi. I agree, to get the flats to do its job is top priority.
I'm using 36mm filters, which are larger than 1.25 inch, but a bit smaller than 2 inch. (36mm is 1.42 inches)
/Daniel
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Ok, I took new flats with shorter exposure and this time I got this result after calibration
(single calibrated sub)
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_CalWithNewFlat.png)
This is a new flat sub. Does it look better than the previous ones?
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Flat_L_New_001.fit
Still not good in other words.
Any ideas? Could it be my light-box that isn't providing an even illuminated surface?
/Daniel
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that is certainly possible - can you try to make some sky flats just as a comparison?
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Yes, but that has to wait until tomorrow. It's dark here now in Sweden.
I'll post the result as soon as I can.
/Daniel
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ok. along the lines of even illumination - what is the light source in the box?
fluorescent bulbs do flicker at the mains frequency, so you need to make sure that your flat exposure is long enough to capture multiple cycles. if it is an EL panel it will flicker at 10khz or 20khz or something like that. less of a problem for CCDs than for DSLRs which can make very short exposures with good illumination.
another pitfall is that some types of shutter give artifacts for short exposures. so if the light source is really bright, you may be getting artifacts from the shutter. my STT-8300M is supposed to have an "even illumination" shutter but flats < 1s do start showing some funny artifacts.
rob
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Daniel, maybe your problem is not in flats. I thing you have a problem inside your system that is causing uneven illumination. Recently I have similar problem. I have spacer ring which was not black, and was causing internal reflections.
Can something in your system cause such a thing?
Maybe your secondary mirror is not big enough, and due to that causes uneven illumination?
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I've not seen anything like this...
Your original flat master vs the new flat you just uploaded - the vignetting has changed!
I can only think of two possibilities:
1) the light box illumination has changed
2) something in the optical train has changed
Your 36mm filters should be fine - are you using the StarlightXpress filter wheel?
The vignetting is not symmetrical which implies something is not aligned in the optical train (but we need to see a sky flat to rule out your light box)
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Hello again.
Quick reply, my little daughter needs to eat dinner soon. :)
This is a sky flat sub that I took.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/sky_25_001.fit
And this is a light sub calibrated with 10 sky flats (plus bias and dark).
(AutoSTF applied)
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_CaliWithSkyFlat.png)
So I still have the same problem…
I'll get back to your other questions as soon as I can.
/Daniel
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To me both flats (light box and sky) match pretty well, so I would discard a problem with them. Also the bias seems to work well in them to remove the pedestal.
The problem seems to be with the lights. Do you have a dark over there? Maybe the dark current is high enough to create a new pedestal signal that is messing the normalization done by the bias.
I substracted the mean of the bias to the image, additionally to the first bias substraction and that worked pretty well. There is still a gradient, but seems to be light pollution or normal sky gradient.
Also there may be a inconsistency with a gain somewhere...
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To me both flats (light box and sky) match pretty well, so I would discard a problem with them. Also the bias seems to work well in them to remove the pedestal.
The problem seems to be with the lights. Do you have a dark over there? Maybe the dark current is high enough to create a new pedestal signal that is messing the normalization done by the bias.
I substracted the mean of the bias to the image, additionally to the first bias substraction and that worked pretty well. There is still a gradient, but seems to be light pollution or normal sky gradient.
Also there may be a inconsistency with a gain somewhere...
Here's the master dark that I used. (it's 20x5 min total)
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/dark-BINNING_1-EXPTIME_300.fit
I don't fully understand what you did (or how) to get a better result, but that sounds good. There probably will be a fair amount of light pollution in the image.
/Daniel
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Daniel, is this the first time you have that kind of problem, or you have vignetting on older images too?
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Daniel, is this the first time you have that kind of problem, or you have vignetting on older images too?
This is only the second image I've done with this camera.
The first actually had the same problem, but I just thought it maybe was because of the full moon that time. I gave up on that image...
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Leo.png)
I have previously imaged with a DSLR and I did not have this problem with that camera.
/Daniel
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I have previously imaged with a DSLR and I did not have this problem with that camera.
With DSLR you have different back focus, so now you camera is in the different position. I still think your secondary mirror could cause this type of vignetting.
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Are the darks already bias substracted? Could you post a raw dark sub?
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Are the darks already bias substracted? Could you post a raw dark sub?
Hi.
I guess the dark gets bias subtracted in the PP script?
Here's a single dark sub. No calibration.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Dark_001.fit
/Daniel
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I have previously imaged with a DSLR and I did not have this problem with that camera.
With DSLR you have different back focus, so now you camera is in the different position. I still think your secondary mirror could cause this type of vignetting.
If that is the case, shouldn't the flats still correct it even though the vignetting has increased?
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To me, all your calibration frames looks ok (although, I would recomment taking A LOT of them. Bias specially, is very noisy, and you can see the same patterns in the dark). The flats from the box and the sky flat are very similar (you can judge by yourself by substracting the bias to them, and then divide one by the another with PixelMath, using a expression like this: "$T*mean(flat)/flat" and disable rescaling.
As I said earlier, the problem seems to be with your lights. They have an extra pedestal, that does not match your bias. If there is nothing wrong with the capture settings, or file format, you may also check for parasital light, although for the pedestal value I doubt that this is the problem.
To fix this, as a workaround, open your main bias. Then, create a new ImageContainer, with all the raw light frames in it. Then use PixelMath with the following equation: "$T-mean(bias)", where bias is the identifier of your master frame image. Do not rescale. And apply this instance to the ImageContainer. It should create new files with the bias pedestal substracted. Finally, run the new light through the batchprocessing script, or the image calibration process.
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I think I understand the asymmetric vignetting now...
http://www.skyandtelescope.com/howto/diy/3306996.html
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To me, all your calibration frames looks ok (although, I would recomment taking A LOT of them. Bias specially, is very noisy, and you can see the same patterns in the dark). The flats from the box and the sky flat are very similar (you can judge by yourself by substracting the bias to them, and then divide one by the another with PixelMath, using a expression like this: "$T*mean(flat)/flat" and disable rescaling.
As I said earlier, the problem seems to be with your lights. They have an extra pedestal, that does not match your bias. If there is nothing wrong with the capture settings, or file format, you may also check for parasital light, although for the pedestal value I doubt that this is the problem.
To fix this, as a workaround, open your main bias. Then, create a new ImageContainer, with all the raw light frames in it. Then use PixelMath with the following equation: "$T-mean(bias)", where bias is the identifier of your master frame image. Do not rescale. And apply this instance to the ImageContainer. It should create new files with the bias pedestal substracted. Finally, run the new light through the batchprocessing script, or the image calibration process.
Hi Carlos.
I did as you suggested and then ran a new round in the batch preprocessing script and now it looks better. As you said it still contains a gradient but that can probably be fixed with DBE. Do you think you could explain why this is working? You first subtract the mean bias and then I suppose bias is subtracted again in the script, am I right?
(https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_mean_bias_subtracted.png)
I don't know what could have happened during capture of my lights. I use Nebulosity to control the camera and it doesn't let me adjust gain or offset, just the exposure time so I don't really know how to fix this issue.
/Daniel
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Do you take your darks with the camera on the scope?
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Do you take your darks with the camera on the scope?
No, not this time, I removed the camera and covered the opening, as well as leaving the shutter closed while exposing.
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Phil, I don't believe there is a problem with the darks. They seem good enough, and they do not require to be on the scope at all.
Daniel, the overcorrection is a clear indicator that there is a pedestal problem somewhere, either on the flats or the lights. The "model" behind our images is that they consist on several sources:
Img_adquired = (Img_ideal+noise)*uneven_illumination*pixel_sensitivity + bias + dark_current + noise
Since we can capture the external sources in the flat, dark and bias frames, we end with the following equation to recover the "ideal" image (of course, ignoring the noise):
img_calibrated = ( img_adquired - bias - (dark-bias) )/(flat - bias)
If we have an additional pedestal value in the lights, we end with:
img_calibrated' = img_calibrated + pedestal/(flat-bias)
(we have the same image as before, plus a gradient from the inverse of the flat... i.e. an over correction).
Also we may have these problems with a negative pedestal in the flats.
I found the value by experimentation. It kind of makes sense that the pedestal is the mean of the bias, since that is the electrical pedestal value, although I don't know why it is doubled in your images. Also I experimented with a gain factor 2x (dividing the lights by two), and then calibrating, but it seems that in that case the bias and dark frames do not match the data.
That's it. I encourage you to investigate this thing further. Try making the capture in another software, and see if the problems still appear.
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Hello Daniel
I don't read the hole thread, but for me it looks like the most usual problem with reflectors... Light which does not travel the right optical path. A Newton for example suffers from "light from behind". If there is any light source (laptop) behind the telescope, it causes strange illumination issues, especially with flats… I always put a shoppingbag on the rear end of the tube and the problem is gone.
Regards
TeeJay
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This is exactly what I was getting at with my question about the darks...
If there is a light leak (for example via the focuser drawtube and/or the rear cell in the case of a Newtonian) it can show up in darks taken on the scope if the shutter is open and it can show up in the flats to a small extent (they are normally very short exposures) and in the lights.
I have had to add some tape to my Baader steeltrack focuser to block a light leak and several other folks on the forum I use have reported the same problem with various focusers and scopes.
The problem was first found by looking at on-scope darks taken with DSLR's which leave their shutter wide open during dark acquisition.
The problem really messes up the image calibration process. Obviously it will vary each time because the incident light finding its way into the scope/camera is always different (e.g. from the moon etc).
I'm not saying this IS the problem here - just that it is a situation where image acquisition appears to have worked correctly but images will not calibrate/integrate correctly.
regards
Phil
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Yes, it may be parasital light, but is really strange that it matches the bias pedestal... Anyway, it is a thing that must be discarded too with some darks in the scope.
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Phil, I don't believe there is a problem with the darks. They seem good enough, and they do not require to be on the scope at all.
Daniel, the overcorrection is a clear indicator that there is a pedestal problem somewhere, either on the flats or the lights. The "model" behind our images is that they consist on several sources:
Img_adquired = (Img_ideal+noise)*uneven_illumination*pixel_sensitivity + bias + dark_current + noise
Since we can capture the external sources in the flat, dark and bias frames, we end with the following equation to recover the "ideal" image (of course, ignoring the noise):
img_calibrated = ( img_adquired - bias - (dark-bias) )/(flat - bias)
If we have an additional pedestal value in the lights, we end with:
img_calibrated' = img_calibrated + pedestal/(flat-bias)
(we have the same image as before, plus a gradient from the inverse of the flat... i.e. an over correction).
Also we may have these problems with a negative pedestal in the flats.
I found the value by experimentation. It kind of makes sense that the pedestal is the mean of the bias, since that is the electrical pedestal value, although I don't know why it is doubled in your images. Also I experimented with a gain factor 2x (dividing the lights by two), and then calibrating, but it seems that in that case the bias and dark frames do not match the data.
That's it. I encourage you to investigate this thing further. Try making the capture in another software, and see if the problems still appear.
Sorry for not responding for a while, but the weather has been bad so I have not been able to try to capture a new set of images.
I did however capture bias frames with another computer (I use a Mac) and other software. I don't think there's a very big difference between them but some of you might see something?
Here's a bias with Nebulosity running on a PC
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/biastestNebPC.fit
Here's a bias captured with Starlight Xpress own capture software (Had to save as tiff, the fits files were unreadable for some reason).
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/biastestsx.tif
And one more thing as well, I had to send the camera back to SX for cleaning of the CCD. It's possible that the big black spot is a paint chip from the shutter (other have had the same problem with this model). So I can't test anything until I get the camera back from England.
/Daniel
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Having exactly the same issue with my F2.8 PowerNext (with ASA corrector).
I have tried everything, but some interesting things to try here.
1. Baader focuser - check I have one - check for light leaks
2. Stray light into rear cell - check, will check this too as heard this can be a probelm too.
3. Darks Bias on the scope - Will re-create my bias and darks on the scope and see if this solves it.
Currently I am having to use Maxim to manually subtract my flats at 5% scale from my lums to resolve my issue.
The problem is NOT PI, it happens in Maxim and DSS as well and it seems the scaling is off by 95% or more in my case.