GSO 8" RC + DSLR need flatteners?

troypiggo (Troy) · #1 30-08-2009, 03:40 PM

Let's say I know this guy who is pretty new to all this, and has already spent quite a lot of dough collecting many, too many, bits and pieces. But this guy can't help himself, and is thinking of getting one of the GSO RC scopes. And he has a 40D that he'd like to put on it. He understands that the RC scopes are supposed to be pretty flat field, but is wondering if that applies to all cameras attached to it or just the smaller chip cameras.

And focus is achieved with a normal T adapter on the focuser? No modding or other attachments needed?

rat156 · #2 30-08-2009, 03:52 PM

I'll let you know in a few days.

This guy I know well has just ordered one to go with a G11 mount that he ordered the other day. He has a D300 Nikon which he may be able to put in the back of the GSO...

Cheers
Stuart

troypiggo (Troy) · #3 30-08-2009, 03:54 PM

Your mate and my mate should talk more in a few days...

Moon (James) · #4 30-08-2009, 08:47 PM

Tell your mate to get one of these and report back how it goes...

You can also tell your mate to ignore the stories about the diameter of the flattener.

troypiggo (Troy) · #5 30-08-2009, 08:54 PM

I'll pass it on

Hmm, wonder how that flattener goes on an ED80? ...

<clickety click />

marki · #6 30-08-2009, 10:47 PM

RC's do not have a flat field it is curved. They have no coma and produce tight stars.

Mark

citivolus (Ric) · #7 31-08-2009, 05:29 AM

Quote:

Originally Posted by troypiggo

I'll pass it on

Hmm, wonder how that flattener goes on an ED80? ...

<clickety click />

As mentioned in another thread, a couple people over on Cloudy Nights seem happy with it. It doesn't seem too bad, especially considering the price. I'd be curious to see someone run it through CCD Inspector, as well as see some flats taken with/without it.

I'm inquiring with Astronomics now about shipping to Australia.

troypiggo (Troy) · #8 31-08-2009, 06:25 AM

Thanks Mark and Ric. Also came across a Stellarvue flattener with good words spoken about it: http://www.skiesunlimited.net/index.php?ProductID=5336

rat156 · #9 31-08-2009, 07:50 AM

Looking at the pictures produced by others, the field looks pretty flat for a DSLR at prime focus. Haven't seen too many pics with focal reducers in though.

Cheers
Stuart

Satchmo · #10 31-08-2009, 11:17 AM

Quote:

Originally Posted by marki

RC's do not have a flat field it is curved. They have no coma and produce tight stars.

Mark

Without a field flattener RC's produce astigmatism off -axis , which given that star images ,as you go off axis, are on one side of focus this means increasingly elliptical star images as you go to the corner of the frame. The 8" GSO's seem ok for an APS size chip but clearly need a field flattener for larger chips. See Peter Wards full ( 35mm ) frame shot of Omega Centauri and APS size crop and that pretty much explains it all.

I would imagine before too long there will be a number of after market flatteners available for them.

pmrid (Peter) · #11 31-08-2009, 05:04 PM

Quote:

Originally Posted by Satchmo

Without a field flattener RC's produce astigmatism off -axis , which given that star images ,as you go off axis, are on one side of focus this means increasingly elliptical star images as you go to the corner of the frame. The 8" GSO's seem ok for an APS size chip but clearly need a field flattener for larger chips. See Peter Wards full ( 35mm ) frame shot of Omega Centauri and APS size crop and that pretty much explains it all.

I would imagine before too long there will be a number of after market flatteners available for them.

MArk, am I right in thinking that this would make an RC an unsuitable candidate for use with an off-axis guider since by their very nature, they're picking stars from the corners of the image.
Peter

Satchmo · #12 31-08-2009, 09:25 PM

Im not sure of the capabilities of modern guiding software. I would think that if you have a sophisticated set up with off axis autoguiding that a field flattener won't be too much trouble. I know that for astrometry purposes the eliptical off axis images aren't a problem as its is easy for astrometry software to find the centroid of an ellipse.

Satchmo · #13 31-08-2009, 09:28 PM

Im not sure of the capabilities of modern guiding software. Bear in mind that you will be able to independently focus a guiding camera so best focus will be more like a cross and seeing will blur that more into a fuzzy blob. Even a cross pattern at best focus should be no trouble for guiding software .

I know that for astrometry purposes the eliptical off axis images aren't a problem as its is easy for astrometry software to find the centroid of an ellipse.

Peter Ward · #14 31-08-2009, 09:55 PM

Dealer hat off for a bit...

I know from experience that SBIG guiders and cameras are not phased at all by non-bell shaped stellar profiles. They guide brilliantly with oblong, liner and cross shaped stars (I've seen them all and then some)

As an after-market flatener, I'd probably look at the latest William Optics unit which would allow easy (ie don't need a machinist) fine tuning of optimal spacing...I am not aware of any other manufacturer who makes a similar product, but would be interested in hearing of better alternatives.

My overall impresion of the GSO RC's is very good. They vary a tad in spherical error correction, but nothing gross to be sure.

The RC design might have astigmatism, but they don't have coma, so within modest limits stars look round (albeit out of focus at the edges due the curved field) which I find much more pleasing than the "seagulls" I have seen in some Newts and SCT's.

...nuff said...

troypiggo (Troy) · #15 31-08-2009, 10:20 PM

Thanks for your input all.

Peter - that latest WO flattener you're referring to, you mean the flattener IV one? Isn't that for refractors? You reckon it'd work?

citivolus (Ric) · #16 01-09-2009, 03:16 AM

Troy, I have the Flattener 4 if you want to swing by and give it a try. I also asked Astronomics about their flattener, and they said that they would ship to Australia, with an estimate of about $30USD for shipping.

One of the flaws with the Flattener 4 is that they didn't include a set screw to lock it, so if you bump the rotating barrel on it while imaging the focal length can shift. This could be easily fixed with some adhesive backed velcro, which would still allow you to adjust the positioning if needed.

troypiggo (Troy) · #17 01-09-2009, 06:31 AM

Thanks for the offer, Ric! Might have to take you up on that. I'm off to NZ this Friday for a couple of weeks, so when I get back I'll let you know.

Still wondering about Peter's suggestion of using it on the RC.

AlexN · #18 01-09-2009, 09:43 PM

Troy - It should work just fine... the principals of flattening a curved field are much the same whether its a refractor or a reflector.. I've successfully used a Baader MPCC to flatten the field of my refractor, despite the fact that its designed to correct coma in a reflector, it seems to correct curvature in a refractor also.. chip to sensor distance is somewhat different though. However. the WO flattener 4 is adjustable in that respect.. You should have no worries flattening the field with the WO flattener 4..

Improvisation often leads to interesting discoveries... If Ric is going to give you the opportunity to give it a fly, I say take it... You may be plesently surprised..