Jump to content


Photo

M33 - The Triangulum Galaxy

  • Please log in to reply
29 replies to this topic

#26 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 5854
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 08 January 2013 - 10:54 PM

Thanks for your time writing this up. So far I've not notice field rotation in my images using OAG. My setup is always portable. I have a A-P Mach1 and it has a cool and quick way to do polar alignment and I can get it done within 10 to 15 minutes and pretty accurately.

I am just a little concerned for difficulty of finding guide stars for images I have not yet image. I live in fairly high light pollution area and was worried the next time I image I won't be able to find a guide star. I cannot easily rotate my OAG because it will collide with JMI MotoFocus. One big plus of ONAG is no rotation is required. Also, ONAG could give me a guarantee that it will easily find a suitable guide star due to huge available FOV in guide port.

Thanks,
Peter

#27 srosenfraz

srosenfraz

    Soyuz

  • *****
  • Posts: 3999
  • Joined: 06 Mar 2011
  • Loc: United States

Posted 08 January 2013 - 11:49 PM

Thanks for your time writing this up. So far I've not notice field rotation in my images using OAG. My setup is always portable. I have a A-P Mach1 and it has a cool and quick way to do polar alignment and I can get it done within 10 to 15 minutes and pretty accurately.

I am just a little concerned for difficulty of finding guide stars for images I have not yet image. I live in fairly high light pollution area and was worried the next time I image I won't be able to find a guide star. I cannot easily rotate my OAG because it will collide with JMI MotoFocus. One big plus of ONAG is no rotation is required. Also, ONAG could give me a guarantee that it will easily find a suitable guide star due to huge available FOV in guide port.

Thanks,
Peter


Well, I can say the ONAG is tops when it comes to ease of finding guide stars. Also, you're correct about not having to rotate it. Sometimes you'll want to rotate the camera for framing purposes (it rotates about the camera port), but the ONAG's orientation stays fixed on the scope. Also, I have a Robofocus, and it doesn't interfere with the ONAG at all (and the Robofocus is pretty big).

#28 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 5854
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 10 January 2013 - 11:01 AM

Hi Scott,

Celestron just announced a new focal reducer for C-8 EdgeHD. The new FR requires a much shorter back focus of 105mm as compared to 133mm at F/10. I was expecting same back focus with or without FR at 133mm. It's going to be impossible to achieve 105mm BF with ONAG. My camera's BF is 14mm and filter wheel BF is 29mm plus add 6mm for ONAG SCT adapter and that gives a total BF of about 115mm (ONAG BF is 66mm). In reality it will be a little more and more like 120mm. I am not happy about Celestron's decision of reducing BF.

I know that you use FR for your non-EdgeHD and requires an optimal BF of 105mm but I believe the BF requirement is less critical than with EdgeHD C-8. Your DSLR has about the same BF as my camera/FW so what's your total BF in your ONAG setup?

Thanks,
Peter

#29 srosenfraz

srosenfraz

    Soyuz

  • *****
  • Posts: 3999
  • Joined: 06 Mar 2011
  • Loc: United States

Posted 10 January 2013 - 11:41 PM

My backfocus is right around 120mm. The ONAG is 66mm, Camera is 44, T-ring is about 10mm (+/-). So, that all adds up to 120mm. For the standard (non-HD) focal reducer, the nominal (f/6.3) distance is 105mm. The net result for me is that it ends up giving me f/5.8 instead.

Before I purchased my ONAG, I was concerned about the effect of having the greater backfocus. I bought a set of T-adapter extension tubes so that I could simulate the longer backfocus. I tested with a 20mm extension tube (giving me 125mm BF), and decided that the FR seemed to work fine like that.

If you're considering the ONAG, you may want to spend about $40 for a set of these spacers:

http://tinyurl.com/bf4xfhl

With the 10mm and 5mm spacer, you can emulate the backfocus you'd have with the ONAG and decide if it works with the Celestron FR.

The other thing you may want to consider is the Astro-Physics CCDT67 .67x FR. Its much less expensive than the Celestron FR, and I have heard that it works well with the HD scopes (obviously, I have no direct experience). I'm not sure what the BF requirements are nor how it responds to a longer than nominal BF. From the description, it sounds like the design works with higher compression ratios (longer BF).

If you try any of these experiments/options, I'd be most interested to hear your results. I have hopes at some point of replacing my C-8 with an HD Edge, so I'm (hopefully) going to confront these very issues at some point in time.

#30 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 5854
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 11 January 2013 - 01:24 AM

Good idea about testing with spacers. I already have lots of spacers. The new FR won't be ready till probably in the Spring so I will have to wait.

Thanks,
Peter






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics