View Full Version here: : Fastar imaging backfocus distance

25-04-2013, 08:33 PM
I picked up a QHY10 from a forum member here (actually more of a lurker since his only posts are in the for sale section), and one of the things I wanted to use it with is an older Fastar setup. With my C8 and Fastar I should get a nice ~400mm focal length for some wide imaging.

The only problem is I can't focus. The mirror gets to the end of it's travel while the stars are still coming into focus. My guess is the spacing between the camera and the Fastar corrector isn't big enough.

Can anyone tell me what the backfocus distance of a Fastar should be? The Hyperstar says 39mm. My QHY10 is 20mm.

Can anyone confirm ~40mm is correct for the Fastar too, if so I need to order a 20mm t-threaded spacer.

26-04-2013, 03:17 AM
The Starizona website has the back focus distance specification listed for each Hyperstar lens. Or it did the last time I looked.

26-04-2013, 07:05 AM
Yes I found that site. It says the Hyperstar back focus distance is 39.8mm for a C8.

I was wondering if there's any difference between the Hyperstar and the (as far as I can tell) far older Fastar system.

26-04-2013, 11:07 AM
The Faststar system was designed for the backfocus distance of the ST-237.

I think it was 16-17mm.

<EDIT: you might find an answer in the Yahoo! group : http://tech.groups.yahoo.com/group/fastar/ >
<EDIT2: Found a gourp specifically for the ST-237 : http://groups.yahoo.com/group/ST-237/ >

27-04-2013, 09:05 AM
Hmmmm this is starting to make less sense. I've seen pictures of the ST237 mounted directly to the front of the fastar so what you're saying must be right, but I've also seen pictures of small Canon DSLR mounted too, and those have a mount-sensor distance of over 30mm.

Somehow I'm trying something in the middle and yet I'm unable to focus my telescope...


27-04-2013, 09:52 AM

Might be worth having a chat to Marc. He has hyperstar on a C11 with a QHY8.


28-04-2013, 09:20 AM
Oops, my apologies. I need some reading comprehension refresher lessons. You wrote Fastar and I read Hyperstar. <facepalm>

Speaking of which, I actually still have a turn of the century vintage C-14 Fastar lens here as well as an ST-237 camera head. Alas the electronics box for the ST-237 got severely water damaged so I threw it out in the trash. Not sure what anyone can do with an ST-237 camera head sans electronics box.

28-04-2013, 09:34 AM
The spacing between the back of the glass and the camera sensor is approx. 59mm. If you're out but more than 10mm your scope won't focus as you've discovered. The standard spacer is 40mm. What's the distance between your CCD and the camera face on the QHY10? You need to subtract that from the 59mm to get your spacer length.

28-04-2013, 10:01 AM
On the QHY10 it's about 20mm from focal plane to front of the camera. I'm still someone perplexed as the ST-237 mounts directly to the front meaning you'll definitely be more than 10mm out as it only has a 16mm spacing between the camera thread and the focal plane as mentioned above.

What I may do it see if I can get a rough picture using a homemade spacer, or simply a steady hand and then if it works get a complete set of T-thread spacers.

And if it doesn't work, throw it in the bin and buy an ED-80 :)

28-04-2013, 04:45 PM
Well a bit of experimentation will have come at the answer. I set up my camera and pointed the telescope through an open window and at a bridge 8km away. Then I held up a white card and measured the distance from the card to the optics at min and max mirror position. I was able to achieve focus between a distance of 38mm and 65mm.

So that's it then. Going to buy a set of t-ring spacers. Now... How the hell does the ST-237 work at just 16mm? Black magic?

28-04-2013, 04:51 PM
That's odd. The 40mm standard spacer should work if your CCD distance to camera face is 20mm. It's 19mm for the QHY9. Did you take into account any nose piece or window/adapter? Any pics of your faststar and camera?

Similar field but probably 15 times more integration time if you're willing to wait. :)

29-04-2013, 06:59 AM
What's one mm between friends :lol:. Anyway I have a set of t-thread spacers on order. Should arrive soon.