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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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Stelios
Carpal Tunnel
*****

Reged: 10/04/03

Loc: West Hills, CA
Why do my stars vary from image to image? new
      #6120034 - 10/06/13 03:49 AM Attachment (26 downloads)

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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Stelios
Carpal Tunnel
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Reged: 10/04/03

Loc: West Hills, CA
Re: Why do my stars vary from image to image? new [Re: Stelios]
      #6120035 - 10/06/13 03:50 AM Attachment (19 downloads)

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

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Maverick199
Postmaster
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Reged: 02/27/11

Loc: India
Re: Why do my stars vary from image to image? new [Re: Stelios]
      #6120037 - 10/06/13 03:52 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.


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: Why do my stars vary from image to image? new [Re: Maverick199]
      #6120333 - 10/06/13 10:22 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.


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WarmWeatherGuy
Carpal Tunnel
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Reged: 08/27/11

Loc: Orlando, FL 28° N, 81° W
Re: Why do my stars vary from image to image? new [Re: Stelios]
      #6120550 - 10/06/13 12:52 PM

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.

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Jeff2011
Carpal Tunnel
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Reged: 01/01/13

Loc: Sugar Land, TX
Re: Why do my stars vary from image to image? new [Re: jrcrilly]
      #6120559 - 10/06/13 12:57 PM

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.


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ebacon
member


Reged: 02/13/13

Loc: Rochester Hills, MI
Re: Why do my stars vary from image to image? new [Re: Jeff2011]
      #6120589 - 10/06/13 01:17 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.

Edited by ebacon (10/06/13 01:17 PM)


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Stelios
Carpal Tunnel
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Reged: 10/04/03

Loc: West Hills, CA
Re: Why do my stars vary from image to image? new [Re: WarmWeatherGuy]
      #6120592 - 10/06/13 01:18 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.


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nodalpoint
Pooh-Bah


Reged: 06/03/13

Loc: FEMA camp
Re: Why do my stars vary from image to image? new [Re: Stelios]
      #6120614 - 10/06/13 01:33 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.


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Maverick199
Postmaster
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Reged: 02/27/11

Loc: India
Re: Why do my stars vary from image to image? new [Re: nodalpoint]
      #6120626 - 10/06/13 01:41 PM

Quote:

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.


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Madratter
Postmaster


Reged: 01/14/13

Re: Why do my stars vary from image to image? new [Re: nodalpoint]
      #6120631 - 10/06/13 01:43 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.


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MitchAlsup
Pooh-Bah


Reged: 08/31/09

Re: Why do my stars vary from image to image? new [Re: Madratter]
      #6120906 - 10/06/13 04:29 PM

Quote:

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!


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timtrice
sage


Reged: 08/01/13

Loc: Katy, TX
Re: Why do my stars vary from image to image? new [Re: Madratter]
      #6120913 - 10/06/13 04:33 PM

Quote:

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...


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gdd
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Why do my stars vary from image to image? new [Re: timtrice]
      #6121072 - 10/06/13 06:09 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


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kbev
super member


Reged: 12/29/10

Loc: Far, far east Mesa
Re: Why do my stars vary from image to image? new [Re: gdd]
      #6121116 - 10/06/13 06:49 PM Attachment (12 downloads)

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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kbev
super member


Reged: 12/29/10

Loc: Far, far east Mesa
Re: Why do my stars vary from image to image? new [Re: kbev]
      #6121122 - 10/06/13 06:53 PM Attachment (8 downloads)

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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Cliff Hipsher
Pooh-Bah
***

Reged: 12/31/08

Loc: North Chesterfield, VA
Re: Why do my stars vary from image to image? new [Re: Stelios]
      #6121206 - 10/06/13 07:52 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.yahoo.com/group/CG5/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.com/c3/support3/index.php?_m=downloads&_a=view&p...

Hope this helps.


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Thirteen
sage


Reged: 07/12/13

Loc: Milford, Michigan
Re: Why do my stars vary from image to image? new [Re: jrcrilly]
      #6121233 - 10/06/13 08:09 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.

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timtrice
sage


Reged: 08/01/13

Loc: Katy, TX
Re: Why do my stars vary from image to image? [Re: Thirteen]
      #6121271 - 10/06/13 08:40 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.

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Stelios
Carpal Tunnel
*****

Reged: 10/04/03

Loc: West Hills, CA
Re: Why do my stars vary from image to image? [Re: timtrice]
      #6121640 - 10/07/13 01:34 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


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