Wide FOVs, fast scope and Coma Corrector

[Bart]

Hello all,

especially those who have spent time buying, selling, and generally trialing eyepieces..

I have a fast Dob, 16" F4.5. Now recently splashed out on a 100* 20mm ES. I like the wide FOV. I say like and not love becasue I'm not sure how much better it is than the 82* of the other EPs I have. Don't get me wrong, the wide FOV is nice and the view is sharp for a good bit of the view and the object is nicely framed alright. The coma (I'm assuming it coma) is somewhat annoying and am considering a coma corrector but not sure. I'm pernickety about the collimation now as I understand it exacerbates coma in F4.5s. I hear that long FL EPs and fast scope show coma even more so. The thing is, I don't seem to notice it as much in the longer FL 30mm 82* as I do with the shorter FL 20mm but larger FOV at 100*. Is it the wider FOV that shows it up more? At first I was dissapointed to find the coma in the new expensive EP, but don't know whether I am getting used to it, getting the collmation more accurate, or what, but am now enjoying the 100*.

However, wondered what effect a coma corrector would have? Would I finally breath a sigh of relief, or realise that at F4.5 coma is a fact of life and start getting used to it. OR, SHOCK HORROR, sell the newly lusted after 100* and go back to 82*

Generally related comments, insights, would be much appreciated

Barry

[Damo636]

Hey Barry. Unfortunately the lower the power & wider the afov the more pronounced the coma will appear. Although I have decided, for now, that I don't need a coma corrector at f5, in your case with f4.5 optics, I am fairly sure one would make a big improvement to the views. A Baader MPCC permanently installed on your 20mm 100° might be worth considering or go all out and spring for the Parracor!

[John]

..... At first I was disappointed to find the coma in the new expensive EP, but don't know whether I am getting used to it......

Coma is generated by the scope, not the eyepiece. Often coma is hidden because wide field eyepieces generate astigmatism in fast scopes and this masks the coma inherent to the scope. When you use premium quality wide field eyepieces (Nagler, Ethos etc) the astigmatism is gone and, hey presto, you can see the coma !

Thats why Tele Vue developed the Parracor coma correctors.

I see coma when I use my Naglers and Ethos with my 10" F/4.8 newtonian but I've decided to live with it.

[Moonshane]

I bought a used older type Paracorr with the green letters in excellent (perfect) condition for less than £200. for my f4 16" dob it was probably the best eyepiece purchase I ever made. it lives in the dob basically and makes it perform like an f8 scope. honestly it does. it's easy to use and no faff at all to change the eyepiece as you just twist the top a little and it's done.

if you can find one at a decent price then I'd strongly recommend it.

one thing to be aware of it takes the focus point in by maybe 20mm and this can affect your ability to focus some eyepieces. I moved my primary to compensate. not sure if this is the case with of coma correctors.

[Mike73]

I've ordered my 16" f/4.5 so it will accommodate a Paracorr even though the bulk of my EP's are 72˚ and probably dont need it but after looking through a f/4.5 with a 26mm T5 and knowing that my 26mm is probably my most used EP I figured it was worth it.

Having my truss poles cut to the correct focal length for a Paracorr will hopefully 'future proof' any EP changes.

[alan potts]

Barry,

I know you have saved and saved for this but it could just be the softer edges that we all know eyepieces other than Televue suffer with. Your scope its a fast F 4.5 you will start to see failings in the Ex Sc 100 and the Meade 100 at this F/Ratio but not in the Televue. Buying a Paracorr will not correct what is caused by the eyepiece, only the telescope.

Sorry, Alan.

[cs1cjc]

In my expereince a coma corrector will make a significant difference on eyepieces with a wide apparent field of view. Televue is not the only make available. I can recommend the Astronomics/Altair Astro/GSO, though I know of nobody stocking it in the UK currently. At the time of writing Agena say they have it in stock for $130 including eyepiece adapter:

http://agenaastro.com/gso-2-coma-corrector.html

Telescope Express also seem to have it in stock for Euro 55, but without an eyepiece adaptor (though they have on available):

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3969_AKTION-2--Koma-Korrektor-fuer-f-4-bis-f-5-Newtons-M48-Anschluss.html

In both cases additonal spacing to the eyepiece of about 20mm is needed (to make the spacing 75mm), though a Hyperion 14mm fine tuning ring may suffice. See my more detailed review, here:

[Moonshane]

that's a great write up Chris. I have never even heard of that particular coma corrector. for me the convenience of the tunable top is worth paying a little extra for via a used TV Paracorr. I am sure they all do a very much similar job.

[KaStern_Former_Member]

Hello Bart,

in an f/4.5 Newt off-axis coma will be easily visible.

The eyepiece focal length does not matter at all.

For example a 20mm eyepiece with 22mm field stop diameter

and a 10mm eyepiece with 11mm field stop diameter will show

the same amount of coma at the edge of field.

Coma increases linear with the distance from the optical axis.

So it will be double at the edge of the 20mm eyepiece.

But magnification in the 10mm eyepiece is double,

so the comatic blur of a star at the edge of field has the same apparent diameter.

The long focal length eyepieces often suffer more from off-axis astigmatism than the shorter focal length eyepieces.

The combined coma (from the mirror) and astigmatism (from the eyepiece) add.

There are some coma correctors, from expensive to quite cheap.

Better avoid 2-lens coma correctors.

In former times there were coma-compensating eyepieces for Newtonians.

University optics had a 25mm "Pretoria" eyepiece and there was a scaled eyepiece with 16mm or so.

And there was a 20mm Brandon too. Unfortunately they all are discontinued.

University Optics had coma-compensating barlows too, discontinued...

Cheers, Karsten

[Bart]

Thank you all for your comments nad help. Have managed to get out quite a bit in the last week or so and have decided while I might try a CC later, it doesn't bother me too much at the moment.

Thanks all.

Barry