Coma correctors

[jetstream]

What amazes me is how good a well collimated fast dob with a large secondary can be on the moon and planets-unbelievable views last night.The moon took an unreal 400x well,with a bit over 300x crisp and sharp,while the good seeing allowed the scopes potential to be released.I use the Catseye sight tube and cheshire to collimate using the partial offset method.Very easy to tweak,and its fun learning the nuances of collimation along the way.The notion that a fast dob is only for DSO is just that-a notion.I am very glad my scope doesn't show the coma as is described by some posts in this thread thats for sure.

[wolfpaw]

What amazes me is how good a well collimated fast dob with a large secondary can be on the moon and planets-unbelievable views last night.The moon took an unreal 400x well,with a bit over 300x crisp and sharp,while the good seeing allowed the scopes potential to be released.I use the Catseye sight tube and cheshire to collimate using the partial offset method.Very easy to tweak,and its fun learning the nuances of collimation along the way.The notion that a fast dob is only for DSO is just that-a notion.I am very glad my scope doesn't show the coma as is described by some posts in this thread thats for sure.

I am too. Theoretically though, I think I'm right in saying that if you stand two Skywatcher 250s next to each other and used the same eyepiece then they should show the same amount of coma. Maybe the optics on your OOUK scope are just vastly superior to the stuff churned out by Synta.

[AndyWB]

... especially as they've already stated that the only 'fault' with it can be miscollimation.

Well, that's a statement to make! Correct me if I'm wrong, but duff mirrors do (occasionally) happen, right? I'm sure I read on a German forum about them finding a 130p with a dodgy mirror, and someone getting a Sumerian built only to have problems with the GSO mirror they were supplied.

[nicks90]

wolfpaw - i really do feel for you and the issues you are having with your scope.

but you do seem to be insisting on hijacking every thread about coma and astigmatism and other aberrations on SGL at the moment. You are having issues, but being aggressive on here is not going to solve this. We dont know for sure if its your scope mirror, misalignment, collimation, eyepieces or even your eyes themselves! Without someone experienced with their own eps visiting you and having a look, we probably never will.

You are legally entitled to a refund under the EU selling laws. If your retailer is refusing to do this - then you need to seek some legal advice. Usually your first consultation is free.

I recommend you use this, get together the facts and also include the EU distance selling directive and also your legal representatives hourly costs and send a very polite letter to the retailer explaining you are within your rights to reject your scope and if they do not accept this, then your legal representative will deal with it for you - and they will be liable for his costs. You will be amazed how accommodating a retailer can be when they realise they HAVE to follow the law.

Get your money back, then go visit a star party or two and look through other scopes and make an informed decision as to what type of glass suits you and your budget.

[wolfpaw]

wolfpaw - i really do feel for you and the issues you are having with your scope.

but you do seem to be insisting on hijacking every thread about coma and astigmatism and other aberrations on SGL at the moment. You are having issues, but being aggressive on here is not going to solve this. We dont know for sure if its your scope mirror, misalignment, collimation, eyepieces or even your eyes themselves! Without someone experienced with their own eps visiting you and having a look, we probably never will.

You are legally entitled to a refund under the EU selling laws. If your retailer is refusing to do this - then you need to seek some legal advice. Usually your first consultation is free.

I recommend you use this, get together the facts and also include the EU distance selling directive and also your legal representatives hourly costs and send a very polite letter to the retailer explaining you are within your rights to reject your scope and if they do not accept this, then your legal representative will deal with it for you - and they will be liable for his costs. You will be amazed how accommodating a retailer can be when they realise they HAVE to follow the law.

Get your money back, then go visit a star party or two and look through other scopes and make an informed decision as to what type of glass suits you and your budget.

So you don't think people considering buying an f/4.7 telescope have the right to know that they can possibly expect to see over 50% coma through the eyepiece? I'm sorry you feel that I 'hijack' every thread about it but if I don't mention this issue with coma then no-one else will. As I've said, if someone had told me that that amount of coma was 'normal' and 'expected' then I wouldn't have bought the telescope in the first place. I did a lot of research before getting my Skywatcher 250PX and the only phrase that kept popping up re. coma was 'edge of the field'. It was not 'half of the field'. I just feel that people have a right to know what they're getting and the potential issues that can arise.

[Moonshane]

at the risk of getting further off topic (albeit related) other than the moons of Jupiter have you had any other issues? I seem to recall you mentioned it was not nearly as bad with e.g. a large cluster? have you repeated the test on the moons of Jupiter with the same effects?  have you let Jupiter and moons drift across the whole field to see where and when the flaring starts? all this sort of thing would help eliminate possible causes but I sincerely wish you lived near to me so I could have a look at your scope and allow you to try my coma corrector. personally I would always recommend a coma corrector below f5 as I have said many times as coma starts immediately when you go off axis and is more pronounced in faster scopes, wider fields and lower powers.

[admin]

wolfpaw - i really do feel for you and the issues you are having with your scope.

but you do seem to be insisting on hijacking every thread about coma and astigmatism and other aberrations on SGL at the moment. You are having issues, but being aggressive on here is not going to solve this. We dont know for sure if its your scope mirror, misalignment, collimation, eyepieces or even your eyes themselves! Without someone experienced with their own eps visiting you and having a look, we probably never will.

.... go visit a star party or two and look through other scopes and make an informed decision as to what type of glass suits you and your budget.

Wolfpaw, nick 90 has offered you good advice. Please read it again then consider adopting a more positive and constructive posting style 

No more thread hijacks please. 

[scarp15]

Suggest that you aim to make direct contact with a local group of amateur astronomers and arrange to meet up, if not where you are located now then on returning, as you have indicated back to the UK. That way you may gain first hand experience and expertise with a variety of scopes and exchange experiences with yours and so perhaps get to the root of the issue. The forum debate has perhaps gone round the block several times and exhausted its ability to further assist.

[AlexB67]

Wolfpaw, I sincerely wish you the best of luck. I hate not seeing people not enjoying the hobby and the joys that a 10 inch Dob can bring you  , because it can I believe, even without coma correction. Whether you experiencing issues through a faulty mirror or not. I admit I don't disagree with regard to comments on coma you have made  per se a lot of the time,  when you really look for it deliberately in a longer focal length eyepieces wide angle eyepieces it can be there quite noticeably, but I also feel there is an element of, do you look at the glass half full versus the glass half empty, and when is coma actually really bothersome, and put in perspective  you get what you pay for and get out of such a scope and what you can actually see with it.

You said you did a lot of research.  I was under no qualms that there isn't enough info on the internet  to understand the topic and get a good idea how much of it will manifest itself in such scopes at that ratio. Everything I have bought to date has pretty much tallied with expectations and the results I've been getting so far. 

Thing is, words are often qualitative and often much is left to the imagination, sites like http://www.telescope-optics.net/ carry a wealth of information to convey the information  so much more accurately, but I appreciate that site is not for everyone.   

I wish you  all the best in the hobby and that something will work out for you eventually.

[sharkmelley]

Returning to the question posed by the original post,  I highly recommend Smith,Ceragioli & Berry's book "Telescopes, Eyepieces and Astrographs - Design, Analysis and Performance of Modern Astronomical Optics" though it is fairly technical.  However they do address this precise question regarding any difference between low magnification and high magnification eyepieces on a fast Newtonian in section 7.5.  Comparing 2 eyepieces with the same apparent field of view (e.g. 50 degrees), they say: 

"With a shorter focal length eyepiece, the blur [from coma] is smaller but the magnification is greater.  Unfortunately the increased magnification excatly compensated for the decreased coma blur size.  Thus, in each eyepiece, the perceived coma appears equally bad, independent of magnification."

Mark

[sharkmelley]

That should read "exactly compensates for"

[mitchelln]

Wolfpaw. Buy a Coma Corrector to see if it solves your problem. Either way, as discussed a lot now, you have 7 days to return it under the EU distance selling regulations without even having to give a reason. At least then you will know one way or the other.

Failing that, I will lend you one if it puts this to bed!

BTW here is one of the very first shots I took of M42 with my f4.9 300P. So no coma corrector or flattener. Full frame shots at prime focus in both orientations with 2" nosepiece.

Between them, I believe these two shots cover the entire FOV. Coma only visible near the edges. None at 50% as far as I can tell and certainly none at 80%. This is before I really knew how to collimate properly as well. Literally the 2nd time I got the scope out.

[AlexB67]

Returning to the question posed by the original post,  I highly recommend Smith,Ceragioli & Berry's book "Telescopes, Eyepieces and Astrographs - Design, Analysis and Performance of Modern Astronomical Optics" though it is fairly technical.  However they do address this precise question regarding any difference between low magnification and high magnification eyepieces on a fast Newtonian in section 7.5.  Comparing 2 eyepieces with the same apparent field of view (e.g. 50 degrees), they say: 

"With a shorter focal length eyepiece, the blur [from coma] is smaller but the magnification is greater.  Unfortunately the increased magnification excatly compensated for the decreased coma blur size.  Thus, in each eyepiece, the perceived coma appears equally bad, independent of magnification."

Mark

I don't agree with that, that is a based on a simplified theory, perceptible coma  in a low mag eyepiece is easier to see. There is a difference between actual calculated coma, and perceptible coma, this is where a lot of the misunderstanding comes from.

The site I linked earlier explains exactly why too but even that neglects seeing conditions, which will be a factor at higher mag in terms of blurring, but it dicusses this in great detail nevertheless. There is how much off axis you see too at a given mag and FOV as well so the percentage of the field affected by coma is a factor also.

Sorry I have never read the book, but that statement is a common regurtitation what is often mentioned. In my experience coma is not independent of magnification to the eye as seen in the  scope, and that is an important point often overlooked in visual observing and when visual acuity and other factors are considered.

There are many factors involved, calculating how the eye perceives coma is not easy by any means, but that common statement you see often quoted is misleading IMHO.

[AlexB67]

Anyway, my scope is cooling outside, time for mars soon, who cares about coma

[RikM]

I went looking for coma tonight. With my 20mm 70° eyepiece in general viewing, stars are no longer pinpoints in the outer 50% of the field. However, this eyepiece shows some slight field curvature and by refocusing as you move outwards, the residual coma is only visible in the outer 25-30% of the field. I personally don't find either of these aberrations problematic or disappointing. Especially when you consider that scope and eyepiece combined cost less than an 80mm Triplet apo refractor. Eyepiece manufacturers have to balance feature and they can do it in different ways. I prefer eyepieces that show very little (I would say none, but there are pedants everywhere!) distortion. To get that, I have to accept a certain amount of field curvature. I could purchase eyepieces from another manufacturer which don't show this field curvature but the price would be distortion of the field (pincusion or rectilinear distortion I am not sure of the distinction), which I detest. I understand that I am not going to get the pinpoint stars performance of a 10" APO refractor (£60,000) for the price of a 10" Newtonian (£450).

Confirming what Alex has posted above, with my 10mm 70° and shorter EP's There is no visible coma or field curvature at all. Actually with the 10, 7, 5 and 3.5 the limiting thing is the atmosphere. Tonight the atmosphere was very stable. I got a single clean airy ring around the stars at 240x and 343x. I also had one of the best views of Mars ever. The disc is really a good size this year. Small polar cap and a good level of surface detail. (Sketch to post tomorrow )

[wolfpaw]

Wolfpaw, I sincerely wish you the best of luck. I hate not seeing people not enjoying the hobby and the joys that a 10 inch Dob can bring you  , because it can I believe, even without coma correction. Whether you experiencing issues through a faulty mirror or not. I admit I don't disagree with regard to comments on coma you have made  per se a lot of the time,  when you really look for it deliberately in a longer focal length eyepieces wide angle eyepieces it can be there quite noticeably, but I also feel there is an element of, do you look at the glass half full versus the glass half empty, and when is coma actually really bothersome, and put in perspective  you get what you pay for and get out of such a scope and what you can actually see with it.

You said you did a lot of research.  I was under no qualms that there isn't enough info on the internet  to understand the topic and get a good idea how much of it will manifest itself in such scopes at that ratio. Everything I have bought to date has pretty much tallied with expectations and the results I've been getting so far. 

Thing is, words are often qualitative and often much is left to the imagination, sites like http://www.telescope-optics.net/ carry a wealth of information to convey the information  so much more accurately, but I appreciate that site is not for everyone.   

I wish you  all the best in the hobby and that something will work out for you eventually.

Thank you. I suspect my problems with the telescope aren't really relevant to the forum any longer so I'll say goodbye, but you've always been helpful and friendly and it has been appreciated!

Clear skies and best wishes

Rich

[AlexB67]

I also had one of the best views of Mars ever. The disc is really a good size this year. Small polar cap and a good level of surface detail. (Sketch to post tomorrow )

Looking rather good right now is us mars, just using 240x right now, scope will be more settled later to pump that mag hopefully. Not as steady as last night but very nice indeed. last night I was doing 300x and in moments of good seeing the views were superb.  Just a brew and back out for more

[mitchelln]

Sky at Night magazine reviews the SkyVision 12" 1200mm f/4 compact truss dobo this month.

They report a sharp image across 80% of the FOV without a CC.

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