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Messages - CraigNZ

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General / Re: Newbie question about darks
« on: 2015 September 11 00:48:56 »
I would recommend taking say 10 darks at different temperatures, call each of them a 'Set'.  For each set combine them to create an average, so one for each set.  Then use statistics to look at each average image and compare them.  If the mean and deviation are pretty much the same then I would not worry about temperature.

It is always best to bracket your light frames with dark frames.  In other words, say a set of 10 dark images before you take your light frames and 10 after.  Again average each set and then take the average of the two set averages which then represents somewhat the average dark frame during the time of your light frames.

Dark frames are also dependent on the exposure time.  A longer exposure will have more thermal noise (Dark frame) in it than a short exposure.  Again, for a given exposure time repeat the tests above and determine if you need to do specific dark frames given the exposure time, or as mentioned above, take a very long dark frame (usually 2x longer than your longest light frame) and use 'optimization' in PI when calibrating an image.

Each sensor and observing session is different so begin by simply taking dark frames in varying conditions and see how your camera does.  If the thermal noise is very low and the exposure times are short than a single dark frame will suffice.  But if you are doing precision photometry work or very critical 'pretty pictures' then you will need more sophisticated calibration and dark frames.

Start slowly and learn, but most of all have fun.  Take pictures and study each one, you will have plenty of time to master it all.  Going too technical can quickly become a downward spiral where we spend more time trying to squeeze that last bit of perfection out of a pixel than we do in simply enjoying the hobby.

I have seen many times that a uncalibrated image of a galaxy or nebula will bring the same 'wow' from family and friends as did several hours, days and weeks trying to get that picture to match the Hubble.

General / Re: Aperture Photometry script: User defined objects
« on: 2015 September 10 08:48:48 »
Hi Andres,

Fair enough, what I can do for my immediate needs is to write a Post Processor which will load in one or more csv files and reformat the data to a new csv file.  In the long run though I see a lot of additions to the Aperture Photometry script, if it is okay with you, I could adopt the script and provide support and updates in the future.


General / Aperture Photometry script: User defined objects
« on: 2015 September 09 21:38:31 »
Is there an easy way to select from an image only the stars I want to use in the Aperture Photometry script?

I have 35 images spread over 3 hours that I want to list the flux for just a few selected stars.

I tried using 1 image, finding the stars I wanted in the output table and then entering the Star ID (column B) into the user defined stars table but it could not find the stars.  For example this star:   210220.7-094031         
 is listed in the table but it cannot find it when I add it to user defined stars.  To do it by coordinates means I have to translate the position in degrees to or which takes quite a while.

Ideally the best way would be to select the first image, output a detected stars image, click on the ones I want and then have the script follow these stars over the 35 images and give me the detailed data.  As a minimum I should be able to add by star id.

General / Re: Astrometric distance between two stars
« on: 2015 September 04 20:00:08 »
I do use SkyX for camera and drive control, but didn't see any way to apply the distance and angle tool to an image or series of images.  The tool seems to work only on the sky displayed or an Image Link image.

What I have done so far is use DynamicPSF and selected the two stars on a series of 10 images.  I then exported this to a csv file and loaded into Excel.  In Excel I then calculated the distance between the two stars on each image and graphed the results.

The precision is very good, down somewhere to 0.01 pixel or so .. haven't quantified this yet.  But if you run a trend line through the data you see a very distinct slope showing the effect of differential refraction.  I will verify this by correcting the position of each star for refraction effects and see if the slope is removed.

The equation for the trend line is: y  = 0.0019x + 68.537 where:
    y - separation in pixels
    x - image #

The image # represents the time because the images were taken as a software controlled series, each image was 45 seconds exposure + 10 seconds delay + a couple of seconds download time.  Or about 1 minute for each image.

You can see the slope is very small resulting in a drift of approx 0.03 pixel over the 10 images (about 10 minutes).

A lot more work is needed in confirming all of this but it does show the level of precision I am trying to achieve.  PI is a far better tool for doing this than SkyX.

General / Re: Image Solver/Focal length calculation
« on: 2015 September 04 02:58:41 »
Agreed, we don't need to create extra work when not necessary.  What we now know is how the resolution is calculated and the precision.  Thanks heaps for providing that.

PI is well designed and forms a base for serious image processing, especially in photometry and astrometry.  I have moved from Mirametrics software to PI for my work and hoping to contribute to its growth.

General / Re: Image Solver/Focal length calculation
« on: 2015 September 04 02:21:46 »
Assuming the data sheet for the KAF 8300 sensor is accurate then we are limited to 4 significant digits for the pixel size because the size of the active sensor area is given as 17.33 mm.  Dividing the active sensor size by the number of active pixels we get o.oo539987973 rounded to 4 significant digits is 5.400 microns per pixel.

If this is the limiting factor in the equation then we can specify the resolution as 1624 mm.

The plate scale would be o.6857 arc seconds per pixel.

We would need to know if the plate scale has a precision of 4 significant digits by examine how it is derived.

General / Astrometric distance between two stars
« on: 2015 September 04 01:58:04 »
I use DynamicPSF to precisely determine the pixel location of stars on an image.  Is there a script for calculating the distance and angle between two stars?  I did not see it in aperture photometry but I guess it could be done using the spreadsheet tables and a macro.

Would be nicer if DynamicPSF could show the distance from the previous star selected to the current star.  If Image Solve has been performed on the image the distance can include any field curvature and displayed in arc seconds.

General / Re: Image Solver/Focal length calculation
« on: 2015 September 04 01:45:29 »
Hi Andrés,
I agree about the limits of precision, in which case we should not display the resolution as 1624.44 which implies 6 digits df precision.  With only three digits we would need to round this to 1620.

General / Re: Image Solver/Focal length calculation
« on: 2015 September 03 19:44:44 »
The order of calculations on the formula above threw me a bit, it would be better to write it as:
  return (resolution > 0) ? (this.xpixsz/resolution) * (0.18/Math.PI) : 0;

Doing this and using the values from Image solver I get:

  fl = (5.4 * 3600 / 0.6865) * (0.18/pi) = 1622.48 mm

which is the same as using the original formula

or, using the values from Image Solver:
  fl = (5.4 * 3600 / 0.686) * (0.18/pi) = 1623.66 mm

So this shows the equation used by PI is equivalent to the one I used, giving the same results.

The discrepancy between the results above and what is displayed in Image Solver is probably due to one of the numbers:

5.4 - this is the pixel size, I am assuming PI is getting this from the FITS header:
  XPIXSZ = 5.4

3600 is the conversion of arcsec to degrees so that should be okay.

0.686(5) - this is the number reported by IS and was verified by measuring two stars on the plate, so it is good.

0.18 is 180 degrees / 1000 um/mm  the 180 is used in converting from radians to degrees, and the 1000 is converting um to mm

pi is a mathematical constant

Assuming the 5.4 is the same value used in both methods then it is likely the plate scale is different.  Working the equation in reverse results in a plate size of:

resolution = (5.4 / 1624.44) * 3600 * (0.18 / pi) = 0.68567

This shows the error is likely due to (a) rounding when displaying the plate scale, and (b) me not being able to measure the plate scale manually to this precision.

Using the value 0.68567 shows a focal length of

  fl = (5.4 * 3600 / 0.68567) * (0.18/pi) = 1624.44 mm

So a very small change in plate scale results in a few mm variance in the focal length.  Could we therefore have the precision of the resolution number in the Image Solver report extended from 3 decimal places to 5 decimal places?

Current: Resolution ..... 0.686 arc sec / pix

Proposed: Resolution ..... 0.68567 arc sec / pix

General / Re: Image Solver/Focal length calculation
« on: 2015 September 03 12:31:02 »

Thank you .. that is exactly what I was looking for.

General / Re: Image Solver/Focal length calculation
« on: 2015 September 02 19:37:06 »
Possibly, I am looking to confirm the method used for calculating the focal length in Image Solver so I can reproduce it at this end.

The reason for the accuracy is for determining the spacing between mirrors in a RC telescope.  If one of the mirrors is moved too far in or out then the focal length will change and hence the plate scale.  I am thinking that Image Solver could be used to determine that the mirrors are out of alignment (separation in this case) by measuring the plate scale exactly and calculating the focal length.

General / Image Solver/Focal length calculation
« on: 2015 September 02 17:42:20 »
Using a sample image I get this report from Image Solver:

Referentiation Matrix (Gnomonic projection = Matrix * Coords[x,y]):
           +0.000190468       -5.53464e-008           -0.319207
          +8.53177e-008         +0.00019046           -0.241338
                     +0                  +0                  +1
Projection origin.. [1676.282351 1266.380252]pix -> [RA:+22 09 12.72 Dec:-09 51 54.40]
Resolution ........ 0.686 arcsec/pix
Rotation .......... 179.980 deg
Focal ............. 1624.44 mm
Pixel size ........ 5.40 um
Field of view ..... 38' 18.4" x 28' 56.1"
Image center ...... RA: 22 09 12.710  Dec: -09 51 54.66
Image bounds:
   top-left ....... RA: 22 07 54.907  Dec: -10 06 22.64
   top-right ...... RA: 22 10 30.546  Dec: -10 06 21.61
   bottom-left .... RA: 22 07 54.987  Dec: -09 37 26.60
   bottom-right ... RA: 22 10 30.398  Dec: -09 37 25.57

The formula for calculating FL from an image is:

FL (mm) = 206265 * pixel size (mm/pixel) / plate scale (arcsec/pixel)

I carefully measured a few star distances on the plate and found a plate scale of 0.6865 arcsec/pixel which is in agreement with Image Solver.

The camera is a SBIG STF-8300M with pixel size of 5.4 u (0.0054 m)
substituting into the above equation yields:
FL = 206265 * 0.0054 / 0.6865 = 1622.48 mm

Image Solver shows 1624.44 mm

Why the difference between the two numbers?  Both are using the same Pixel size and plate scale.
Curvature of the plate?

I have been using the trial version of PI for two weeks with no problems.  But in the last two days when I run the program I get this error.  If I close the error and try again sometimes it works okay and other times I have to try and run it three or four times before it finally works okay.

Can someone explain why I get the error and then when I try again it works okay?  Something wrong with the server or the network?


General / Re: Open DSLR raw file shows wrong pixel values
« on: 2015 July 09 01:05:19 »
I ran a quick test, using an image with 16 bit it showed max as 14787.  In 14 bit it showed 3696.  The difference of 2 bits is a divide by 4, so 14787 / 4 = 3696.  So setting the drop down box simply takes the pixel value and scales it by the number of bits selected.  That makes sense now.

General / Re: Open DSLR raw file shows wrong pixel values
« on: 2015 July 09 01:01:29 »
You are right .. I changed the setting from 14 bit to 16 bit and now the values look correct.  Though it is a 14 bit camera I guess because they are stored as 16 bit values the 14 bit setting was throwing off the calculation.  Thanks heaps for that.

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