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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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Darren1968
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Re: M33 - The Triangulum Galaxy new [Re: fco_star]
      #5608740 - 01/06/13 04:16 PM

Gotta be one of the best DSLR M33's that I've seen.

Very nice!

Darren


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mpgxsvcd
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Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: M33 - The Triangulum Galaxy new [Re: fco_star]
      #5610269 - 01/07/13 01:47 PM

Great image. Thanks for sharing.

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Erk1024
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Re: M33 - The Triangulum Galaxy new [Re: mpgxsvcd]
      #5610651 - 01/07/13 06:15 PM

This is a lot of imaging time, processing work, and a great result!

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Peter in Reno
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Re: M33 - The Triangulum Galaxy new [Re: fco_star]
      #5612462 - 01/08/13 06:27 PM

Hi Scott,

You are one of the very few I know of that uses ONAG. I checked all of your images using ONAG and they are all excellent.

December 2012 issue of Sky & Telescope has a very positive review about ONAG. The author mentioned that he saw some kind of ghost reflection when a bright star was at or near center of the image but the reflection disappeared after he moved the bright star a little bit off center. He could not prove the reflection was caused by the cold mirror of ONAG. Have you ever notice any kind of refelctions like the author described?

I have been eyeing on ONAG. I like the idea of guiding on a star near the center of the DSO. It makes me wonder that there would be less noticable star trails around the center due to field rotation if the guide star was at the center of DSO in case the polar alignment was less than ideal.

I have been using Hutech OAG/Lodestar with C-8 EdgeHD at 2000mm focal length with great success. I give Lodestar more credit because it's so sensitive that it finds at least one suitable guide star in OAG's guide port FOV without ever hunting for one. I am wondering if I would get better guiding results with ONAG than OAG since guide star can be at the center of DSO instead of off-axis.

Thanks,
Peter


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srosenfraz
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Re: M33 - The Triangulum Galaxy new [Re: Peter in Reno]
      #5612724 - 01/08/13 09:10 PM

Quote:

Hi Scott,

You are one of the very few I know of that uses ONAG. I checked all of your images using ONAG and they are all excellent.





Thank you very much, Peter.


Quote:


December 2012 issue of Sky & Telescope has a very positive review about ONAG. The author mentioned that he saw some kind of ghost reflection when a bright star was at or near center of the image but the reflection disappeared after he moved the bright star a little bit off center. He could not prove the reflection was caused by the cold mirror of ONAG. Have you ever notice any kind of refelctions like the author described?






I haven't noticed anything like that, but I can't recall having tried to image anything with an unusually bright star near the center of the FOV. So, I don't think I could say it wouldn't happen. I've imaged a lot of objects with it, so I can say that whatever internal reflection problems it may potentially have don't affect the vast majority of targets.


Quote:


I have been eyeing on ONAG. I like the idea of guiding on a star near the center of the DSO. It makes me wonder that there would be less noticable star trails around the center due to field rotation if the guide star was at the center of DSO in case the polar alignment was less than ideal.

I have been using Hutech OAG/Lodestar with C-8 EdgeHD at 2000mm focal length with great success. I give Lodestar more credit because it's so sensitive that it finds at least one suitable guide star in OAG's guide port FOV without ever hunting for one. I am wondering if I would get better guiding results with ONAG than OAG since guide star can be at the center of DSO instead of off-axis.







To the extent that you have some misalignment causing field rotation, having a guidestar at the center would generally be preferable for preserving more of the FOV. A significant percentage of the objects I usually image don't fill the entire FOV. If I had field rotation, rotation about the center (where my target is) would mean that it would generally be sharper than if my center of rotation was towards the corner of the field. So, I think your hypothesis is accurate.

That having been said, one of the big advantages to the ONAG is that it allows you the entire FOV to choose a guidestar. As a result, I don't always use a star near the center of the field. I usually look for a reasonably bright star (anywhere in the FOV) and then move the guider stage to find that brighter star.

Of course, I'm not concerned about field rotation because my scope is permanently mounted. I think the aspect of having a centered guide star would be more attractive to me if I were constantly taking my scope in the field and trying to do new PA each time.

I agree with you that the Lodestar definitely helps with guide star choice. The difference in star brightness between my Lodestar and the SSAG is quite remarkable.

As far as what improvement you might yield from an ONAG over your Hutech OAG - I think you've correctly identified the potential advantage (centered guide star). If you're commonly seeing field rotation in your images, then the ONAG could help (alternatively you could spend more time during setup with your PA, but that could mean lost imaging time). If you were considering going from a guide scope to an OAG or an ONAG, I wouldn't hesitate to recommend spending the extra $$ for the ONAG. If the Hutech OAG is working well for you, I'm not sure if you'll see much advantage (other than the field rotation). The one other big advantage to the ONAG is that its MUCH easier to find guidestars than an OAG. But, if you've already mastered that technique, it may not be as useful for you.

Hope this helps.


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Peter in Reno
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Re: M33 - The Triangulum Galaxy new [Re: srosenfraz]
      #5612866 - 01/08/13 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


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srosenfraz
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Re: M33 - The Triangulum Galaxy new [Re: Peter in Reno]
      #5612954 - 01/08/13 11:49 PM

Quote:

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).


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Peter in Reno
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Re: M33 - The Triangulum Galaxy new [Re: srosenfraz]
      #5615252 - 01/10/13 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


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srosenfraz
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Re: M33 - The Triangulum Galaxy new [Re: Peter in Reno]
      #5616511 - 01/10/13 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.


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Peter in Reno
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Re: M33 - The Triangulum Galaxy [Re: srosenfraz]
      #5616590 - 01/11/13 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


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