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06-18-2020, 01:54 PM   #1
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Astrotracer consistancy with a 360 mm lens

I haven't seen this topic discussed in existing threads. The problem I am seeing is that, shot-to-shot, I see huge variations in the tracking accuracy. In the summary image attached, I'm taking consequetive 20 sec exposures with a ZenithStar61 360mm refractor. Each image of the 36 I took (of an area near M-51, which is currently near the zenith) shows a 200% crop of a region near the center of each image.

Because of a peculiarity in the way that photoshop brings in a group of images, the image start in the second column going down, then 3 column and so on. The final 6 exposures are in the first column.

As you can see, the tracking errors change drastically shot to shot.

Maybe this happens all the time, and is only noticible at longer FLs? I've gotten good results at 135 mm.

The focus seems to change, but I doubt that is what is actually happening, If I shoot the region about Polaris (astrotracer turned off), the star shapes a perfect and the smallest stars are about 1 pixel across.

Attached Images
 
06-18-2020, 02:55 PM - 1 Like   #2
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What camera? How is telescope attached to camera - K-mount adapter, I presume.

How are you staying centered from frame to frame? What is your camera support? If you are moving the camera+lens between frames, there may be variable backlash and/or sag in your tripod (or whatever). Your rig is probably at least 3 or 4 kilograms (the lens alone is listed at 1.7 kg).

How close to the zenith are you really? If you are within a few degrees, the tracking might get a bit funky. At/through the zenith, alt/az tracking has a singularity (the azimuth changes by 180 degrees instantaneously). I think we've never actually figured out just what tracking motion Pentax does with the sensor. If it mimics hour angle/dec tracking, then going through the zenith should (?!?!) be OK.
06-18-2020, 04:41 PM   #3
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QuoteOriginally posted by AstroDave Quote
What camera? How is telescope attached to camera - K-mount adapter, I presume.

How are you staying centered from frame to frame? What is your camera support? If you are moving the camera+lens between frames, there may be variable backlash and/or sag in your tripod (or whatever). Your rig is probably at least 3 or 4 kilograms (the lens alone is listed at 1.7 kg).

How close to the zenith are you really? If you are within a few degrees, the tracking might get a bit funky. At/through the zenith, alt/az tracking has a singularity (the azimuth changes by 180 degrees instantaneously). I think we've never actually figured out just what tracking motion Pentax does with the sensor. If it mimics hour angle/dec tracking, then going through the zenith should (?!?!) be OK.
This is a K-1. Heavy duty Oben tripod. I have a gear head - no sag after adjustment, but I only readjusted 2 times. Cam + lens is 8 pounds. M-51 was at 316 and 75 degress elevation. This whole series of exposures only took 20 minutes.

But I don't see how any of that matters (except crossing the zenith, which I didn't). Wherever I point, I should see consistant results, whether they be good or bad. At least that is what I expect....

I forget to mention that, looking at the exif data, I see the 'photographic direction' jumping around by up to 10 degrees.
06-18-2020, 05:59 PM   #4
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I trust you did coarse and fine calibration? It is mandatory to get good results with Astrotracer.

06-19-2020, 12:38 AM   #5
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When/if I run into problems like this (K-70 + O-GPS1) I find a re-calibration often helps. I usually work "in the field" for my astro-work, so I'm using a car. Standing too close to the car when calibrating definitely affects results!

The 'photographic direction' problem implies an external magnetic influence, to my mind.

Presumably the correct focal length of the telescope is being entered into the camera? This data is required by the Astrotracer function as well as the Shake Reduction. Just 'cos you're on a tripod doesn't make this information redundant in this circumstance

My relatively limited experience suggests that 20sec @ 360mm is "pushing the boundaries" a bit, though this would be less of a problem on a full-frame camera.

I've taken to making a single test exposure for critical examination before committing to an extended series, just in case I've not managed to get things exactly right first time. Extra fiddling about, I know, but better than making an hour's worth of exposures only to find when I get home that they haven't worked

Good luck

Last edited by kypfer; 06-19-2020 at 12:40 AM. Reason: punctuation
06-19-2020, 07:12 AM   #6
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QuoteOriginally posted by kypfer Quote
My relatively limited experience suggests that 20sec @ 360mm is "pushing the boundaries" a bit, though this would be less of a problem on a full-frame camera.
My experience is the same as you. FWIW, it seems astrotracer work reliably up to 3-4 stops above what the 500 rule would predict. As a rule of thumb, I use a maximum exposure time of 10 x 500 rule. In this case, 500/360 = 1.4 sec which translates in 14 sec. with astrotracer. At 20 sec., I would expect to observe variations between shots and see trails in some, if not most, of them. YMMV depending on the shooting conditions.

If the goal is to combine multiple exposures, I would suggest to do some tests to determine what is the longest exposure giving reproducible results with the lens. It will be much shorter than what astrotracer will allow you to use.
06-19-2020, 08:29 AM   #7
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You are in the ball park for what one can reilably get. I will run a 400mm and can reliably get 20 second exposures, 30s with a 300mm. There are a lot of thigns that need to be right for astrotracer to work well. Not only the calibration process but also the tilt sensors need to be accurate. Changing magnetic field, lots of iron, shake all can throw things off. If you have eliminated all of that ensure good fresh fully charged batteries in the camera and if you are using it the O-GPS1.

I have my stuff on a wooden tripod, use the 2 second mirror up delay, use a release cable but even then I still get bad frames. My recommendation for that is to deal with them when stacking. I stack using Deep Sky Stacker (DSS) and have it use the best 50% to 75% of shots which eliminates most of the bad shots. Additionally I use the sigma clipped stacking method with a sigma of 3 and 5 iterations which further eliminates trailing so long as it isn't always in the same direction.
06-19-2020, 09:27 AM   #8
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You might want to check this out on a star field near the celestial equator - that's where you will get the greatest movement in R.A., and longest star trails. If you can get optimized for that, then it will work fine elsewhere in the sky.
Also, I have an older Williams Optics scope, and its focuser bearings needed a tweak on the Allen screws to tighten it up. There was slop in it. I also lock down the focuser. If the temperature changes more than about 2-3C, you may have to refocus.
If you still find a problem, a high-res image would be useful to see what is going on across the field.
Lastly, do you have the field flatenner for this scope - it is known for distortion across the field without it.

06-19-2020, 08:46 PM   #9
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QuoteOriginally posted by jbinpg Quote
I trust you did coarse and fine calibration? It is mandatory to get good results with Astrotracer.
Of course. And don't forget that some exposures are perfect - completely round stars. Astrotracer has to be doing something.

---------- Post added 06-19-20 at 08:48 PM ----------

QuoteOriginally posted by kypfer Quote
When/if I run into problems like this (K-70 + O-GPS1) I find a re-calibration often helps. I usually work "in the field" for my astro-work, so I'm using a car. Standing too close to the car when calibrating definitely affects results!

The 'photographic direction' problem implies an external magnetic influence, to my mind.

Presumably the correct focal length of the telescope is being entered into the camera? This data is required by the Astrotracer function as well as the Shake Reduction. Just 'cos you're on a tripod doesn't make this information redundant in this circumstance

My relatively limited experience suggests that 20sec @ 360mm is "pushing the boundaries" a bit, though this would be less of a problem on a full-frame camera.

I've taken to making a single test exposure for critical examination before committing to an extended series, just in case I've not managed to get things exactly right first time. Extra fiddling about, I know, but better than making an hour's worth of exposures only to find when I get home that they haven't worked

Good luck
Well, I can only choose what I'm given. So 350 mm. But once again, AT is erratic. Some exposures are good, many are not, and the ones ttat aren't good are bad in different ways.

---------- Post added 06-19-20 at 08:50 PM ----------

QuoteOriginally posted by MossyRocks Quote
You are in the ball park for what one can reilably get. I will run a 400mm and can reliably get 20 second exposures, 30s with a 300mm. There are a lot of thigns that need to be right for astrotracer to work well. Not only the calibration process but also the tilt sensors need to be accurate. Changing magnetic field, lots of iron, shake all can throw things off. If you have eliminated all of that ensure good fresh fully charged batteries in the camera and if you are using it the O-GPS1.

I have my stuff on a wooden tripod, use the 2 second mirror up delay, use a release cable but even then I still get bad frames. My recommendation for that is to deal with them when stacking. I stack using Deep Sky Stacker (DSS) and have it use the best 50% to 75% of shots which eliminates most of the bad shots. Additionally I use the sigma clipped stacking method with a sigma of 3 and 5 iterations which further eliminates trailing so long as it isn't always in the same direction.
I'm mostly wondering why the results are inconsistant. If they were all poor in the same fashion, well, maybe it's something I can fix.

---------- Post added 06-19-20 at 08:53 PM ----------

QuoteOriginally posted by ProfessorBuzz Quote
You might want to check this out on a star field near the celestial equator - that's where you will get the greatest movement in R.A., and longest star trails. If you can get optimized for that, then it will work fine elsewhere in the sky.
Also, I have an older Williams Optics scope, and its focuser bearings needed a tweak on the Allen screws to tighten it up. There was slop in it. I also lock down the focuser. If the temperature changes more than about 2-3C, you may have to refocus.
If you still find a problem, a high-res image would be useful to see what is going on across the field.
Lastly, do you have the field flatenner for this scope - it is known for distortion across the field without it.
Yes I have the field-flattener. And the soope is brand new. Shooting the field around Polaris (with AT off) gives perfectly round disks (or points for the faint ones) for the stars closest to the pole
06-20-2020, 02:36 AM   #10
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Are you using a remote shutter/intervalometer, 2/10sec delay and mirror up? Don't walk around the tripod, don't sneeze or cough when shooting (I could be joking, but I'm not), any vibration will affect the end result. 20sec @360mm is pushing it, at least on an APS-C, I guess the K-1 should be able to do it with perfect conditions. At least, the star trails should not be as long as yours. I shot this with my K-70/Da*300, 10sec subs.



This is 50sec subs with an M200 f4 lens

06-20-2020, 03:29 AM   #11
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I wonder whether atmospheric variations could have an effect? I.e., swirls of very localized air at varying degrees of temperature and pressure could be changing the refractive index of the air between you and the stars. Twinkling, you know.
06-20-2020, 09:17 AM   #12
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QuoteOriginally posted by davidwright2000 Quote
I haven't seen this topic discussed in existing threads. The problem I am seeing is that, shot-to-shot, I see huge variations in the tracking accuracy. In the summary image attached, I'm taking consequetive 20 sec exposures with a ZenithStar61 360mm refractor. Each image of the 36 I took (of an area near M-51, which is currently near the zenith) shows a 200% crop of a region near the center of each image.

Because of a peculiarity in the way that photoshop brings in a group of images, the image start in the second column going down, then 3 column and so on. The final 6 exposures are in the first column.

As you can see, the tracking errors change drastically shot to shot.

Maybe this happens all the time, and is only noticible at longer FLs? I've gotten good results at 135 mm.

The focus seems to change, but I doubt that is what is actually happening, If I shoot the region about Polaris (astrotracer turned off), the star shapes a perfect and the smallest stars are about 1 pixel across.
New results that are pretty astonishing.

I was able to take a lot of images last night. I'll show only the final result, so as not to overwhelm you with data. But everything I say; I have images to back it up.

So at the end of a frustrating evening, I took images of the region about Polaris. These images below are are 4, 30 second exposures. The sequence was: AT on, AT off (Manual mode) , AT off (Manual mode) , AT on. When AT was on, the star images are TRAILED!! When it is off, the stars are perfect (since they don't move much in 30 seconds). One additional note: I recently updated to firmware 1.52, to get the electronic compass. It worked then, and it works no, but last night, it didn't!
Attached Images
 
06-20-2020, 09:49 AM   #13
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Firmware or hardware problem? K-70 works as expected

Just some follow-up data. but my hypothesis is that either the new firmware has a bug, or my hardware for the electronic compass is defective.

The seqquend of events last night is this: I started with the K-70/135 mm Rokinon lens. AT was already calibrated on that cam, and I just dialed in the alt and az settings for M-13; Az using the electronic compass, alt using the dial on my gear head. M-13 was in the field of view on the first image, but a little low, so I adjusted and took 9, 20 second exposures. They were all identical in terms of tracking accuracy, as shown by these 200% crops:
Attached Images
 
06-20-2020, 10:00 AM   #14
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K-1 is not OK

Then I switched to the K-1/360 mm lens. I was able to find M-13 nearby, and took 9, 20 second exposures. (Below, 200%). They are not terrible, but not good either. At this point I had to adjust the view, and (possibly) I touched a button, because after this the electronic compass was not available, nor was the exposure time setting in AT mode. I redid the coarse and fine calibration, which made no difference, but noted that, to get exposure time adjustment, I had to press the green button, which cconfused me because it placed a sign 'off' next to the AT icon. The upper display, in the area for shutter, read 'bu' - I couldn't find any info on that in the manual. I fussed around for a while with no success, and finally moved over to Polaris - the images I showed earlier.

It seems crazy to me that AT on messes up tracking near Polaris.(The first set of images I showed)

(I would have put all this in one post, but it wasn't clear how to add text past an image.
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06-20-2020, 04:34 PM   #15
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I agree that Astrotracer is not working on those last M13 images.

---------- Post added 06-20-20 at 04:35 PM ----------

Just another question: did you actually turn on the GPS engine via the small button on the right side of the viewfinder hump? Many people miss this necessary step.
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