Jump to content


Photo

EdgeHD back focus is pretty long

  • Please log in to reply
5 replies to this topic

#1 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 5898
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 06 August 2011 - 04:14 PM

I just received Celestron 8" EdgeHD OTA last night. The white paint tube looks pretty high quality. I am replacing CPC0800 with the EdgeHD and soon-to-be-delivered Astro-Physics Mach1GTO GEM in September.

Anyway, from the link:

http://tinyurl.com/EdgeHD-BackFocus

the back focus is 133mm from the rear of visual back adpater to CCD sensor.

I assembled the spacers, OAG and camera and the best I can do is about 130mm. Which is better: a little too short or long back focus? Is 130mm good enough? I setup so that the camera won't be too far back behind OAG and the guide camera to be close to OAG pickup prism. This was done by inserting spacers between the scope and front of OAG. The closer the guide camera to pickup prism the brighter and rounder the guide star.

Thanks,
Peter

Attached Files



#2 Mike Wiles

Mike Wiles

    Viking 1

  • -----
  • Posts: 950
  • Joined: 04 Feb 2009
  • Loc: Goodyear, AZ

Posted 06 August 2011 - 06:54 PM

http://www.celestron...leid=2473&nav=0,247

Buried deep in their knowledge base, Celestron has a pretty good .pdf file that answers this. I don't think the 3 mm is going to be an issue for you though. The document I've listed addresses the C11, but I suspect that the same story holds true for your 8", just a slightly different optimum distance. Looking forward to seeing what you can do with that scope.

Mike

#3 Nocturnal

Nocturnal

    Surveyor 1

  • *****
  • Posts: 1538
  • Joined: 14 Sep 2005
  • Loc: CT, USA

Posted 07 August 2011 - 11:46 AM

Thanks for posting that link Mike. I've found with my HD 11 that back focus isn't terribly critical with my QHY8 with APS-C sized sensor. Due to the weather my number of exposures has been limited though.

#4 Dr Benway

Dr Benway

    Viking 1

  • *****
  • Posts: 656
  • Joined: 27 Mar 2008
  • Loc: Abilene, TX

Posted 07 August 2011 - 11:58 AM

I just checked the back focus on my Edge HD 800 with the Celestron #93644 T-Adapter. Total was 135mm to the CCD plane with my QHY9M and filterwheel. The adapter is two-piece. The second piece is 29mm. This might allow you to use your AO unit and stay within the parameters.

John

Attached Files



#5 Nocturnal

Nocturnal

    Surveyor 1

  • *****
  • Posts: 1538
  • Joined: 14 Sep 2005
  • Loc: CT, USA

Posted 07 August 2011 - 12:22 PM

That Celestron T adapter is made specifically to put DSLRs in the focal plane. Depending on your needs you're probably better off with the AP visual back that gives you an excellent 2" compression fitting or something like this SCT to T2 adapter http://agenaastro.co...table-t-02.html and the appropriate T2 extensions. An OAG needs a lot of room, even if it's thin. Depending on how close you can get the guide cam to the prism and how big it is with regards to clearing the focus and mirror lock knobs on the back of the SCT.

#6 ADBjester

ADBjester

    Viking 1

  • -----
  • Posts: 611
  • Joined: 26 Sep 2010
  • Loc: Reading, PA

Posted 07 August 2011 - 11:53 PM

I assembled the spacers, OAG and camera and the best I can do is about 130mm. Which is better: a little too short or long back focus? Is 130mm good enough?


I asked such questions when contemplating my Edge 9.25 purchase, and was referred to this link.

In a nutshell, unless you're using a ridiculously large sensor chip like a U16M (4096 x 4096 at 9 microns, or 52mm diameter), you don't need to worry about a few millimeters.

Your SXVR-25 has a chip that's identical to my QHY8L -- 26.x mm diagonal. That means that the furthest point from the center of the chip is 13.5 mm distant, thus the 13.5 mm line on the above chart is what matters most to you. As you can see, even within 5 mm there's almost no appreciable difference.

It's only when you vary by a huge margin with a moderate sensor size like ours, or when you vary by a smaller margin with a huge sensor size like 52 mm (where the 26 mm line in the chart applies, showing rapid curvature as you drift from the sweet spot of back focus).

That chart is for the EdgeHD 11", but the same principle applies to the other apertures. For our sensor size, don't sweat it. Get within 5 mm, and call it good. You did, so you're set.

Jester






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics