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AweSIM

Should I get a coma corrector or a different scope? =(

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Hi,

I am a very proud owner of SkyWatcher PDS-200 F5 newtonian reflector and I've been using it for many years now. I think I understand the process of collimation well enough. However, even with my best efforts, I still can't get crisp stars. I've tried to ensure that my collimation is good. I've tried to ensure that my focus is good. But even then, my images suffer loads of coma.

One of the best images I've been able to snap is the following. If you zoom the image up to 100%, you'll be able to see how bad coma is at the edges of the frame. I am aware that newtonian scopes suffer from coma, but is it really as bad as in this image?

For a long time, I've been mulling over whether I should get a coma corrector (how is it different from a field flattener, btw?) or a different scope that doesn't suffer from coma. But I don't have answers to the following questions that pop up in my head:

  1. Should I get a coma corrector? If so, how should I go about choosing one for my scope? Can you recommend a suitable coma corrector for my scope? Should I also get extension tubes for the scope's focuser?
  2. Or should I get a different scope altogether? If a good coma corrector is gonna cost as much as a scope, I'll probably be better off getting a more portable scope. If so, which type of scope should I be looking at?

I know any discussion about possibilities for a scope is very open-ended, but I really just wanna get your perspectives on this problem of mine. Is my current scope good and am I using it wrong? Or is it bad but can be improved to perfection with a coma corrector? Or is it a lost cause and I should rather invest in a better (and more portable) scope?

Also, I would really appreciate if any of you guys can guide me how could I improve this image I've shot. This is one of my best attempts but it is nowhere near the awesome images I've seen elsewhere on this forum. I took this image from a really really really dark sky site. I shot it with a Nikon D5100 attached to my SkyWatcher PDS200 F5 mounted on an NEQ6 Pro mount which was polar aligned and tracked but without any PHD autoguiding (cuz I'm not very learned on the subject with regards to hardware required). The image is a stack of 10 images, each 30 seconds long (cuz my DSLR doesn't support computer controlled exposures longer than 30s). Any tips and guidance is most welcome. I would love to be shown how can I improve my skills.

Thanks,

Asim Sohail

NGC869_DoubleCluster_3.jpg

Screen Shot 2019-11-22 at 1.24.22 AM.png

Edited by AweSIM

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Coma is an error that's inherent in the optical train -- off axis point sources are never focussed correctly.  A flattener might well be used even if focus were good but the image plane wasn't flat (so the focus is good on off-axis stars, just not quite where you want it).  My limited experience is that the two look quite similar in images, but it's only quite recently that I've paired an OTA (frac) and camera that make it obvious the field is far from flat, so I may be missing something there.

The fact that you're using a DSLR and therefore have a fairly large sensor probably makes it look even worse.

I know others have posted that they're happy with the results of using the Skywatcher coma corrector with the 200PDS and a DSLR, so that does seem like it would be worth trying.

James

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@JamesF thanks for that valuable insight. I'll look into skywatcher coma corrector. :-)

@Ronclarke and @Grimbles. Would you be kind enough to post a link to the coma corrector you're using? Perhaps if you could share pics that could show the difference your coma corrector makes to an image vs one without it.

And if I understand you correctly... You're saying that my images would be s lot better if I invested in a coma corrector and that I don't need to switch to a different scope, right?

Finally, I see that coma correctors have very different prices, some cheap, some expensive. What would you recommend for my scope? My scope has a 2" focuser. And would I need focus extension tubes to attach my dslr? Perhaps if you would kind enough to share a picture which shows how the CC fits into your optical train... :-)

Thank you so much for your responses. :-)

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ill give you my opionion

I have never owned or  used coma corrector on any scopes, even on my F/4.5 dob.

saying that tho you have an f/5 system so its gonna have that its standard so for visual it never bothered me in the slightest. Your pic is I think decent however your 2nd pic blown up does show it BUT it only cause its soo blown up tho. many people buy f/4 reflectors for imaging so it will even be worse in those but at f/5 I think image is ok

I guess the issue tho comes cause you want to do imaging so that's a whole different ball game. almost every scope that people buy for imaging will need a focal reducer or like your coma corrector or flate field. this is part of getting into this part of the hobby. There are some scopes that don't need these extra accessiors BUT cause they have those items built into it. The price is more expensive tho.

you are correct coma corrector are expensive even the badder ones are so expensive that I wouldn't even get one here where iam the badder one is $285 +49 shipping then tax that's $377.42. yet that's the cost pretty much of the whole ota tube.

To me as I said in another post I would never buy an accessiory that costs as much as the scope itself,cept maybe eyepices where they can be used on every scope ( I own 11 or 12 scopes right now about) so that makes sence.

so I guess if you plan on imaging that's kinda what you need to do tho.

joejaguar

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Note that Skywatcher do two coma correctors at very different prices. The expensive one is the Aplanatic, which is optimised for the Quattro f/4 scopes. I can vouch for the fact that this one works.

NIgelM

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I have a quattro and use the baader cc and it seems to work okay for me. Do not though that as the cc is attached to the image train as soon as you remove you camera and place it into another scope not adjustment is needed. Its just the spacing from the sensor to the cc.

Switching my gear from the quattro to the 130pds makes not difference. Just note that the baader cc will not reduce but the skywatcher one will.

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I have the same scope as you, and am using Skywatcher's 0.9 reducer/CC.  I have never used the scope for imaging without it, so can't offer any with/without comparisons.  One advantage of this CC is that it will work with other scopes too - a 130PDS for example.  Got such one a couple of months ago, for wider field work, but regrettably weather hasn't permitted me to use it even once yet.

Below are some shots with the 200PDS, on an unguided HEQ5Pro, using a Nikon D7000, with 30-sec exposures stacked, so a setup very similar to your own:

 

M95 - M96 DSS2.jpg

M101d.jpg

M106 DSS3a.jpg

M31 DSSc.jpg

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19 hours ago, joe aguiar said:

ill give you my opionion

I have never owned or  used coma corrector on any scopes, even on my F/4.5 dob.

saying that tho you have an f/5 system so its gonna have that its standard so for visual it never bothered me in the slightest. Your pic is I think decent however your 2nd pic blown up does show it BUT it only cause its soo blown up tho. many people buy f/4 reflectors for imaging so it will even be worse in those but at f/5 I think image is ok

I guess the issue tho comes cause you want to do imaging so that's a whole different ball game. almost every scope that people buy for imaging will need a focal reducer or like your coma corrector or flate field. this is part of getting into this part of the hobby. There are some scopes that don't need these extra accessiors BUT cause they have those items built into it. The price is more expensive tho.

you are correct coma corrector are expensive even the badder ones are so expensive that I wouldn't even get one here where iam the badder one is $285 +49 shipping then tax that's $377.42. yet that's the cost pretty much of the whole ota tube.

To me as I said in another post I would never buy an accessiory that costs as much as the scope itself,cept maybe eyepices where they can be used on every scope ( I own 11 or 12 scopes right now about) so that makes sence.

so I guess if you plan on imaging that's kinda what you need to do tho.

joejaguar

Hi! Thanks for your advice. Yes you're right. Whereas the coma is not problematic for visual work, but it is for imaging. The second image I uploaded was the same image but at 100% zoom. Since most galaxies and nebula appear quite small on the camera frame, I usually have to crop an image to allow more detail to be seen. This is where coma becomes a lot apparent. When I initially got my OTA, I didn't know that much about coma and I frankly didn't get it why would so many people settle for high quality, small aperture refractors when you could have an 8" newtonian for far less cost. But I have learnt my lesson. 😃 =P

In Pakistan, hardly any company sells high quality astronomy equipment so I'm stuck with my newtonian (which is an impressive instrument in its own). Over the years I've learnt the importance of having a small setup that is easy to transport to dark sky sites than having a big setup that stays stranded under my light-polluted urban skies. Maybe in the future, I would invest in a fast high quality refractor. But at this point in time, I'm wondering if a coma corrector can bring me peace and satisfaction of having crisp stars. Judging by so many comments on this post, I'm actually very hopeful that I won't be disappointed. So most certainly, I will invest in a CC very very soon. 😃

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6 hours ago, Erling G-P said:

I have the same scope as you, and am using Skywatcher's 0.9 reducer/CC.  I have never used the scope for imaging without it, so can't offer any with/without comparisons.  One advantage of this CC is that it will work with other scopes too - a 130PDS for example.  Got such one a couple of months ago, for wider field work, but regrettably weather hasn't permitted me to use it even once yet.

Below are some shots with the 200PDS, on an unguided HEQ5Pro, using a Nikon D7000, with 30-sec exposures stacked, so a setup very similar to your own:

 

M95 - M96 DSS2.jpg

M101d.jpg

M106 DSS3a.jpg

M31 DSSc.jpg

Hi Earling!

Thanks a lot for sharing those images. Seeing that you have the same setup as myself, the images you shared are so much insightful! Just recently, I managed to go to a dark sky site and had the chance to photograph the Andromeda myself. I was constrained on time so took 5 minute stacks of 30s exposures each and there's a good chance my focus wasn't perfect as I bring my Bahnitov mask along. I've shared my final stack below. I can immediately see a few things:

  1. The coma in my pic is noticeable clearly even though I haven't cropped any part of the frame. In your pic, I cannot see any coma and certainly not even close to what I have in my stack.
  2. Your image processing skills are way better than mine as I've blown the core of Andromeda and you've managed to keep it intact.

To me, #1 is a clear indication that I NEED to be get my hands on a Coma Corrector now. It should get me much better stars across the field. Even then, I have a lot more to learn. Can you share how long your Andromeda stack was? When you say "unguided", I'm assuming you meant tracked but not auto-guided. At the moment, I'm also thinking about getting an auto-guider setup for myself. Seeing how better your pics are than mine, I'm getting doubtful if I'm overspending and that I simply need to get a CC and not invest in an guider scope/camera and a dedicated astronomy camera. What would your thoughts be about it?

I live under VERY light-polluted urban skies, so much so that my city images suffer from a massive pink/brown overcast. What sort of skies did you take these images of yours? If you live under as light-polluted skies, then that's a reason for me to stop doubting my equipment and work on my skills, instead of investing in a dedicated cooled camera and auto-guider setup and narrow-band filters to allow me to cut light pollution and be able to take longer, tracked exposures that will allow me to see more detail in my images.

 

M31_AndromedaGalaxy_3.jpg

Edited by AweSIM

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Hi Asim,

Thanks for your kind words regarding my images; I'm still just a beginner, fumbling my way around, having done this for about a year now.  Yes, images were tracked, but not guided.  I'm trying to get guiding up and running, as I'm pretty much restricted to 30-sec exposures as it is now - trying to increase the exposure times results in too many lost subs, due to tracking errors (A HEQ5Pro is probably at its limit with a 200PDS).  I'm blessed with having Bortle 4 skies where I live, so all shots were from my garden.

With regards to processing, I'm very much experimenting, still learning. All has been done with Astroart 6, and Andromeda wasn't particularly hard. It has a filter called 'DDP', which is described as 'making digital look like film'.  I find it invaluable for galaxies, as it makes for a smooth transition from the core to the fainter outer parts. Don't know if it goes under different names in other image softwares.

Andromeda pic was made by using 219 out of 250 subs, each 30-sec, so total time is 109.5 minutes to be exact. Stacked with DeepSky Stacker.  Used 20 darks and no flats.  Images shot at ISO 200.

In your case, I would go for a CC first, and then look at guiding, but remember that I'm just a beginner, so take it with a pinch of salt :)

As mentioned, I'm trying to get guiding working myself.  Have the necessary hardware ready, but weather has been most uncooperative the entire autumn.  Also have a new (to me) astro modded, cooled Canon DSLR, that I'm dying to try, but the weather.. :clouds2:

Best of luck with your continued endevors,

Erling

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I've recently bought a skywatcher 200pds and I bought a SW coma corrector with it. I've only been able to use the scope twice, just visual not imaging. It didn't seem to make any difference with visual use and the stars at the edge of the field were still elongated.  Afterwards I did a bit of research and found something about having to adjust the distance between the CC and the eyepiece in order to get the best out of the CC.  Does anyone know anything about that?

Cheers

Steve

 

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I have noticed the extact same effect on my images as well around the edges with the 200p (not the PDS). I would be interested if the coma corrector would work for the 200p as well?

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