Potential coma in a F4.5 Scope

Potential coma in a F4.5 Scope
Hi there,
I ended up with a F4.5, 8 inch Newt when I purchased a new mount two years ago. It was one of then special package deals where the mount alone would have cost more than the package. Despite having no wish for a Newt ( I had planned to sell it immediately), I have now grown to like the Orion Optics UK scope. Its one of these scopes that came with a nice cert, Strehl ratio of 0.992. I have even come to grips with Colimination. (As Daves recently highlighted on another posting here, these Orion Optics are nice scopes). This OTA is lightweight and it does not dew up like the SCT. I am sure it would be great for Astro work, but I am only a visual observer.
However I believe I am suffering from Coma as I only have a Baader 8- 24 Zoom eye piece. This Zoom was purchased to use with my F10 SCT and is a perfect match at F10.

I have been following some debates on Cloudy Nights and came across the following information. I am not sure of the math but for my scope the following would be the results:

The coma free region, the diffraction limited diameter, can be computed:

CFR = 0.022 x F^3 mm where F is the focal ratio of the scope. So, an F/4.5 scope has a Coma Free Region of:

CFR = 0.022 x 4.5^3 mm = 2.0mm *

If you had an eyepiece with a 2.0mm field stop, the view would be free from coma.

You can compute the angular diameter of the coma free region by dividing by the focal length and converting from radians to degrees:

CFD = 57.3 deg/rad x CFR / FL scope.

So a 200mm F/4.5 scope has a Coma free diameter of:

CFD = 57.3 deg/rad x 2.00mm/200*mm/ 4.5 = 0.1273 degrees.

You can determine the size of the coma free field at a given magnification simply by multiply the CFD by the magnification. At 200x the above example has a coma free AFoV of 0.1273* deg x 200= 25.46 deg. It is only the centre of the field of view that will be sharp.

These calculations seem to reflect what I am seeing.
The Baader has an AFOV of between 52 to 68 depending on the zoom and who is quoting the figures. Values for guidance only.

So my questions:
1. Am I looking at buying 'better' quality eyepieces or do I need a coma corrector with the zoom?
2. What make /model of ‘better eyepieces’ would folk suggest?
3. I only have 1.25 inch eyepieces, should I be using 2 inch with this scope?

I believe the Philips Toucan Pro camera has a sensor size of 2.68mm, so would a good Barlow be ok for planetary work, if I head in that direction so as to match my 2.0 mm CFR with the sensor size of 2.68 mm?

Donal

Donal, Just a couple of points.

If you use 2" eyepieces, you will just be effectively widening the field stop so the coma will be more apparent towards the edge. If you want to use 2" (and you really should) you would really need to use them in conjunction with a coma corrector. The televue paracorr is apparently the best but I've used the baader MPCC photographically and it is also very good. I understand you can use it visually too with the correct adaptor.

To be honest I've always felt that eyepieces designed for one particular focal length are best, I've no direct experience with zoom eyepieces but I can't see how they can give optimal performance given that they have to cover such a wide range of f/l.

just my 2c

Hi Dave,
Thanks for the info. Had not appreciated coma being more apparent at the edge with 2 inch EP's.
The emperical calculations do back up what you say.
The popularity of the Baader Zoom seems to be reasonable preformance at around F10, ie SCT's. It does suffer at the faster Focal Ratios.
Anyway back to the original issues, so you reckon I should be after eyepieces that mention performance specifically at F4.5 or that general area!I appreciate all scopes are different but are you suggesting a coma corrector may not be required with'good quality' 1.25 eyepieces? I presume that would explain why coma corectors all seem to be 2 inch.
Donal

Donal,

As usual I didn't explain very well When I said that its better to use eyepieces that are designed for a particular focal length I really meant that of the eyepiece not the scope. However that does raise an interesting point. I have read reviews where people talk about eyepieces performing better at a particular f/d (of the scope). Perhaps it might be worth a bit of digging around google to find some eyepieces which have been reviewed with something closing on your f/d ratio.

In an ideal world a coma corrector will mean that lot of the coma is diminished before the light gets to the eyepiece. Therefore matching up some eyepieces with a CC would definitely be the best way to go. Any eyepiece will perform a lot better with a good CC like the televue or MPCC.