#1  
Old 25-04-2013, 07:33 PM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
Fastar imaging backfocus distance

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.
Reply With Quote
  #2  
Old 26-04-2013, 02:17 AM
frolinmod's Avatar
frolinmod
Registered User

frolinmod is offline
 
Join Date: Dec 2010
Posts: 573
The Starizona website has the back focus distance specification listed for each Hyperstar lens. Or it did the last time I looked.
Reply With Quote
  #3  
Old 26-04-2013, 06:05 AM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
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.
Reply With Quote
  #4  
Old 26-04-2013, 10:07 AM
OICURMT's Avatar
OICURMT
Oh, I See You Are Empty!

OICURMT is offline
 
Join Date: Apr 2010
Location: Laramie, WY - United States of America
Posts: 1,438
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/ >
Reply With Quote
  #5  
Old 27-04-2013, 08:05 AM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
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...

Reply With Quote
  #6  
Old 27-04-2013, 08:52 AM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 4,021
Chris,

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

http://www.iceinspace.com.au/forum/s...ghlight=fastar
Reply With Quote
  #7  
Old 28-04-2013, 08:20 AM
frolinmod's Avatar
frolinmod
Registered User

frolinmod is offline
 
Join Date: Dec 2010
Posts: 573
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.
Reply With Quote
  #8  
Old 28-04-2013, 08:34 AM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,138
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.
Reply With Quote
  #9  
Old 28-04-2013, 09:01 AM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
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
Reply With Quote
  #10  
Old 28-04-2013, 03:45 PM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
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?
Reply With Quote
  #11  
Old 28-04-2013, 03:51 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 18,138
Quote:
Originally Posted by Garbz View Post
On the QHY10 it's about 20mm from focal plane to front of the camera.
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?

Quote:
Originally Posted by Garbz View Post
And if it doesn't work, throw it in the bin and buy an ED-80
Similar field but probably 15 times more integration time if you're willing to wait.
Reply With Quote
  #12  
Old 29-04-2013, 05:59 AM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 639
Quote:
Originally Posted by multiweb View Post
That's odd. The 40mm standard spacer should work if your CCD distance to camera face is 20mm. It's 19mm for the QHY9.
What's one mm between friends . Anyway I have a set of t-thread spacers on order. Should arrive soon.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 02:28 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
OzScopes Authorised Dealer
Advertisement
Celestron Australia
Advertisement
Bintel
Advertisement
Meade Australia
Advertisement
SkyWatcher Australia
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
Astronomy and Electronics Centre
Advertisement