GSO RCA-200 f/8 Astrograph, opinions?

[peeb61 (Paul)]

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 (Dave)]

Paul, have a look at Peter Ward's image in this thread:

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 (David)]

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]

Originally Posted by bojan

(and, final question: Bintel or Andrews?)

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 (Bratislav)]

Originally Posted by 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.

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...7440#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.