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Why do my stars vary from image to image?

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#1 Stelios

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Posted 06 October 2013 - 02:49 AM

I am a beginner, so I expect to take lousy pics. But I don't understand why, in the same session (using Backyard EOS to take 18 consecutive lights) the shape of the stars varies from very elongated ones on one image (M57a1) to (for me) reasonable (M57b1). Nobody touched the scope, camera or laptop between those exposures. There was some light wind, but I doubt that could've done it. Most puzzling, the *better* image came later than the poor one.

Both are single 45-second light frames, Canon T3i unguided, ED80 on a CG-5 (ASPA-aligned).

Another question is--should I bother stacking images that different, or should I manually review them and edit out the ones with the elongated stars?

This is the one with the real bad stars, M57a1 (a crop of a larger image):

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#2 Stelios

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Posted 06 October 2013 - 02:50 AM

... and this is the better one M57b1, about 10 frames later (no human intervention):

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#3 Maverick199

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Posted 06 October 2013 - 02:52 AM

Due to periodic error and Mount not properly polar aligned. After the ASPA, do a Polar alignment by following instructions on the hand controller. That will get you round stars on your 80mm for 45 secs without issues with perhaps an odd Sub or two thrown out.

Also check balance.

You should check all light frames manually and toss out the one's which show star streaks etc., and then stack them.

#4 jrcrilly

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Posted 06 October 2013 - 09:22 AM

You must discard the bad frames.

Declination drift from polar misalignment should be a constant across all frames - and the trails shown above are in RA (RA is up and down in that image).

Periodic error in RA will be different in each 45 second frame so that's the most likely candidate. Good PEC training will reduce that to the best the mount can do. Then select the best frames and see what you can do with them.

#5 WarmWeatherGuy

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Posted 06 October 2013 - 11:52 AM

If you rotate your camera so that North is up then any streaking left/right will be due to periodic error and any streaking up/down will be due to bad polar alignment.

#6 Jeff2011

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Posted 06 October 2013 - 11:57 AM

Stelios,

I have been there. Before I got my autoguider, I was throwing out over 50 percent of my frames. You have learned the hard lesson that unguided ap is not very repeatable especially with low end mounts like ours. For unguided, try to get the best polar align and work on your scope balance. The good news is that the frames will be short so throwing bad ones out won't be as painful as it would for 5 minute subs. You are doing well. Keep going and you will get there.

#7 ebacon

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Posted 06 October 2013 - 12:17 PM

Stelios,

It takes time for gear lash to settle after slewing the mount. That might be part of the reason why your later image is better than your earlier one.

My mount takes about 6-8 minutes for gear lash to settle. Your time will be different.

#8 Stelios

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Posted 06 October 2013 - 12:18 PM

@Steve: Great tip, thanks!

@John & Maverick: Thanks for the analysis. PEC isn't correctable on a CG-5 from what I've read, so I guess it's try harder on polar alignment, then pick frames...till I can afford a better mount.

#9 nodalpoint

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Posted 06 October 2013 - 12:33 PM

How do you know you need a better mount? Use a reticle eyepiece, work on better polar alignment and figure out how to maximize what you have. Search for that mount on Astrobin and you'll find great photos made with it.

Never been on a site where spending more money is regarded as such a magic bullet. Lol.

#10 Maverick199

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Posted 06 October 2013 - 12:41 PM

How do you know you need a better mount? Use a reticle eyepiece, work on better polar alignment and figure out how to maximize what you have. Search for that mount on Astrobin and you'll find great photos made with it.

Never been on a site where spending more money is regarded as such a magic bullet. Lol.


Agree with this reply. I have seen some extremely impressive images with this mount. All you need is patience and decent skies.

#11 Madratter

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Posted 06 October 2013 - 12:43 PM

Unless you have a LOT of money, you aren't going to get a mount that corrects periodic error well enough to do without guiding at all except at very short focal lengths. It is MUCH cheaper to get into auto-guiding. The alternative is to go old school and manually guide. It is worth doing once or twice to figure out why almost no one does it anymore.

As for money being a magic bullet, all the money in the world won't make you a better imager if you don't know how to use the equipment. BUT, better equipment IS the only answer to some problems.

#12 MitchAlsup

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Posted 06 October 2013 - 03:29 PM

It is MUCH cheaper to get into auto-guiding. The alternative is to go old school and manually guide. It is worth doing once or twice to figure out why almost no one does it anymore.


Brilliant!

#13 timtrice

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Posted 06 October 2013 - 03:33 PM

The alternative is to go old school and manually guide. It is worth doing once or twice to figure out why almost no one does it anymore.


Coming from a man who sleeps while he photographs! ;)

I can't imagine those who did it manually AND film...

#14 gdd

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Posted 06 October 2013 - 05:09 PM

The mount may be bouncing between the worm threads due to backlash (you need some backlash to prevent binding). The bouncing can be minimized by NOT perfectly balancing the load on the mount. The RA axis should have a bit more weight on the east side so the pressure is always on the same side of the threads. You will here this called "east side bias". This problem is worst when imaging near the meridian and causes the symptoms you reported.

Gale

#15 kbev

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Posted 06 October 2013 - 05:49 PM

I agree with those that say the mount is capable, I know because that's what I started off with. Here is a picture using a similar setup - AT72ED with a T3/1100D on a CG5, the only difference is that I did use an autoguider (the Orion mini-guider package with the SSAG camera):

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#16 kbev

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Posted 06 October 2013 - 05:53 PM

And here is a crop from the center at full-scale to show the stars. Disregard all of the noise in the image, the data wasn't the best and I hope to do better next go-round.

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#17 Cliff Hipsher

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Posted 06 October 2013 - 06:52 PM

I don't own a CG5, but I've done some digging, and depending on your hardware, you MAY be able to use PecTool to measure and "train" your mount.

Check these links: http://tech.groups.y...G5/message/1799

Here is a link to the hand controller firmware updates, motor controller firm ware up dates, Nex Remote, and PecTool: http://www.celestron...ds&_a=view&p...

Hope this helps.

#18 Thirteen

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Posted 06 October 2013 - 07:09 PM

Correct me if I'm wrong here, but you shouldn't have to manually sort through these. I am assuming you will use DSS. In my experience DSS will not be able to calculate the offsets on the streaked one and will have to toss it. When it can't calculate, it will just exclude it from the stack. If any do sneak through, they will be graded very low and you can just tell it to stack 95% or so of the best. I fought some long battles unguided, but I never flipped through every frame. That could be considered torture. Short subs means lots of subs.

#19 timtrice

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Posted 06 October 2013 - 07:40 PM

Jason, I flip through every single image. I don't care if I have 20 or 100. I generally do them as I take them but at times I have fallen asleep I make sure to do it as soon as possible. You're image is only as good as your worst data.

#20 Stelios

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Posted 07 October 2013 - 12:34 AM

Thanks all for the ideas and suggestions.

To answer some: I'm not really trying to "throw money" at anything. I've received advice right here from the very start that the CG-5 was not a great mount for AP. I'll stick with it for the next year or two at least, but that doesn't mean I can't dream of something better (not something without PEC, but perhaps something that tracks on both axes, has PEC training available, and can support a larger scope comfortably as well). Whether dreams ever materialize is another story. Meanwhile I plan to get as good images as can be got out of my particular CG-5.

The auto-guider is something I'll buy in the near future, once I learn a bit more about post-processing. The mini-guider package on Orion seems just the ticket.

I will learn how to do perfect polar alignments, although I don't think this was really the culprit--later in the same session I got near-perfect stars when photographing M2.

I will make the balance a bit off on the East side, this will require getting a lighter counterweight that fits the CG-5 shaft (anyone know of a source?)

So far I find AP to be shrouded in layers of confusion. Every layer that I penetrate leads to another. But I've taken some pictures, I'm having fun, and I have the collective wisdom here to help. I'm sure I'll get there, albeit kicking and screaming all the way :)

#21 Maverick199

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Posted 07 October 2013 - 09:07 AM

I recently got a 2 kg ioptron weight that I use with the Z71 on CG-5. You need to insert a small iron pin to tighten against the CW shaft. However you can do the east bias with your present CW?

#22 SergeC

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Posted 07 October 2013 - 09:23 AM

I think you'll fix most, if not all, of your trailing issues with an autoguider. Your image train is well within the capabilities of a well tuned, balanced and guided CG-5 (speaking from experience).

Since you plan on keeping the mount for a while, give serious thought to hypertuning it. If you're reasonably handy with tools it's not that difficult. You can get a kit and complete instructions for DIY hypertuning from Deep Space Products, or send it in and have Ed tune it if you're not confident in your own tinkering abilities. Either way, the mount will perform light years better, and balancing it properly will be much easier and more accurate.

It's also possible to train PEC on a CG-5, 'tho it won't be permanent (meaning you'd have to train it each time after you turn it off). To do that you'd use PECTools from Celestron and hook up the handbox to you computer via a serial-to-usb interface cable. That might be more trouble than it's worth if you do hypertune the mount.

Whatever you do, it will get easier and your results will improve the more you use it and establish a routine.

CS

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#23 CounterWeight

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Posted 07 October 2013 - 11:20 AM

Getting good at polar aligning your mount is something that will always be of use on any mount you use as it is determining your tracking baseline of the mount 'in the raw'. The particulars of making the mount do it best will also always help. Auto-guiding is a fantastic help, but the better you are polar aligned (and therefore tracking)the less the auto-guiding has to do (less tracking error to correct for), and that is IMO where it works best.

I had to laugh about the layers of mystery (in agreement with my own experience) - it is for a while just a bunch of questions that get re-arranged. Get those answered and a bunch of new ones pop up ;) the key is having a good time with the process overall.

#24 Stelios

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Posted 07 October 2013 - 01:25 PM

I recently got a 2 kg ioptron weight that I use with the Z71 on CG-5. You need to insert a small iron pin to tighten against the CW shaft. However you can do the east bias with your present CW?


Not really--if I understand correctly :question:, east bias means on the scope side. Scope, finder and camera just about balance the scope with the 11 lb counterweight in its closest position, but I need to pull the counterweight about 1/2" away else it hits the mount casing.

If I misunderstood "east bias" it wouldn't be the first time. :tonofbricks:

#25 nodalpoint

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Posted 07 October 2013 - 01:30 PM

Yes, with the 11 lb counterweight it needs to be almost all the way to the mount. I need to find a 5 lb. counterweight so it can be slid down further.

If you are using the C80ED scope it might be very tail heavy. I added a 2.5 lb. ankle weight to the dew cap on my scope to get it correctly balanced in that axis. Check this right away because it can be a major headache for your tracking.






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