Jump to content


Photo

coma corrector: Baader vs GSO

  • Please log in to reply
16 replies to this topic

#1 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5481
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 29 April 2013 - 12:59 AM

the new Baader MPCC is now rated down to f3.5

but GSO claims that their version will work down to f3!

anybody used the GSO with a fast scope and can post some star images?

#2 dtripz

dtripz

    Explorer 1

  • *****
  • Posts: 90
  • Joined: 01 Apr 2008
  • Loc: Brooklyn NY

Posted 29 April 2013 - 10:14 AM

To add to the posters question, can these be used for visual, the paracor 2 seems quite expensive.

#3 MikeBOKC

MikeBOKC

    Aurora

  • *****
  • Posts: 4512
  • Joined: 10 May 2010
  • Loc: Oklahoma City, OK

Posted 29 April 2013 - 12:36 PM

I have the Astro Tech coma corrector which I suspect is pretty much the same as the ones mentioned above and it is fine for visual in my 4.6 Dob.

#4 cjc

cjc

    Ranger 4

  • -----
  • Posts: 354
  • Joined: 15 Oct 2010
  • Loc: Derbyshire, England

Posted 29 April 2013 - 05:26 PM

The GSO was optimised, I understand for F/4.5, so F/3.0 would surely be pushing it, but I know of no published spot information. Roger Ceragioli did the optical design for the Astro-Tech/GSO corrector and is also one of the authors of Telescopes, Eyepieces and Astrographs: Design, Analysis and Performance of Modern Astronomical Optics which contains a possibly related corrector design also optimised for F/4.5 but said to be suitable for mirrors slower than F/3.5.

The issue with visual use is setting up the correct spacing to the focal plane with eyepieces that are not parfocal. The Paracorr includes a turntable to set the spacing. The GSO, with its back focus of 75mm will be more forgiving than the Baader at 55mm.

#5 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22875
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 29 April 2013 - 05:41 PM

I can't speak for the GSO corrector, but the Baader, at least, is more of a photographic coma corrector than a visual one. It adds appreciable spherical aberration, if you read all the myriad of CN posts and threads over the last decade (likely to be a significant problem for resolution at higher powers, but not an issue in imaging), and a fair number of spacers of differing lengths will be necessary to place the lens the correct distance from the eyepiece (since it is threaded onto the bottom of the eyepiece or extension tube).

The GSO corrector and TeleVue Paracorr function like barlows, in that the eyepiece is inserted in a tube, so spacing the eyepiece can be more easily done with parfocalizing rings (on the GSO) or by merely sliding the Tunable top (on the Paracorr).

The current Paracorr keeps the comatic star image smaller than the Airy disc down to f/3.5 and does a credible job below that. I don't believe any other corrector currently corrects as completely to such a low f/ratio. The Paracorr also flattens the field slightly, improving star images for eyepieces that have less-than-flat focal planes. It's expensive, but easier to use and to produce better results with. I can see the owner of a less-expensive scope might look for a lower-priced alternative. You should also look at the pile of threads on the GSO/Astrotech coma corrector here on CN, too, as it might point out what you're getting yourself into, where adjusting the corrector for different eyepieces is concerned. Do note that you only have to do this once, though.

#6 cjc

cjc

    Ranger 4

  • -----
  • Posts: 354
  • Joined: 15 Oct 2010
  • Loc: Derbyshire, England

Posted 30 April 2013 - 02:10 AM

I can't speak for the GSO corrector, but the Baader, at least, is more of a photographic coma corrector than a visual one. It adds appreciable spherical aberration...


This applies to the previous version. The new Baader Multi Purpose Coma Corrector (MPCC) Mark III coma corrector is described here.

#7 beatlejuice

beatlejuice

    Surveyor 1

  • *****
  • Posts: 1590
  • Joined: 05 Apr 2011
  • Loc: Hamilton, ON,Canada

Posted 30 April 2013 - 02:21 AM

This applies to the previous version. The new Baader Multi Purpose Coma Corrector (MPCC) Mark III coma corrector is described here.



My Bitdefender just blocked me from going to that site because of malware!

Eric

#8 cjc

cjc

    Ranger 4

  • -----
  • Posts: 354
  • Joined: 15 Oct 2010
  • Loc: Derbyshire, England

Posted 30 April 2013 - 02:23 AM

Don is of the view that eyepieces should be parfocalised so the corrector gives the best possible correction for all.

With the GSO, I use eyepieces that are close to parfocal with a compromise setting, described here.

#9 cjc

cjc

    Ranger 4

  • -----
  • Posts: 354
  • Joined: 15 Oct 2010
  • Loc: Derbyshire, England

Posted 30 April 2013 - 02:29 AM

My Bitdefender just blocked me from going to that site because of malware!

Eric


Apologies! I hope this link is better.

#10 beatlejuice

beatlejuice

    Surveyor 1

  • *****
  • Posts: 1590
  • Joined: 05 Apr 2011
  • Loc: Hamilton, ON,Canada

Posted 30 April 2013 - 08:49 AM

Apologies! I hope this link is better.



Thanks Chris, it is.

Eric

#11 precaud

precaud

    Apollo

  • *****
  • Posts: 1493
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 30 April 2013 - 08:51 AM

Don is of the view that eyepieces should be parfocalised so the corrector gives the best possible correction for all.


I think Don's approach is the most precise one and I will end up doing that, once I get/make enough parfocal rings to do all of my EP's.

Until then, the "compromised" approach is working really well. All of my EP's except two are within 2mm of the target 75mm for my High Point/GSO CC and the results are excellent. Having said that, I did a brief experiment last night to if I could see the difference between 73mm and 75mm on a 25mm EP, and the answer is a definite yes, correction was better edge-to-edge with the proper distance. So I will definitely be making the proper spacers.

#12 Metalmanstan

Metalmanstan

    Vostok 1

  • -----
  • Posts: 120
  • Joined: 02 Jul 2012
  • Loc: Oneonta, NY

Posted 30 April 2013 - 10:32 AM

I have the astrotech coma corrector and it definitely works fine in my 12" f4.9 scope. It's a lot cheaper then the other brands and it does the job just fine. If you don't want to spend that much money, I would definitely go with the astrotech, it will save you money!!

Stash

#13 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22875
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 30 April 2013 - 12:11 PM

Well, regardless of spacers or parfocalizing rings, you only do it once, and then you get the benefits of coma correction thereafter. It's sort of like the pain of installing a new piece of audio visual equipment. After the initial "installation", everything is easy.

And the really EZ thing is you only need to have one eyepiece perfectly positioned and all your other eyepieces follow from that.

Using a Paracorr with its tunable top doesn't parfocalize your eyepieces (though the tunable top does so in action), but the other coma correctors sort of force you to do that to dial in each eyepiece to the corrector.

As for adapters and rings to do so, many sellers have these things (e.g. Scopestuff, telescopeadapters.com). Even blank filter casings come in handy when you only need to change the insertion of an eyepiece a few millimeters.

#14 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5481
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 30 April 2013 - 10:48 PM

Actually I was thinking of AP use. Here's what I thought:

1) you can get an f/4 newtonian and buy the very expensive Keller reducer to get it down to f/3

2) you can pay (a lot of cash) for an f/3 newtonian and then use one of these coma correctors.....?

At the end of the day, I don't know what is more expensive, a garden-variety 8" f/4 or 10" f/4 with the Keller reducer, or pay someone like Royce (presumably a steep sum) for an 8" f/3 or 10" f/3 mirror.

#15 bratislav

bratislav

    Viking 1

  • *****
  • Posts: 518
  • Joined: 07 Sep 2006

Posted 01 May 2013 - 12:10 AM

How many fast Newtonians have you critically collimated (I mean collimated well enough so it does not show focal plane tilt and decenter on a decent size CCD chip) ? If answer is "not many" (as I expect), my suggestion is to stay with f/5 Newts until you learn the ropes first. 6" f/5 is cheap to play with (and it WILL produce great images to boot!) and you can keep your corrector for later when you get your 8" or 10" f/fast. In fact if you buy the 6" second hand you may not even lose any money in the process.
Believe me it gets very, very far from trivial once you get below f/4.

#16 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5481
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 02 May 2013 - 05:13 AM

trouble is f/5 is no better than my refractor.

was looking for more speed.

#17 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6386
  • Joined: 28 Apr 2003
  • Loc: Atlanta, GA

Posted 02 May 2013 - 06:35 AM

How about one of these:

Fastar F2 imaging

Jarad






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics