Coma Corrector Advice

[Rod]

Hi

I would like to buy a coma corrector. I was originally thinking of a televue type 1 but they have been discontinued. The type 2 is out of my price range. I notice Bintel has an inexpensive one for around $160. Has anyone used this? I assume it is a GSO one rebadged.

I want one for visual use with F4 and F5 newtonians.

Thanks

Rod.

[mozzie]

hi rod i have a 2" bintel coma corrector i use it with my 16" lightbridge...there is difference and the coma around the edge of fov is much crisper.i use naglers 82 deg....
haven't used it for ages so if your interested i will sell it..

[Rod]

Hi Peter

Thanks for the offer. At this stage if I were to buy a cheaper coma corrector I would prefer to get a new one. I am pleased to hear you found it useful though. Any reason why you don't use it now?

Rod.

[mozzie]

no reason rod,i just don't use my lightbridge all the time...the corrector is brand new only used a dozen times..i was offering it as it isn't being used ...a waste

[mental4astro]

Hi Rod,

There's a couple of things to consider with coma correctors. They will help deal with coma but not astigmatism in eyepieces. In astigmatic EPs, the abberation will remain unaltered as this occurs post corrector. They also won't correct for abberations like pincushion.

The other thing is you WILL loose light. While coatings & lenses in correctors have improved, you will always loose light.

In the end the choice is yours.

As a user of f/4 & f/4.5 Newtonians, I did give coma correctors a go, but gave them up because of this last point. I'm happy to deal with the small abberation that coma really is, even with my 36mm EP in my 8" f/4 Newt. I've been slowly changing over my EPs to ones that have astigmatism & other abberations better controlled. For some abberations it is also a matter of a mismatch between the EP & the scope as in pincushion.

For me, coma is a very subtle thing. I understand why it happens, and with all other abberations controlled I'm prepared to live with it in my low power EPs. It only happens at the edge, which is an area that serious observing is NOT done - you move the scope for this, even if you are using a corrector.

There is one last thing. Coma does become more pronounced when ultra wide angle EPs are used. Comparing the edge of two EPs of the same focal length, one 68* AFOV & the other 100*, the latter will show more the effects. Having said that, in my 36mm 72* Aspheric Hyperion, coma is still very, very subtle along the edge in my 8" f/4. Astigmatism, while not totally eliminated from this EP/scope combination, is still more pronounced than coma & it is very well controlled in this extreme combination.

Please consider.

Mental.

[Rod]

Thanks Alex

I have access to quite a few short focus instruments at my club which has an observing site near where I live. Mostly I have used very wide field eyepieces and a Paracorr with them. I like that kind of view and would like to replicate it in my own scopes. I really was just trying to find out if a cheap coma corrector would do the job as well as the type 1 Paracorr.

Rod

[ausastronomer]

Hi Rod,

I suggest you hunt around for a 2nd hand Televue type 1 Paracorr. These are the best for visual observing. There are 3 different versions of the type 1 paracorr. The first was non adjustable, avoid it. The 2nd was adjustable with single locking screws. This works just fine. The last of the type 1 paracorrs has double locking screws on all the adjustments. This is the best one mechanically but both of the latter type 1 paracorrs work just fine down to F4. I have both of the later type 1 paracorrs, the newest one is a touch nicer mechanically but they do the same job optically. The original non adjustable paracorr isnt as good as the later versions and should be avoided IMO.

There are conflicting opinions on whether the paracorr "steals precious photons". On the central axis you certainly dont need it. About 30% or further from the central axis the paracorr will allow you to see dimmer stars than without in scopes faster than F5. I dont need to go into the physics of why that is in this thread but take it as a given. There is plenty of literature out there on why this is. If you are only worried about observing targets in the centre of the FOV you are better off with simple minimum glass narrow field eyepieces like orthos and plossls and no paracorr. If you are going to use 60 degree plus eyepieces and use all of the FOV and your scope is faster than F5 you are generally better off with a paracorr and you will certainly see dimmer stars and features than without a paracorr the further off axis you go in sub F5 scopes.

Cheers,
John B

[Rod]

Thanks John

I had wondered about the early Paracorr so it's good to get your perspective on the different versions.

Rod.

[alistairsam]

Hi,
I use a Baader MPCC for AP, but have also tested it with an EP and its very good.
I've read that a member here tested the Bintel one's and they don't correct enough at F4. again, for AP.
I use a GSO SV EP with an F4 and its quite good even without a CC. There is coma at the edges but minimal.

[ausastronomer]

Hi,

What eyepiece(s) did you try the Baader MPCC with?

There are a whole number of reasons why the Baader MPCC isn't a good choice for visual astronomy.

1) The Baader MPCC uses a different and more simple design than the Televue Paracorr, the MPCC is based on a Ross Corrector design. This actually does a marginally better job of coma correction, however, it introduces a number of aberrations into the system, which are not desirable for visual astronomy, most notably spherical aberration. This isn't noticeable with long focal length eyepieces but it is noticeable with shorter focal length eyepieces.

2) The Baader MPCC has a clear aperture of about 36mm therefore it will vignette with any eyepiece having a field stop diameter > 36mm. Depending on the eyepiece the vignetting may or may not be noticeable. The Televue Paracorr has a clear aperture of about 38mm or 39mm.

3) The Baader MPCC threads on to the indiviudal eyepiece with special spacers, a bit like a filter. Different eyepieces require different length spacers making it very cumbersome to change eyepieces and re attach the MPCC. Once the TV Paracorr is inserted into the focuser you just change eyepieces as if they were going straight into the focuser.

None of the above are issues for astrophotography and for that purpose the Baader MPCC is an excellent choice.

One thing which draws people to the MPCC is the fact that it does not increase the effective focal length of the telescope by 15% like the Televue Paracorr does. This however is something which is necessary to eliminate spherical and other aberrations from the system.

Cheers,
John B

[alistairsam]

Hi John,

What you say makes sense and I'll retract my statement as mine was a very brief test with a GSO 26mm and I may not have paid close attention to detail. Just the initial impression I got that I thought it worked well.

I've tried a different brand 22mm EP with the same F4 scope and the coma was very severe. So the 26mm and mpcc might still have exhibited issues but from what I remember, it wasn't bad at all.
as you mention, its excellent for AP, that I can vouch for.

Cheers