Coma, astigmatism and Newts - some useful notes ... perhaps

[AlexB67]

Hello,

Often coma and astigmatism regularly come up in various topics with regard fast and slow telescopes. It is an area that can get quite technical, and it is also an area often surrounded by some confusion and mist, thus lacking clarity. All too often have I stated something that leaves much to the imagination and I wish I never said it in a particular way, but it is also an area where it's easy to get the wrong idea without writing a long essay !.

Over time I've learned more and more about eyepieces and telescope design, with of course much still to learn. I am all too familiar with the saying The more I learn, the more I realise there is more to learn

The following resources are useful if  you want to read a bit more in your on time and help in making decisions when you ask yourself the type of questions: ShouId I invest in a coma corrector,is a coma corrector worth it ? Should I spend 200 - 300 pounds on that TV. Naturally, it can also be a subjective topic depending on the demands of the observer.

I have my personal views to date,  but it is not for me to say what an observer should or shouldn't buy, in the end of the day each to their own.  If asked I would recommend what I think, but in my short observing career so far, my standards and demands are creeping up. What I recommend today may change next week, and what I recommend next week may change next month. In the end we can only form our own opinions.

I leave you with the following resources that l've found useful in gaining an understanding in this area.

• http://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

IMO, this page is the absolute bible, but it can be heavy going. Bolster up, have a coffee, but it is full of all important information  . Even if most of the page is  heavy going, the paragraph starting with a summary contains some very useful info and formulae, it gives a good indication when you are likely to see coma in an eyepiece given its focal length and knowing the focal ratio of a scope, ultimately something you'll will be more interested in, as opposed to the coma free field is, which can paint a more pessimistic story.

My personal feeling is that in many cases in it can be hard to distinguish astigmatism from coma, you really need to look carefully. In faster scopes, say roughly below f/5 say, the bottom line is that you need to pay a lot of money for wide angle eyepieces to get rid of aberrations or better put, minimise them, only in such eyepieces will coma  show in its true form if it lies inside the visible field.  

We don't tend to speak much about coma caused by eyepieces, since it is relatively easy to eliminate by design. Most eyepieces I have looked through so far to date in an f4.7 Dob in the mid price range contain at least some element of astigmatism and.field curvature,  it is more a question of how much and whether you find it acceptable. 

• http://www.rfroyce.com/mirror_performance.htm

This article is somewhat easier going, but IMO misses out on some of the more  important subtle points, it is much easier to digest however.

• http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5232029/page/8/view/collapsed/sb/5/o/all/fpart/all/vc/1

A very useful discussion on cloudy nights that also links to the above

• I attach a further document from a telescope builder, it touches on other topics too, but has useful  information on coma, focal ratios, and the implications of coma correctors, their pros and cons in some situations, and implications on collimation.
• http://adsabs.harvard.edu/abs/1935ApJ....81..156R  this is probably more than anyone would ever need, I certainly never digested most of it but will provide the link for anyone interested.
• A useful little app to play with, though it is beta and rather old, not all of it works. http://aberrator.astronomy.net/html/mdibeta.html  
• My own calculator application which is work in progress has some stuff in it relating to this too, If I ever release it before growing a grey beard I will announce it. !

The most important thing, have fun and don't lose sleep over it  .

Finally in closing, we have some very knowledgeable moderators on here with long term experience. Typically, when the likes of Michael Wilkinson and John say something and offer advice, not to single anyone out or names that elude me right now, just to name a few, I can assure you it is good advice. It is also through them and many others where I gained knowledge as well as reading around.

Clear skies to you all

coma.pdf

[Naemeth]

Some interesting stuff there Alex, I agree with you - not to lose sleep over coma, it is a hobby after all where the purpose is to have fun, and/or learn/gain something or understanding .

[todd8137]

Great read thanks for posting

Pat

[Tonys]

The Telescope-Optics site is brilliant for anyone interested in the technical aspects of scope optics, so much so I printed it all out (it fills a ring binder). It is very technical in parts though, so recently I got a couple of Ipad apps to help me understand. They are Ray Optics and RayLab. The first is a basic intro to simple lenses and mirrors. The latter is at an earlier stage of development but with it you can make virtual telescopes and even change things like the type of glass used in optical elements to see the effects.