GSO RCA-200 f/8 Astrograph, opinions?

[bojan]

Hi folks,
I am considering the GSO RCA-200 f/8 Astrograph soon..
Are there any hands-on experiences with this OTA? (and, final question: Bintel or Andrews?)

[TrevorW]

At f/8 would it really be an astrograph ??

[bojan]

That's how it's been advertised ;)

[TrevorW]

The f8 Astrograph is the RC design at $1300 sorry I misunderstood as they also do an f4 and f5 Newt, a number of guys have the RC's inclduing myself and they produce good results for astrophotgraphy

[jjjnettie]

Yup, my RC is cooling down outside as i write.
Not done much imaging through it yet, but from what i've achieved so far, i'm happy.

[DavidU]

Paul has done some very nice images on his 8" RC
http://paulhaese.net/IC2944BokGlobules.html

[Moon]

Bojan,
I've had mine for a while now and I give it the thumbs up. From memory you have an EQ6? I think this is about the limit in terms of weight and focal length for that mount.
My attempts at images: LINK (http://deepspaceplace.com/images.php?sort=Date&filter=GSO8RC)
Some thoughts : LINK (http://deepspaceplace.com/gso8rc.php)

Other random thoughts:
- At f/8 the focus is surprisingly forgiving and therefore a robo focus is not mandatory. The focuser doesn't 'sag' but it's also not 100% repeatable with a robo focus on it either. Having said that, it does keep focus and collimation from night to night.
- The 10 inch verison has a better primary support design, but it's too heavy for an EQ6
- You will spend time collimating on those rare clear nights. If you don't have a permanent setup, I seriously recommend considering a refractor instead.
- IMHO CCD Inspector is just about mandatory.
- I can't comment about the quality of the mirrors - except that the central obstruction size and seeing are probably going to be a bigger issues that any mirror defects you might have.
- There is supposed to be a 12" and 16" version in the pipe. You might pick up a cheap second hand 8" version if/when these are released.

James

[Astrobserver99]

While I haven't actually used the 8" GSO RC, I did consider purchasing one when I was changing scopes. I went for the LX200 ACF SCT for the following reasons:

1. Closed OTA Design, Requires minimum maintenance and has a long life
2. Keeps collimation
3. Lots of accessories available
4. Excellent support
5. Similar design to the RC - Coma Free

The only advantage I can see for the GSO RC is that it is lower in cost. But you probably still need to upgrade the focuser for imaging...

[Alchemy]

I have not got one, my impression is it's good for bright objects, giving good value for money, I can't say I am keen on the camera distance extensions, yet to see really faint stuff done well ( yet),
Having used a newt for a while, I'd rather have closed cell optics, the dust is just such a pest. Happy with the refractor for now.

[Moon]

Of course your mileage may vary...this is from my own personal experience having owned 8 inch SCT (f/10) , 8 inch Newt (f/4) and 8 inch RC (f/8). I think f/8 is about the sweet spot for a managble 8 inch scope for imaging.

Benefits of RC when compared to SCT:
1. Faster cool down with open tube
2. No dew problems on the corrector plate
3. Fixed primary
- can be collimated
- holds collimation better
- better pointing accuracy
- way less differential flexure
4. Faster / shorted f/l - easier to guide.

Disadvantages of RC when compared to SCT:
1. Harder to collimate

Advantages of SCT and RC when compared to fast Newt:
1. Focuser and camera are in a more stable position (i.e. not attached to thin tube wall)
2. No offset secondary - defocused stars are always symmetric.
3. Backfocus distance and vignetting are easier to manage.

Some people will look the spot diagrams and make the decision based on that. Fair enough, but spot sizes don't count for much if you can't get the focus, guiding or collimation right in the first place.

James

[bojan]

Thank you all for you inputs :-)

From your answers, my only conclusion is: it's not such a bad idea to get one :-)

And yes, my EQ6 can still manage it.
At first, I was thinking of mounting my 10" Newtonian on it but 13 kg+ does not seem to be easy to carry (my biggest concern was and still is that dovetail flange.. maybe it is OK for such a weight, but I was always afraid to try it, despite some people saying it should be right..

So, GSO 8" RC was just decided :eyepop:

[Paul Haese]

Just caught up with this thread. Nice scope, I am happy with it. It has its own little issues but overall the optics are very nice. Not seen many images that are not good. Usually operator error in those cases.

[bojan]

Hi Paul,
Seeing images you did with it was a major factor in my decision :-)

BTW, what is it's weight?

[Paul Haese]

To be honest I don't know 100% what it weighs. However, I think it is around 5 kgs. It is very light and you are going to marvel how light it is when you pick it up.

[AlexN]

Just weighed mine, its 7.5kgs with my moonlite focuser on it.

Just to add to everyone elses comments, I've now owned two of them, I sold the first one to try my hand at wide field nonsense, but have since found my way back to an 8" RC... There are advantages and disadvantages to the RC just like any other telescope, but overall I find the GSO RC to be a very capable little scope. The focuser that comes with it really is laughable however... I strongly recommend a moonlite or feather touch..

[bojan]

Thanks again for information and suggestions :-)
Re focuser, most likely I will make my own.. I already designed helical focuser for my 10" (two threaded tubes, (and additional threaded ring to tighten the whole lot), but it will go to RC instead.. or it will be used on both.

[peeb61]

Really daft question,
I have just found this thread as I was thinking about the 200mm version. Do you need to use a Coma Corrector when imaging? I have the QHY8L.

Thanks in advance
Paul

[Peter.M]

The RC design is coma free, it does suffer from field curvature however so you need a field flattener on the qhy8

[peeb61]

Thanks Peter for your reply.
I figured, I am trying to shy away from the use of Coma Correctors/flatteners...like to have a 'no other glass' kind of scope.

Paul

[naskies]

Given the 1625 mm focal length, the field curvature's not as intense as with say a short focal length refractor. I have a KAF-8300 chip CCD too - the curvature is noticeable in the corners if you look for it, but it's mild enough that you could probably get away no field flattener.

[peeb61]

Hi David,
Many thanks for your input, it is indeed got me thinking. If it is hardly noticeable then the image could be cropped? Could you point me in the direction of some examples?

Thanks again.
Paul

[naskies]

Paul, have a look at Peter Ward's image in this thread (http://www.iceinspace.com.au/forum/showthread.php?t=48028):

http://www.atscope.com.au/BRO/images/GSO/gsom8.html

An 8300 chip would only cover the central portion of that image... you can see that the field is reasonably flat there.

[et]

G'day,
There are 2 staff that have there own observatory's,and the other 2 may as well have there own observatory's.
Bintel will make sure it will be collimated before it leaves the shop and they give good service.
Andrews,I've brought an EQ6 of them years ago , pretty cheap.
It's a no brainer for me.
best Dave

[clive milne]

I would recommend that you give both distributors a call and ask them their policy with respect to defects. Specifically, as an experiment tell them that you intend to send the OTA to a credible third party for an optical test report and if the optics are less than 1/4 wave would they exchange it?
You probably don't actually have to go through with the testing (because I think the optics will be fine) but the exercise will reveal to you which distributor will stand by you in the event of receiving a problem child in the post and which one will leave you crying in to your beer. The last time I did this the responses of the two retailers you mentioned were polar opposites in their attitude. One of them is a factory backed dealer with credible, experienced and helpful staff that knows how to service the equipment they sell. The other might possibly be described as a retailer that sells grey import astronomy equipment (amongst other things).

caveat emptor.

[bratislav]

Actually, it is as "intense" as in a short FL refractor. That is why many people find that refractor field flatteners work well with their RCs.
<in a Cassegrain radius of field curvature is proportional to difference in radii of primary and secondary mirrors. Flat field systems have R of primary similar to R of secondary; but that necessitates huge obstructions so it is not done often; and definitely not in GSO RC case.>

Using the data measured by Wolfgang Rohr

http://www.astro-foren.de/showthread.php?11753-Neues-GSO-8-quot-f-8-RC-Ritchey-Cr%E9tien-203-1624&p=47440#post47440

I find radius of curvature of the field to be around -320mm, which is pretty much equivalent to a refractor of approx 800 to 900mm focal length.

Main problem BTW with those RCs is vignetting. You have about 5-10mm (at best) of unvignetted and then it drops of fairly rapidly as cone gets cut off by baffles. So for most of your frame you are operating more at f/10+ after you factor in the obstruction. Large chips will be even worse.