Coma corrector

[clive milne]

Came across a curious thing today.

Here's a picture of the GSO coma corrector sold at Andrew's:
http://www.andrewscom.com.au/images/...acorrector.jpg

Here's the product description:

COMA CORRECTOR (CO-001) just arrived! Proven ideal for 8", 10" and 12" Newtonians. Coma corrects SCTs well and improves their field of view, too! Customers report very impressive results. Notably improves visual performance of GSO RC telescopes, too. Not for refractors.

The idea that a Newtonian field corrector will improve an otherwise aplanatic optical system such as an RC seems a bit suspect to me. It has been a while, but my experiments using ray tracing software trying to find a two mirror cassegrain (DK or RC) that would benefit from an off the shelf coma corrector left me with the conclusion that it was a complete waste of time. I'd be curious to know the basis of the claim in the product info.

Also, it appears that the GSO coma corrector on Andrew's web site is not the same as the one everyone else is selling (pictured below) you can tell by the length of the lens housing.

http://www.opticstar.com/images/astr...or-594x235.jpg

Incidentally, the corrector is made from 4 lenses in two groups. The slight magnification (1.1x) suggests to me that this could be a derivative of the paracorr which has the same lens groupings. There is certainly enough optical degrees of freedom in that configuration to make a superb corrector, with any luck GSO might have come up with a winner.

fwiw) This is not a paracorr clone, the magnification is slightly different as is the back focal distance, which is slightly longer.

[clive milne]

<edit>

Just had a look at GSO's web site... the coma corrector they show is of the same mechanical construction as the one on Andrew's web page. If I was to take a guess, I would say it is an old Ross style corrector.

One criticism I think needs to be fairly levelled at GSO is the lack of specs on this product. Things like; how much in focus does it require, how far is the focal plane shifted from the native focus? Spot diagrams or at least some indication of field correction and maybe vignetting curves wouldn't go astray either?

[Satchmo]

Clive - I think the Andrews one is the same as the Bintel- its just that the andrews picture shows the eyepiece holder screwed off to reveal the t-thread adapter. Still - a 4 element coma corrector for $149 is amazing value. I'd love to give one a 'road test' at least for visual purposes.

http://www.bintel.com.au/Astrophotog...oductview.aspx

[mswhin63 (Malcolm)]

Originally Posted by clive milne

One criticism I think needs to be fairly levelled at GSO is the lack of specs on this product. Things like; how much in focus does it require, how far is the focal plane shifted from the native focus? Spot diagrams or at least some indication of field correction and maybe vignetting curves wouldn't go astray either?

I think it is all about cost, I don't think there is ever enough effort on product information on low cost manufacturers. It is a trade-off. They most probably buy a generic unit from a manufacturer then rename it and provide basic information (They may not even know the difference). There are a lot of consumers that are not interested is detailed specs and that is where they get their money from. People like you and me try to track down relevant information and spend a lot of time to see if the price is worth it otherwise give-up and buy a more expensive unit with detailed specs.

[clive milne]

Hey Mark,

Yeah, superb value hey?

And I'm sure that the corrector on Andrew's site is not the same as Bintel's.
Compare the length of the lens housing below the safety recess on the barrel.
Bintel's is longer.

[AG Hybrid (Adrian)]

Well, Ive actually got this GSO Coma Corrector. I can report it works quite well. Gives sharp stars to the absolute edge of field in my f5 12" dob with a 30mm 82 degree ES eyepiece. Only increase magnification by 10%. Doesn't work at all for Refractors. It looks very similar to my 2 inch ED barlow. It had a 1.25" adapter.

Corrects down to F4. So if your thinking of getting a GSO or skywatcher F4 Newt for whatever reason. Just get one of these too and consider it as part of cost of the scope.

I'll post a picture of it your interested?

[troypiggo (Troy)]

I'd be interested to know more about the specs in terms of back focus distances and tolerances etc.

[troypiggo (Troy)]

Just found this where there's a diagram of spacings http://www.astromart.com/articles/ar...article_id=773

[clive milne]

Cheers guys...

Adrian, there is one piece of information you could share that would help me out... How much change is there in the position of the focuser when you insert the coma corrector.

If you want to take a photo, take one of the ES 30 at focus without the coma corrector and another one at focus with the corrector...

regards,
~clive.

[AG Hybrid (Adrian)]

Originally Posted by clive milne

Cheers guys...

Adrian, there is one piece of information you could share that would help me out... How much change is there in the position of the focuser when you insert the coma corrector.

If you want to take a photo, take one of the ES 30 at focus without the coma corrector and another one at focus with the corrector...

regards,
~clive.

Sure I'll look into it. Might not get around to setting up till the weekend though.

[stringscope (Ian)]

Ok, I bought one from Andrews Comms last week, $129. Tried it out briefly tonight compared against a TV Paracor tunable top T1. It was only a brief session using my 10"4.5 dob. Testing in the 16" will have to wait. I made sure the scope was accurately collimated using a cheshire and auto collimator, rechecked during the session.

I have purchased this one for the 10" F4.5 dob I use for public outreach nights. This scope has a dedicated eyepiece set (WO UWAN's) and I was looking for an older non tuneable top Paracor to complete the kit but I am taking a punt that the GSO one will do the job.

Initial impression was "heaps of IN focus required". I investigated this first and measured with a vernier caliper I noted the GSO coma corrector (CC) required 17mm "in focus" travel and the TV Paracor required 8mm "in focus" travel. This in focus requirement is a big issue for me using focuser units with only 30mm of total travel. Probably not such a big issue for GSO dobs due to the greater available focuser travel. I have plenty of travel within the mirror cell for this dob (by design) so not a show stopper.

I used a 26mm T5 Nagler, 17mm T4 Nagler and 16mm WO UWAN (ep with known field curvature) eyepieces. Due to limited time I didn't use any shorter length eyepieces. Initial impression with all ep's was that the GSO CC worked and cleaned up most of the visible coma. Neither CC helped the 16mm UWAN with its field curvature though .

I would need to do more testing before I could say if both CC's delivered the same performance.

In sumary

Similar weight - GSO feels slightly heavier if anything.
Both clean up coma quite nicely @ F4.5.
The GSO sits higher in the focuser - about 20mm.
The GSO required 17mm additional "in focus" on my dob.
Mine did not come with a 1.25" adapter.

Cheers,

[AG Hybrid (Adrian)]

Originally Posted by stringscope

Mine did not come with a 1.25" adapter.

yeah... I forgot to mention this. I had to buy a separate GSO T-Adapter to allow the use of 1.25" eyepieces.

In regards to the in focus required. Can't say my mass produced single speed crayford that comes with my skywatcher dob has an issue. I suppose, if you have an aftermarket (JMI, Feathertouch and Moonlite) short focuser you may have an issue.

[stringscope (Ian)]

For a variety of reasons during my recent visit to Wiruna I took my 10" F4.5 "Outreach" dob kit instead of the 16" with all the accessories. I was extremely lucky and had two clear nights. These two "all nighters" allowed me to have a good look at the GSO coma corrector. I didn't have my Paracor with me so was not able to directly compare the two units.

The scope is built around an old Meade 10" F4.5 primary that appears to have quite a good polish and figure although the coating is getting a little thin and should be redone. The secondary is a 2.6" Protostar - unheated. The finders and eyepieces are all heated. This lack of secondary heat caused me grief with a sudden ending to the first nights observing @ 0430 by suddenly dewing after a night of very heavy dew. The filter slide (also unheated) had given in to condensation at around midnight. The second night was essentially dew free .

The eyepiece set for this scope comproses a 30mm Vixen NLVW and 16mm, 7mm and 4mm UWAN eyepieces. F4.5 is a challenge for any eyepiece and the UWAN's certainly handle it better than the NLVW. Comparing the image on various types of objects with and without the GSO CC in place it was clear the GSO CC made a pretty good attempt at minimising the visible coma. I did not make any attempt to fine tune the images by moving the eyepiece with respect to the CC. Instead I just inserted the EP's to the shoulder and used them as is. This is how I set them up during outreach sessions.

Prior to leaving home I had moved the primary mirror cell 15mm closer to the secondary to allow for the GSO CC infocus requirements.

The 7mm and 4mm UWANs became essentially sharp almost to the edge with the GSO CC in place although the seeing never allowed the 4mm to show its true capability. The 16mm cleaned up nicely but the remaining field curvature meant the outer 1/4 of the field was not as sharp as it could be. OK for general use though. The NLVW was also cleaned up by the GSO CC however some residual coma and astigmatism was still visible in the outer 1/4 of the field.

Frome previous experience with this scope kit and the Paracor I would say the only difference I have noted is the 30mm NLVW shows a slighty better image with the Paracor as opposed to the GSO CC. The 3 WO eyepieces appear to perform the same with either CC.

While I didn't accurately measure it the GSO CC appeared to give an apparent increase in focal length of about 10% based on estimated FoV reduction.

Would I buy the GSO CC again? Sure would as it works just fine for me for the intended task.

Cheers,