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Egg shaped stars on test image

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#1 JimCase

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Posted 18 April 2021 - 11:58 AM

I experimented with stacking 10 subs (5 minute exposures ) using the Optolong extreme filter with my DSLR  (Pentax K-70 with long exposure noise reduction enabled) with the Meade 80 mm APO refractor on the Meade LX85 mount to get this image. My question is how to avoid the egg shaped stars? Is it due to an alignment error needing a careful drift alignment? Do I need to invest in a guide camera? Take more shorter exposures? Other suggestions? The attached image is a screen shot from my phone of the larger image.)

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  • 95BA5B0A-B772-4B69-B87E-612259C660D5.jpeg

Edited by JimCase, 18 April 2021 - 12:02 PM.


#2 kraegar

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Posted 18 April 2021 - 12:00 PM

5 minute exposures will require guiding, and good polar alignment.  No way around it.


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#3 VincenzoZito

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Posted 18 April 2021 - 12:03 PM

Hi,

DSO= guide camera

no guide camera  = shorter exposures


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#4 ravenhawk82

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Posted 18 April 2021 - 12:24 PM

Yep, that's pretty typical for unguided 5 minute exposures on any but the best mounts. Fortunately, a guide scope and camera is relatively cheap compared to a new mount and will clean those stars up quite a bit. 

Shorter exposures (and more of them) will help in the meantime. You'll get a better SNR out of stacking longer exposures because there won't be as much read noise to contend with, but without a guide scope you just have to find how long your mount can track before the stars start to smear. On my Atlas, I can manage about 30s unguided exposures close to the equator and 60s exposures at ~60* declination. Just for fun I took some unguided 5 minute shots of Polaris the other night while waiting for Virgo to clear some power lines and they came out beautiful, but that's the absolute best case because apparent star motion is at a minimum near the pole.



#5 chanrobi

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Posted 18 April 2021 - 12:52 PM

Just take shorter exposures,

 

Unless you want to spend money and time on guiding


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#6 Stelios

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Posted 18 April 2021 - 02:41 PM

A guide camera and guide scope is only about $230--an investment that will pay dividends many times over, especially as you get better.


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#7 gdavis

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Posted 18 April 2021 - 04:10 PM

I just started playing around with some DSO imaging using my DSLR and an old atlas mount with a basic clock drive (no goto or PEC). I took a bunch of 30sec exposures and when I step through them, I can see the stars dancing back and forth due to the periodic error.  Some of the images had round stars, some of them had trails depending on which part of the cycle it happened to be in.



#8 Northrim

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Posted 18 April 2021 - 08:02 PM

I've had pretty good luck with the Orion Magnificent Mini AutoGuider Package.  It's  around $350.  I have it on a ES ED80 mounted on a Celestron  AVX.  My guiding needs a lot of workelly but even now I'm  taking 180 sec exposures an sometimes 300 sec.



#9 Stelios

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Posted 19 April 2021 - 03:20 AM

One of the least expensive--but good--options is to get the Orion $79 guidescope and the ASI120MM-mini camera ($149). It is the same scope as the Magnificent Mini and better camera for $120 less. 

 

Nothing wrong about the Magnificent Mini except the usurious price. I bought it years ago and still have it. But the world changed since then.


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#10 asanmax

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Posted 19 April 2021 - 11:39 AM

1. No noise reduction for astrophotography is needed, the stacking software will take care of it.

2. 5 min subs is too long even with guiding, your stars will get overexposed and will lose their colors. 120 sec will do the job.

3. Take more exposures, the more you give to the stacking software the smoother your stacked image will be.

4. On a strict budget? Buy a used guidescope and camera, you will never regret it. It's a learning curve but well worth it.


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#11 sbharrat

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Posted 19 April 2021 - 12:01 PM

One of the least expensive--but good--options is to get the Orion $79 guidescope and the ASI120MM-mini camera ($149). It is the same scope as the Magnificent Mini and better camera for $120 less. 

 

Nothing wrong about the Magnificent Mini except the usurious price. I bought it years ago and still have it. But the world changed since then.

I have been meaning to ask about this for a while now. I use this scope (actually a variant with the helical focuser) and camera (non-mini) combination. But the scope attaches to a finder shoe and the bracket to the shoe is an integral part of the scope.

guide.jpg

 

But all the recommendations all over CN are to NOT use a finder shoe but to attach the guide scope with rings at two separate points. Can I even use this scope for that (how?) or do I need to replace my guide scope.



#12 TimN

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Posted 19 April 2021 - 12:31 PM

Don't replace your guide scope if it is working. Yes, its true that attaching the finder scope to the finder shoe can cause shifting problems when compared to rings. However, that doesn't mean that you will have problems.

 

I had a similar setup to yours some 10 years ago when I started imaging. I was also told not to use the finder shoe. I decided to use it temporarily. Since then I've added a new mount, a couple more telescopes, another DSLR and 2 mono ZWO cameras. The only original piece of equipment that I have is my same guide scope attached to my main scopes by the finder shoe. I tell myself not to worry - its only temporary.

 

So, I would keep an eye on it, understanding if its not attached solidly it could cause problems. However, there is so much to fix in this hobby that does go wrong, I've not found it necessary to look for problems.


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#13 Stelios

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Posted 19 April 2021 - 03:30 PM

I have been meaning to ask about this for a while now. I use this scope (actually a variant with the helical focuser) and camera (non-mini) combination. But the scope attaches to a finder shoe and the bracket to the shoe is an integral part of the scope.

attachicon.gifguide.jpg

 

But all the recommendations all over CN are to NOT use a finder shoe but to attach the guide scope with rings at two separate points. Can I even use this scope for that (how?) or do I need to replace my guide scope.

Yes. The recommendation is sound. And unfortunately, it has been misinterpreted as "if you use the finder shoe, your images will be bad." 

 

The trouble with the recommendation, is that a good solution is expensive. Most medium-priced scopes have no facility *as sold* for attaching a guidescope correctly, and many if not most guidescopes will require replacing the mounting hardware they come with. 

 

If you have an extra $300 lying around for perfecting your guiding, by all means do so--but you will be MUCH better off switching to OAG in such a case, for better guiding, easy portability between scopes, and complete elimination of differential flexure. 

 

Meanwhile, using the finderscope shoe is MILES better than not guiding at all. 

 

Note also that some guidescopes are sold with two rings, which attach to a base... which attaches as a single point. That doesn't do much. THIS is the ideal mounting--and note that the top dovetail bar on your scope is extra, and may need replacing the rings or other mounting hardware your scope came with. 


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#14 asanmax

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Posted 19 April 2021 - 04:09 PM

I have been meaning to ask about this for a while now. I use this scope (actually a variant with the helical focuser) and camera (non-mini) combination. But the scope attaches to a finder shoe and the bracket to the shoe is an integral part of the scope.

attachicon.gifguide.jpg

 

But all the recommendations all over CN are to NOT use a finder shoe but to attach the guide scope with rings at two separate points. Can I even use this scope for that (how?) or do I need to replace my guide scope.

I use the same guidescope and I always attach it to my telescopes by using a guidescope dovetail base. It's true that you may get some flexure issues, but I have never had any problems with my setups, even shooting longer than 10 minute subs. This is for example how it sits on my RC scope.

 

post-300294-0-35946800-1617852603.jpg



#15 sbharrat

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Posted 19 April 2021 - 04:21 PM

Don't replace your guide scope if it is working. Yes, its true that attaching the finder scope to the finder shoe can cause shifting problems when compared to rings. However, that doesn't mean that you will have problems.

 

...

 

I use the same guidescope and I always attach it to my telescopes by using a guidescope dovetail base. It's true that you may get some flexure issues, but I have never had any problems with my setups, even shooting longer than 10 minute subs. This is for example how it sits on my RC scope.

 

The problem is that I do have a problem... At 3m subs, I can see the out-of-round on bigger stars. I have no idea if this is what is causing it, but my guiding is not terrible (1" RMS total mostly, with my image scale at 1.96"/px) so I was going to fix this part first... if there was an easy way to fix. Doesn't sound like it though.



#16 sbharrat

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Posted 19 April 2021 - 04:24 PM

...

 

If you have an extra $300 lying around for perfecting your guiding, by all means do so--but you will be MUCH better off switching to OAG in such a case, for better guiding, easy portability between scopes, and complete elimination of differential flexure. 

 

Meanwhile, using the finderscope shoe is MILES better than not guiding at all. 

 

... 

Yes, I have been toying with just going the OAG route. But then that is not just the OAG but also replacing the 120mm if I have been understanding some other guiding threads on CN frown.gif



#17 Stelios

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Posted 19 April 2021 - 05:57 PM

Yes, I have been toying with just going the OAG route. But then that is not just the OAG but also replacing the 120mm if I have been understanding some other guiding threads on CN frown.gif

Yes, you need an ASI290MM-mini ($299) at a minimum. 

 

OTOH, if you are buying from scratch, $130 or so for the ZWO OAG + $300 for the mini is only $430--compare with the $350 for magnificent mini.

 

And unless you are OK living with the guidescope shoe, or find a way to upgrade the mounting hardware cheaply (if you do, please post, it will help others far more than vague advice), it may still make sense to sell your current system and upgrade to OAG. 


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