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DSLR-compatible software/utility for image analysis (x,y star coordinates)?

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#1 deansjc

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Posted 23 September 2020 - 05:02 PM

Can any CN users advise on a software or utility that allows one to track star movement within a camera frame while tracking.  I suspect that most guiding software can do this, but for only determining mount tracking accuracy, purchasing a guide-scope, camera and software package is a bit too large of an investment.

 

Metaguide has this functionality, but is very specific as to cameras, and is certainly not DSLR friendly.  My goal is to use my DSLR while aligned, including PA, to obtain hard data such as x, y coordinates of a star that the software locks onto over a period of time.

 

Any contribution would be appreciated, with my thanks in advance.

 

John



#2 meansrt

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Posted 23 September 2020 - 05:07 PM

Do you have a go-to mount? If so, you could use a program like Astro Photography tool to examine the image your camera takes and determine exactly where it's pointing. From here you can have APT adjust a go-to mount so that you are pointed exactly at your target. Without autoguiding I think that you could take several images then have APT readjust to center your target every once in a while. 



#3 photoracer18

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Posted 23 September 2020 - 05:11 PM

How do you plan to account for atmospheric distortion? Some mounts take this into account and some don't. So your X-Y coordinates will not be accurate the lower in the sky you image unless you add this factor in.



#4 deansjc

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Posted 23 September 2020 - 05:19 PM

So some more details.  Yes, this is for a Celestron SE mount with GoTo.  I will look at APT.  As for atmospheric distortion at low altitudes, while of course it is a factor, when one is simply looking at how one's mount is moving over long exposures relative to ideal tracking, I would have thought that this is a secondary consideration.  I will bear this in mind though.  Thank you.

 

I suppose that Metaguide might be unique in this capability, but I doubt it.



#5 Alex McConahay

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Posted 23 September 2020 - 05:44 PM

Any photo editing software (PixInsight, Nebulosity, Photoshop, whatever) allows you to overlay one image with another. Do NOT register the frames (which moves the stars around on a frame) and simply align one frame over another. Determine where the center of a given star is in one frame in pixels compared to where it is in the other frame. Multiply the difference in pixel position by the arc seconds per pixel, and you have your answer. 

 

Another way to do it......REGISTER the frames. Check the edges wherever you have one frame showing and the other not represents how far the frame moved.....COunt the pixels, and multiply by arc second per pixel. 

 

A third way......Plate Solve (any one of a number of programs will plate solve). The difference in pointing is how much something has moved. 

 

A fourth way....Use astrometric software (like Pinpoint to measure the position of any given star.

 

A fifth way......check the elongation of your stars (the number of pixels in one axis versus the other). The difference is how many pixels it moved (improperly) during the exposure. 

 

Try PECPREP  http://eq-mod.sourceforge.net/pecprep/ to check periodic error.

 

But, what is it you are really trying to do? Check how your scope moves during an exposure? For what purposes?

 

Alex  



#6 rkinnett

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Posted 23 September 2020 - 05:51 PM

Do you need to evaluate on the fly while tracking, or can you shoot photos and process them later?

 

The functionality you're looking for is called plate solving.  The simplest way to use it is to upload individual photos to astrometry.net.  It will tell you the precise coordinates of each frame. 

 

If you need to do the evaluation while tracking, and have your computer hooked up to the camera, then you can either download individual frames and process them through a standalone plate solver (i.g. All Sky Plate Solver).  Or use the plate solving tools baked into pretty much all full-featured astrophotography suites, including APT, SGP, SharpCap, N.I.N.A., Indi/Ekos.



#7 deansjc

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Posted 23 September 2020 - 08:11 PM

Alex, what I am trying to do is determine how often and by how much my mount appears to be "jumping".  Within the same session on the same side of the meridian, I experience frames of the same duration that have streaking of stars and that do not.  The image streaking is typically east west.  These frames with "jumps" seem random, but perhaps are not.  Ideally, I can take a video and playback the data - haven't tried that yet.  This is an SE on a wedge.  Everything is solid.  I strive for balance that engages the Az gearing as advised by many.  I can live with a few but want more good exposures and longer exposures.  My setup is very routinized yet there must be a variable.  (I know the SE mount is not of great quality.) 

 

So to answer the question, yes, I am looking to see how the mount moves, since there is an undesired movement taking place.

 

You've provided several ideas for me to purse.  Thank you. 



#8 meansrt

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Posted 23 September 2020 - 08:41 PM

How are you triggering the camera? Are you physically pressing the button? If so that would explain the streaking



#9 deansjc

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Posted 23 September 2020 - 09:07 PM

Hi Robert,  I use a PC running Backyard Nikon or a wireless remote shutter release.  I take care that there is no cable drag as I use a USB cable to connect the camera to the PC.  Nikon allows a .5 second delay between mirror up and the exposure.  I thought this feature might also help minimize shake or vibration.  It makes no difference.  In fact, my best night of imaging where almost all 40 second images were usable, I inactivated the feature.

 

I appreciate the question,



#10 TelescopeGreg

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Posted 23 September 2020 - 09:28 PM

Alex, what I am trying to do is determine how often and by how much my mount appears to be "jumping".  Within the same session on the same side of the meridian, I experience frames of the same duration that have streaking of stars and that do not.  The image streaking is typically east west.  These frames with "jumps" seem random, but perhaps are not.  Ideally, I can take a video and playback the data - haven't tried that yet.  This is an SE on a wedge.  Everything is solid.  I strive for balance that engages the Az gearing as advised by many.  I can live with a few but want more good exposures and longer exposures.  My setup is very routinized yet there must be a variable.  (I know the SE mount is not of great quality.) 

 

So to answer the question, yes, I am looking to see how the mount moves, since there is an undesired movement taking place.

 

You've provided several ideas for me to purse.  Thank you. 

Ah.  Sounds like your mount isn't tracking smoothly.  What sort of mount is it, what sort of scope do you have on top?  Everything may seem solid, but this is a hobby where microns matter, and microns are very very very small.

 

Instead of a real time movie, perhaps a better approach is to take a bunch of images over a period of 5-10 minutes, and string them together in a short movie.  That will speed up the motion and make it pretty clear what's going on.  I did that, before understanding what Periodic Error was, and what an autoguider did.  Very enlightening.  If it looks like this, that's what's happening, and what the solution is.

 

https://www.dropbox....acking.mp4?dl=0


Edited by TelescopeGreg, 23 September 2020 - 09:32 PM.


#11 Alex McConahay

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Posted 23 September 2020 - 09:40 PM

One suggestion---not fancy analysis, but perhaps instructive......simply put your camera on liveview, magnified as much as possible, with the crosshairs centered on a star, and let it rip for a while. What is the motion of the star? North-south drift is bad polar alignment. East-West is what you are looking for. 

 

 

Here is another experiment you should run. 

 

Decrease your gain, or ISO, or aperture (simply cut a round circle maybe one fourth of your aperture diameter in a piece of cardboard and tape the cardboard to the front of the aperture).  Point is, I'm asking you to take a long exposure, and don't want it overexposed. 

 

Polar MISalign your mount. That is, make sure you are ten degrees or more off on polar alignment. 

 

Now take a ten minute exposure, or fifteen if you can without blowing out the background. 

 

What do the stars look like? Are they pinpoint (they should not be with polar misalignment), a continuous streak north-south (this is good tracking with polar misalignment), or are they jerky streaks (this would be some form of bad tracking....) ? 

 

The purpose of polar-MISaligning the mount is to get the stars to move consistently north or south of where they were. 

 

The purpose of exposing a long time is to see if you are getting something periodically. That is, you get it, then you don't get it, then you get it again. You may not be able to tell this in a shot that is not as long as the period of the error. (That short shot may be while the mount is behaving, but the next shot may be when the mount is NOT moving correctly.) You need a longer shot to analyze it all. 

 

If you did not misalign the mount, the movement might be back and forth, retracing pixels that had already been exposed. But with the misalignment, the star has shifted north of south. 

 

I don't know if it will help, but it is worth a try. 

 

 

Alex


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#12 deansjc

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Posted 23 September 2020 - 09:52 PM

Thanks for this instruction Alex.  I've heard of it but your description helps a lot.  One question: ideally where to I point the scope during the very long exposure, or does it matter.  E.g., meridian, off meridian, equator, off equator, etc.

 

Thank you.

 

Greg, your video looks very familiar.  Did guiding resolve these movements for you?  I am using an SE mount with a short refractor on it.  A very small payroll with a DSLR at prime.



#13 hcf

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Posted 23 September 2020 - 10:27 PM

You could use platesolving to do this quantifiably.

 

Take pictures at say 30 second intervals, and platesolve the images, which will tell you the RA/DEC coordinates of the image center. Looking at these over time you can measure by how many degrees/min your image center is moving.

 

This is best done on a linux box, like a raspberry pi connected to the DSLR, which uses gphoto2 to take pictures and astrometry.net downloaded locally to platesolve. All you then need is a simple script to call these in a loop and print out the coordinates of the center of the image.


Edited by hcf, 23 September 2020 - 10:30 PM.


#14 TelescopeGreg

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Posted 23 September 2020 - 10:29 PM

Thanks for this instruction Alex.  I've heard of it but your description helps a lot.  One question: ideally where to I point the scope during the very long exposure, or does it matter.  E.g., meridian, off meridian, equator, off equator, etc.

 

Thank you.

 

Greg, your video looks very familiar.  Did guiding resolve these movements for you?  I am using an SE mount with a short refractor on it.  A very small payroll with a DSLR at prime.

Yes, what I needed was auto guiding, and that worked like magic (within the abilities of my mount and monster scope on top).  It was like turning on image stabilization in my digital camera. 

 

A small guide scope and ASI120MM Mini camera, attached to a Raspberry Pi (if you don't already have a laptop at the mount) should do it for you.


Edited by TelescopeGreg, 23 September 2020 - 10:30 PM.


#15 Alex McConahay

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Posted 23 September 2020 - 10:38 PM

I would point to meridian right at celestial equator. But, as long as you avoid the far north, where things get messy anyway and it is hard to tell if things are tracking well or just moving slowly, you should be okay. 

 

And, by the way........your best bet is to somehow hook up an autoguider. Beg or borrow one from a friend and stick it in your main tube. Run PHD2, Disable the commands to the mount, and start guiding. Watch where the graph goes, and how the target stars are distributed. 

 

Next best bet is PemPro (costs money) or PECPREP (free). I believe both work with your main imaging camera.  Can anybody confirm that.....it has been a while since I used either. 

 

Alex



#16 spereira

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Posted 24 September 2020 - 07:29 AM

Moving to DSLR ...

 

smp




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