Doh!.....

Scott Badger

Well-known member
The first two paragraphs are just me needing to vent, so you probably want to skip them to the actual question in the third.....

The last couple weeks have been a bit of a slog; a conjunction of unfortunate events. Bad weather, short nights, and bright moon (when the weather wasn't bad). The learning curve of adding guiding for the first time (OAG). The complication and hassle of getting all the right adapters, extenders, and spacers needed to add the OAG. First time through putting it together, binding and bending an extender that put everything on hold waiting for it to be replaced. Getting it all together, finally, to discover I really needed parts with a wider aperture (and a much wider price...) due to the vignetting what I got was causing. A collimation attempt that led to a loose and spinning secondary mirror assembly, which in turn, but indirectly, led to an 'advised' shift of the corrrector plate (advice I followed, but still not sure if advise-able....), and finally a clear night that, to my novice eyes, looked really good.....until the next morning when I pulled up the images and......questioned everything.

Then last night it all came together. Completely clear, and all indications that the seeing was much better than the previous night. My polar alignment was.....well, not terrible, PHD2 was, for the most part, willing to put up with me and my mount, I checked a couple of the first images on my camera screen and they looked really good, so I got everything going, went to bed, woke up after a couple hours to change the camera battery and check all systems, and woke up again before dawn to pack up and minimize dew issues. Did another spot check of a couple images with FastStone before heading to work, still looked good and congratulated myself all day, got home, opened up Pixinsight and loaded a random image and.....it opens in color.....which it's not supposed to, until I glance at the title and see .jpg.......and on further inspection, it's my camera's smaller jpeg format to boot......somehow the file quality setting was changed and I didn't notice. I blame BYEOS......probably an act of retribution for doing my PA and then shutting it down to use a simple intervalometer to run the imaging instead....

Anyhow, I have about 30 of them, so a small stack, but will be getting more, and raw files going forward. My question, is it possible/advisable to combine the jpeg's with future raw files, or do I just stack them alone as sort of a preview to the main event?

Cheers,
Scott
 
i'd just consider anything you make with jpegs as a preview. they are so "cooked" by the camera that they are really not suitable for processing as astro images...

rob
 
I did that once on an all-nighter on Centaurus A back in my DSLR days. I know how you feel.
 
Thanks Rob, that’s what i thought but I was hoping......

Another question, the images from the previous night, clear but poor seeing, are super soft but have low FWHM values, like in the 0.6 range, which is close to the Dawes limit for my setup, if i’m calculating that properly......does that make sense? On the other hand, the jpegs from the better night of seeing, plus better PA and better guiding, were in the 1-2 range. More reaonable given my mount (CGEM) and novice skills.

cheers,
Scott
 
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well if the FWHM is measured in pixels and the jpegs are of reduced size, then probably it's just an artifact of the downsizing of the jpegs. if you converted both to arcseconds you'd probably get similar FWHM numbers for both.

rob
 
Even though I'm pretty sure the FWHM was measured in arcsecs (by the PI subframe selector, I'll check when I get home tonight), I think you may still be right about them being jpegs. The image scale I entered is based on the pixel size reported by Canon, and that is based on the sensor pixel dimensions, right? So wouldn't the jpeg at about half the pixel dimensions of the raw file result in the same artifact you mention even if the units were consistent between the two measurements? That said, given the significantly poorer seeing, PA and guiding of the first night, I'm still surprised that the FWHM measurements are realtively equal......

Cheers,
Scott
 
if you've entered a correct image scale into SFS then yes, it should convert to arcseconds, however, to get correct FWHMs for all of your images, you would have had to enter the FWHM twice in two separate runs, since the jpegs have been scaled down and the CR2s have not. if all the files are converted with a single image scale, then one of the groups is just wrong. yes, the actual conversion it is a linear operation (just multiplication by the scale factor) but i suspect that by downsizing the jpegs the stars are just going to look tighter when subjected to PSF fitting.

i guess why i'm saying this is because an FWHM of 0.6 as would be like chile-level, or maybe even hubble-level seeing... and that is pretty extraordinary. the apparent quality of the jpeg images seems more likely to me to be a measurement/interpolation artifact.

edit; oh i think i got it backwards; you are saying the CR2s have phenomenal FWHM values. i wonder if somehow SFS has measured a bunch of hot pixels or something? at any rate as mentioned if you throw them all into SFS then half of them will have a wrong conversion to arcseconds no matter what you do...

rob
 
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Sorry, I was a bit confused about the jpeg scaling too....the jpegs are half the pixel dimension of the raw files, so pixel size and image scale are effectively doubled, right? and when I plug in the corrected image scale of 1.34 for the jpegs (vs 0.672 for raw), the FWHM values are in the 3 to 4 range.

In any case, I looked through the raw data I've been collecting over 9 nights, unguided for the first 4 and guided since, and noticed that all the unguided images had FWHM weights in the 2 to 4 range, and all the guided images were in the 0.6 to 1 range, except the last night of guided imaging where I collected the jpegs and saw the same range (when scaled) as pre-guiding. Now, I'd expect guiding to help, but like you said the numbers I'm getting are clearly anomalous, and that doesn't explain the last night.....but, there was one other difference that does track all the way through.

When I took the non-guided images, I had the Celestron T-adapter between the scope and my camera which a has a more than 2" aperture, but for the first 4 nights of guiding, I used M48 (much smaller aperture) adaptors and extenders instead to allow the addition of an OAG. Because the M48 parts caused so much vignetting, I ordered a custom PreciseParts piece with an equivalent aperture to the Celestron adapter and used it for the first time that last night.....so, could low FWHM values be an artifact of the narrow aperture optical train?

Cheers,
Scott
 
yes the image scale should be doubled if the image size is halved.

i guess one thing you might try is to use the script called FWHMEccentricity (under Image Analysis) to analyze your vignetted images. if it finds enough stars, you can click the button labeled "Support" and it will generate maps showing the eccentricity and fhwm across the field. that might tell you if somehow the corners are messed up due to the vignetting. since SFS is just showing you the average FWHM across the whole frame, it might be reporting overly optimistic numbers if the corners are bad but the center is OK.

the eccentricity of the guided vs. non guided images might be useful to look at as well. it could be that if the eccentricity is bad the FWHM measurements are not as meaningful as an absolute indicator of image quality.

rob
 
When I ran the FWHME script on both the un-guided (wide aperture, little vignetting) and guided (narrow aperture, lots of vignetting) raw subs, it only picked up 80 to 90 support stars, not enough to produce a map. On the other hand, when I ran the script on one of the the jpegs (guided, wide aperture) it picked up 10x as many, over 800, but that image doesn't have the additional vignetting, so the map isn't as instructive.

Cheers,
Scott
 
FWIW, here's a little more data (see screenshots). While the aperture size/vignetting does seem to correlate with the scale of FHWM values, within each group of images there's of course still variance due to other factors, and often that distortion (like a cloud passing through) seems to drive a lower FWHM value, not raise it One of the lowest values I saw was for a sub where the mount skipped and made every star a double star, but the second one much smaller. Anyhow, I also included the results from last night where I used my new custom larger diameter part with the OAG for the first time (and remembered to check my image settings, that is.....).

In case it matters, the images I ran through SFS are not calibrated yet, and are all the images from those nights, including those I would have tossed. The un-guided images are 120s subs (ISO 2000) and the guided images are 300s subs (ISO 800). Of the three nights, last night was the best; clear sky, good seeing, no wind, stable temp, and no moon. I'm in a Bortle 3 location.

Cheers,
Scott
 

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yep - mount-bumped images can definitely fool these tools. as you noticed what happens is you have a very short exposure of the stars together with a longer one and the very short exposures stars are very small and have low FWHM. i usually run Blink on my subs first to eliminate any obvious outliers (like guiding problems/bumps and clouds) before going into SFS.

it is probably a good idea to run SFS on calibrated images so that the optical vignetting is sorted out.

what is the eccentricity of the unguided images?

rob
 
I do calibrate and blink before SFS, just haven't finished acquisition and started processing M51 yet. If I can get out tonight or tomorrow, that'll be the last night, though if I can't I'll probably make do with what I have. It's been a bad luck galaxy for me......:)

Over four nights worth of unguided subs (319), again not yet calibrated or blinked, the eccentricity ranged from 0.319 to 0.862 with a median of 0.63. Like with FWHM, the values at both ends are images with star distortions. The 0.319 sub is another skip, but with equal double stars.

As a first 'blink', I usually go through the subs with FastStone which has a magnifying glass tool that makes it easy to check for star roundness. I usually toss a little more than a third of the unguided. And then a few more of the outliers in SFS. Here's Bode's unguided and 63 120s exposures: https://www.astrobin.com/kem4rz/?nc=user

Two nights ago, with my new wider aperture extender and guiding, the eccentricity was 0.5 to 0.63, median of 0.57. I have a ways to go towards improving my guiding and aligning, though. The stars aren't as bloated as un-guided, but they aren't as round either..... There might also have been a balance issue with the mount that I've corrected since but haven't tested yet. Additionally, the mount is a 6 year old CGEM and though it appears to have gotten very little use before I got it, maybe that's worse than regular use? In any case, will probably get the hyper-tune kit and see if I can improve things from that end too.

A side question, in the logs, why does PHD2 report such different PA error numbers for different guiding sessions, even though it's the same night, same object, and no adjustments to mount alignment between? Two nights ago, the log viewer shows PA Errors of 0.6', 0.1', 0.8', and 5.6'. FWIW, the 5.6' error was the last session where it had lost the guide star completely due to the rising sun about an hour before I woke up and shut everything down.

Cheers,
Scott
 

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The first two paragraphs are just me needing to vent, so you probably want to skip them to the actual question in the third.....

Then last night it all came together. Completely clear, and all indications that the seeing was much better than the previous night. My polar alignment was.....well, not terrible, PHD2 was, for the most part, willing to put up with me and my mount, I checked a couple of the first images on my camera screen and they looked really good, so I got everything going, went to bed, woke up after a couple hours to change the camera battery and check all systems, and woke up again before dawn to pack up and minimize dew issues. Did another spot check of a couple images with FastStone before heading to work, still looked good and congratulated myself all day, got home, opened up Pixinsight and loaded a random image and.....it opens in color.....which it's not supposed to, until I glance at the title and see .jpg.......and on further inspection, it's my camera's smaller jpeg format to boot......somehow the file quality setting was changed and I didn't notice. I blame BYEOS......probably an act of retribution for doing my PA and then shutting it down to use a simple intervalometer to run the imaging instead....

Cheers,
Scott

I feel for you... I had a similar experience just last month. I visited my parents in Scottsdale, AZ. I haven't been to see them in AZ in over 10 years (they visit us in the eastern US). I couldn't take the whole AP rig but thought the desert would be a good location to try night-time landscape astrophotography from a tripod. One night my father and I drove 2 hours to Sedona, AZ and were rewarded with a great location. There were several cool views to image. I took hundreds of images with different landscapes (at the bottom) of the images while my father sat in the car reading. After a few hours we left and it hit me as we were heading home. Doh! I switched the camera from RAW to JPG earlier in the day when I was taking some picture of the three of us. I used Sequator to stack the JPEG images and get something good from afar but far from good.
 
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