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How bad is coma on f/4.9 10" DOB?

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Hi all,

First time scope buyer and I put in an order for an 8" DOB, but it's on back order, and as I wait I'm reconsidering if I should get the 10" DOB instead as I've read a lot of people think galaxies & DSO's really start coming to life at 10" and above.  I do not want anything bigger than a 10" for a first scope for sure.  The $200 cost difference between the 8 & 10 is doable and I think I can manage the extra weigh of the 10", however I've read that faster scopes (the 10" being f/4.9) can have something called a coma when viewing things.  And from what I've read people say it's one of those things where it bugs some people, others it may not.  Since this will be my first scope and I've never even looked through a Newtonian scope before I really have no idea what this is.

Can someone who really understands this please let me know in layman's terms what this really means, what I can expect from a 10" DOB at f/4.9 far as coma goes, and how important this could be?  From what I loosely gather objects around the edges of the FOV may have trails or something, hopefully not this bad http://umich.edu/~lowbrows/reflections/2007/dscobel.27a.gif but again I have no first hand experience.  Few YouTube videos I've seen of people posting pictures & videos through their 10" DOB's I don't notice anything odd around the edges, so...  From what I've read the 8" DOB at f/6 shouldn't have this problem, and only 10" or bigger DOB's have this issue I think.

Ultimately my decision is either to stick with the 8" DOB I have on order and hope I can see enough DSO to a satisfying amount and don't regret not getting the 10".  Or go for the 10" instead and hope this "coma" thing isn't a big deal.  I see many discussions where people compare sizes, the pros & cons, and generally I see people say go as big as you can manage and afford, but I see very little discussions about coma, so I'm not sure if it's a big deal or not?  But if it's a big deal and if I'll need expensive corrective lenses to compensate, then I'd prefer to stick with the 8" and take my chances on the brightness being enough


Thanks everyone! 

Edited by dcobb
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"For DSOs go with the largest aperture you can manage" - is the mantra given for a reason:  8" for me got me to view all the Messier objects at a darkish site however when I moved up to 12" the difference is vast in what I could actually see and giving to actually wow moments in visual observing.  Training the eye and observing technique is also a big part of visual astronomy as well.  10" would serve you very well - the difference in light gathering from 8" is pretty large so you will benefit greatly.  Don't worry that much about coma  - when you build your eyepiece collection, add eyepieces types that at least correct some or don't make it worse + you can always purchase used to keep the price down...

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Hello dcobb, welcome to SGL.  I say go for the 10", it will give much better DSO performance than any 8" visually.  Unless you are particularly picky, coma visually is no big deal, for imaging it would be an issue but easily corrected with add- ons.  You would always wonder "what if" if you went for the 8".       🙂

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Just to say I agree with everything Davesellars says above.

I think for visual I coma is subjective and somthing you have to experience yourself to know whether it bothers you enough to worry about.

For me in my scopes and visually, try as I might, I don’t notice it so it’s not an issue.

in the 200p it’s immediately apparent in photography but easily corrected.

You’ll be very pleased with the 8” but given the choice go for the 10”.

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For visual coma just doesn’t bother me and a dob is for visual after all.

The 10” will have 56% more light gathering ability than the 8”. The 10” will show more subtle detail on DSOs but won’t be as noticeable on planets.

Would definately recommend the 10” over the 8” if size and budget allow.

Edited by johninderby
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Another vote for the 10 inch if you can physically carry it in 2 parts, as most people can.  For many, a 10 inch is the "sweet spot" between what a scope shows and portability.

The 10 inch even without a coma corrector should show you more than an 8 inch with a coma corrector.

How much coma there is depends on the focal ratio.  The lower this is the more coma there will be.  As 10 inch scopes usually have lower focal ratios than 8 inch ones, this is why they have more coma - not because of the increased aperture.

As said above, coma bothers some observers more than others.  It's most pronounced in eyepieces with a large apparent field of view because it's worse the further you get from the centre.

You can always add a coma corrector in the future.  Plus if you buy an 8 inch you'll always wonder if you should have gone for the 10 inch!


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Another vote for the 10", I have the one in my signature line.
You in the right condition can really enjoy deep sky with 10" and bigger, 
but 10" is manageable, especially in two parts.
But if you think it will be too heavy or awkward, the 8" is a lifetime scope for many.

As to Coma, keep away from very wide field eyepieces and it should be no issue,
however if you have astigmatism, then that needs correcting with glasses before you look through a scope,
I know as what I blamed on coma was my own Astigmatism.

Whatever you buy, get out look up and explore and enjoy the majesty and wonder of the sky above.

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