Coma Correctors

[Scooot]

I'm thinking of getting a low power wide field eyepiece. Maybe a 30mm 82 deg AFO. I'm wondering if I do whether I might need a coma corrector.

I've read about the televue paracorr and the baader mpcc, both appear quite good. The televue magnifies by a little bit and is more expensive. The baader doesn't seem to be quite so user friendly as it requires spacers.

I wonder from anyone that has one how often you actually use it and in what circumstances.

[Damo636]

Richard

For use with one eyepiece I'd go with the Baader MPCC and leave it permanantly attached. I intend to do this with my 21 Ethos as its the only eyepiece I find the coma slightly distracting.

[Scooot]

Richard

For use with one eyepiece I'd go with the Baader MPCC and leave it permanantly attached. I intend to do this with my 21 Ethos as its the only eyepiece I find the coma slightly distracting.

That makes a lot of sense, thank you. Does it improve the views through the non coma affected area at all. From what I've read about the paracorr that's meant to but this could be sales talk of course.

[estwing]

there's a mk1 paracorr on ebay at £125

[estwing]

http://www.ebay.co.uk/itm/Televue-Komakorrektor-Paracorr-Visual-Tele-Vue-/281080840168?_trksid=p2045573.m2042&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D6471705071069336787%26pid%3D100033%26prg%3D1011%26rk%3D2%26sd%3D200906616895%26 there you go

[Scooot]

there's a mk1 paracorr on ebay at £125

i can't find it Doh, could you please post the link.

[Scooot]

http://www.ebay.co.u...d=200906616895 there you go

thanks

[estwing]

glad to be of help,just hope someone's there to get me the mk2 paracorr when i need one...which will be soon.!!

[Scooot]

there's a mk1 paracorr on ebay at £125

It went for £277, (not me) anyone got a view on the price.

[Moonshane]

that looks more like a Mk2?

I use a Paracorr all the time in both my f4 scopes. it makes a big difference to all my eyepieces and makes the scope look like and f8. the effect is most pronounced with eyepieces of 20mm or more focal length or very wide fields (e.g. 13mm Ethos) but is fine without for e.g. 15mm and below albeit still better with to my eyes. the effect is dramatic with my 26mm Nagler. I didn't really need one in my old 12" f5.3 OOUk dob.

[Moonshane]

I bought my Mk 1 for less than £200 used.

[estwing]

don't say it was a mk2...please...no

[Moonshane]

well my Type 1 looks like the one shown here compared with T2

the one in the ebay ad looks different to both actually and might be a leter T1

[estwing]

not sure if the mk 1 was made with the ethos in mind..and i intend to get the 21mm,so i'm still in the hunt

[cs1cjc]

There are good coma correctors for much less than the Paracorr. Once set up, the Astro-Tech/Altair Astro/GSO mdeol with optical design by Roger Ceragioli works well. I use one almost all the time in my Newtonians.

Although coma is more visible at lower magnification, its impact on image quality is largely independent of that.

[Scooot]

1364743884[/url'>' post='1886949']

There are good coma correctors for much less than the Paracorr. Once set up, the Astro-Tech/Altair Astro/GSO mdeol with optical design by Roger Ceragioli works well. I use one almost all the time in my Newtonians.

Although coma is more visible at lower magnification, its impact on image quality is largely independent of that.

Do you use it all the time for visual use, which is my interest?

[cs1cjc]

Do you use it all the time for visual use, which is my interest?

Yes. Review here:

[Scooot]

Yes. Review here: http://stargazerslou...and-user-guide/

Thanks Chris, that's very informative.

[estwing]

not sure if it would be ok at F 3.9,so i'll stick with a paracorr..

[cs1cjc]

not sure if it would be ok at F 3.9,so i'll stick with a paracorr..

I cannot see why you would change a Paracorr for a cheaper coma corrector unless you were in dire need of the money. However if you have an F/3.9 and have no coma corrector then you are certainly in dire need of one and there are cheaper good products than the Paracorr!

The GSO was I understand optimised for F/4.5, but it will work with a F/3.9. All Newtonian mirrors are the same shape so to get an F/4.5 mirror from an F/3.9, you could simply mask off the outer 13% or so of the radius, about 25% of the area (suppose the focal length is 900mm then an F/4.5 is 200mm and an F/3.9 is about 231mm). So a corrector designed for F/4.5 is optimised for 75% of the area of an F/3.9 and it practice it will do a reasonable job on remaining 25%.

I am sure a Paracorr II which is I understand optimised for F3.5 will do better, with spot sizes here: http://www.televue.c...2_spotsizes.pdf . It is a pity there are no spot sizes for the GSO design by Roger Ceragioli. However the book Telescopes, Eyepieces and Astrographs: Design, Analysis and Performance of Modern Astronomical Optics by Gregory Hallock Smith, Roger Ceragioli and Richard Berry gives details of a related design which is said to be satisfactory for all Newtonians down to F/3.5.

[estwing]

mask off!!!!! do you how long and hard i've saved to get 18" app i'm gonna try to use every last mm !!!!! i understand what your sayin thro chris, money(i wouldn't say is no object!) is not the issue as i'm comin into some soon

[Scooot]

Yes. Review here: http://stargazerslou...and-user-guide/

Chris my scopes f4.8 so I would think the Altair/GSO one would work very well if set up correctly. I've read your review and I'm trying to work out whether I need spacers or not with my Delos.

Is the eyepiece shoulder the point where the barrel meets the reference surface?

The 17.3mm barrel below the reference surface is 28mm, and the field stop (am I right in thinking this is the focal plane) is positioned 6 mm above this, so 34mm in total. So if the lens and corrector were 45 mm, my focal plane would be 79mm from the lens, near enough to the desired 75mm.

I'm not sure if I've understood this aspect so your help would be greatly appreciated.

[cs1cjc]

Hi,

What Tele Vue call the reference surface, see diagram at: http://www.televue.com/engine/TV3b_page.asp?id=214 is what I have called the eyepiece shoulder, the top of the barrel.

The Delos has a focal point at -0.23", about 6mm above the shoulder. The GSO coma corrector 2" eyepiece adaptor gives a spacing of 47mm (45+2), to which must be added any spacing from your 2" to 1.25" adaptor, perhaps 10mm, giving perhaps 57mm spacing in total. A Baader fine tuning of 14mm would take the spacing to 71mm which is well within tolerance for that eyepiece.

For general use you may need to compromise. Other eyepieces will be 2" so not have the spacing given by the adaptor or have their focal point nearer to the reference surface or even perhaps below it. I note that Tele Vue do not make it easy and that most of the Delos range has a spacing a focal point of +0.25", over 8mm below the shoulder and 14mm from the focus position for the 17mm! You need to pick spacers to put the spacing for all your eyepieces within +/-10mm of 75mm if practicable. It is a bit of work to get right but once done there needs to be no adjustment in the field. For me ES82, ES68 and Baader Aspheric all achieve this nicely, but focal point height has become a factor in my selection of eyepieces, which is not easy because the only company which publishes this information is Tele Vue.

[Scooot]

Hi,

What Tele Vue call the reference surface, see diagram at: http://www.televue.c...page.asp?id=214 is what I have called the eyepiece shoulder, the top of the barrel.

The Delos has a focal point at -0.23", about 6mm above the shoulder. The GSO coma corrector 2" eyepiece adaptor gives a spacing of 47mm (45+2), to which must be added any spacing from your 2" to 1.25" adaptor, perhaps 10mm, giving perhaps 57mm spacing in total. A Baader fine tuning of 14mm would take the spacing to 71mm which is well within tolerance for that eyepiece.

For general use you may need to compromise. Other eyepieces will be 2" so not have the spacing given by the adaptor or have their focal point nearer to the reference surface or even perhaps below it. I note that Tele Vue do not make it easy and that most of the Delos range has a spacing a focal point of +0.25", over 8mm below the shoulder and 14mm from the focus position for the 17mm! You need to pick spacers to put the spacing for all your eyepieces within +/-10mm of 75mm if practicable. It is a bit of work to get right but once done there needs to be no adjustment in the field. For me ES82, ES68 and Baader Aspheric all achieve this nicely, but focal point height has become a factor in my selection of eyepieces, which is not easy because the only company which publishes this information is Tele Vue.

Many thanks for that, I understand now. It's typical though, I sold a Hyperion 14mm tuning ring with an eyepiece earlier in the year.

[Scooot]

Well what a stroke of luck!

I received the 2" GSO coma corrector from Teleskop Express yesterday and I've been pondering the best way to set it up to try it out. I only bought the corrector itself for 55euro plus shipping.

I have an Astro Engineering Barlow. The bottom of it unscrews so it can be attached directly to an eyepiece to create a 1.5 Barlow. The remaining eyepiece holder is exactly 70mm.

I also have an Altair Astro 2 to 1.25 self-centering adapter with a filter thread. The Gso coma corrector screws on to the bottom of the eyepiece adapter and the top of the Barlow just fits into it without touching the lens of the corrector, giving me exactly 70mm to the shoulder of the Barlow.

As I only have 1.25" eyepieces at the moment it should work fine but I haven't tested it yet. It will be interesting to see what difference it makes, if any, to the view of my 72 deg Delos. I'll update the post once I've tested it.

When I buy a longer focal length 2" I'll try to set up in a similar way with a 2" eyepiece adapter.