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Equipment Discussions >> Reflectors

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orlyandico
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Reged: 08/10/09

Loc: Singapore
ASA 2" 0.73X Keller reducer on AT8IN new
      #6103790 - 09/27/13 09:57 AM

Yes, it can be done.. no, maybe it wasn't such a good idea..



I am a newcomer to newtonians since my long-vanished Dobsonian (that I never properly collimated) 10 years ago.

I got this AT8IN and Keller reducer because I was fed up with 20-minute subs using my AT90EDT. Anyway I just did first light.

Prior to putting in the corrector I took this photo of a defocused star:



Since my laser collimator hasn't arrived yet, I just collimated with a Kodak film can, and then (tried) to collimate on a defocused star, because that's how I do it on my SCT. The Newtonian is a PITA to collimate this way because adjusting the primary moves the star around.

And here's a 10-second sub (yes my location is that light polluted) with the reducer in place. The camera is a QHY8 so APS-C size sensor (this is just a stretch and straight conversion to JPG):

https://dl.dropboxusercontent.com/u/63497702/M57-AT8IN-2KORR-QHY8-10s.jpg

Note that stars aren't round across the field, but the distorted stars are asymmetric. There are actually pretty decent stars in the 7 o'clock corner.

My questions:

1) is this just a collimation issue?

2) or.. is the cheap Astro-Tech focuser sagging and producing tilt of the sensor? but so many people are reporting good results imaging with the AT8IN.. the focuser can't be that bad can it? (it's one of those GSO affairs with a steel track but no rack and pinion)

I used to fight tilt all the time on my old Orion 100ED with WO focuser, until I banished it permanently with the 2.7" rack-and-pinion of the AT90EDT. I had forgotten about that nastiness..


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Peter10
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Reged: 03/31/07

Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6104056 - 09/27/13 12:07 PM

I am using the same CC/FR on my 8" f/5 Newtonian Over the past year, I have worked through some of the same problems you are experiencing. It is my opinion that you have more than collimation issues to deal with. The odd shaped stars you are seeing often result from several factors that work together to compromise image quality.

In my case, the underlying concerns were shown to be inadequate collimation and focal reducer/camera tilt. I discovered this, by eliminating the other possible concerns through testing. You need to examine:

1) Mount Tracking
2) Polar Alignment
3) Focal Plane to Coma Corrector Spacing
4) Alignment of Secondary Mirror
5) General Collimation (Catseye tools or HQ Laser Collimator)
6) Tilt

I have found that the ASA corrector is unbelievably sensitive to tilt. I am still working through tilt issues with my system and am hindered by the general lack of rigidity in most of these inexpensive commercial Newtonian telescope tubes. Though you claim to have combated tilt in your focuser, I am not convinced that you are feeding the CC/FR at an acceptable angle. I had to "tune" this adjustment based camera images. Changing the tilt angle of the focuser even by small amounts can swing the star quality through an amazing range of shapes. You should use the full up system (telescope, Focal Reducer, Camera) to convince yourself that tilt is properly compensated for. The path to success will not be easy.

Peter


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orlyandico
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Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: Peter10]
      #6104075 - 09/27/13 12:23 PM

1) and 2) are non-issues in the image I posted, because it's a 10-second exposure, and I've done 10-minute unguided shots at this focal length (with my refractor) on the same mount, which hasn't moved in months so it's very well polar-aligned.

Spacing to the corrector is more or less correct - about 65.7mm as measured with my digital caliper.

I only did a sight tube collimation because my laser hasn't arrived - as I said this is my first newtonian in about 5 years. So I will have to fix that.

However your input on the ASA reducer being sensitive to tilt makes sense. I will probably end up buying a focuser that costs more than the entire OTA at some point..

And I didn't say I have fixed tilt with the cheap focuser. I meant, I have suffered through focuser tilt in the past, and I only succeeded in fixing it when I got the AT90EDT with its huge focuser. But now I'm using this newtonian, I have to fight a losing battle against tilt again.


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Peter10
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Reged: 03/31/07

Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6104130 - 09/27/13 12:49 PM

It seems that collimation, focus and FR/CC tilt are all coupled and must work well together. The introduction of the CC complicates the issue since we generally collimate without the CC installed. I collimate my Newtonian using the Catseye system which generally works well - but all bets are off once I install the ASA CC.

Since we are working around f/3, this gets rather difficult. We should keep in contact to exchange "lessons learned" as we work through these issues. Good luck with tour equipment.

PEter


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: Peter10]
      #6104424 - 09/27/13 03:10 PM

Found this thread - http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,8/Number/48...

So between better collimation and a cheesy sheet of aluminum, the issues are reduced/resolved.

Peter, have you done any of these mods? are you using a Moonlite or Feathertouch focuser? I'd like to avoid a focuser upgrade if I can, seeing as the focuser costs as much as the entire Newtonian tube (and the reducer cost 3X as much, but I digress...)


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orlyandico
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Loc: Singapore
Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6104467 - 09/27/13 03:43 PM

I cut out 300x300 pixel areas at the corners of my first image:



It's bad... but not as bad as my early attempts at guiding, with field rotation and declination drift. I have just gotten so used to perfect results with the AT90EDT.. :P


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orlyandico
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Loc: Singapore
Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6107269 - 09/29/13 11:25 AM

Surprisingly, I think I have the problem licked! the only things I've done are

- re-did the collimation, still by eyeball using a Kodak film canister, since my laser is still a week or two out;

- I reinforced the focuser with an aluminum plate underneath

- I relocated the locking knobs for the main mirror (drilled and tapped new holes right next to the collimation knobs) so that tightening the locking knobs doesn't throw the collimation off as much as it used to

Here's a single 5-minute unguided sub of M27:
https://dl.dropboxusercontent.com/u/63497702/M27_300s_Ha_001.fit

Stars are "round enough" over most of the field. What this tells me is, the 2" Keller reducer's performance drops off fairly rapidly once you get beyond the 22mm image circle.

ASA has always said the imaging circle is 22mm, but my sensor is APS-C (24x16mm, QHY8) and it's obvious that the stars streak out in the extreme corners.

but Powernewts (which use the same corrector) claim to cover APS-C. It's obvious to me that they don't, at least not to the very corners.


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Peter10
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Reged: 03/31/07

Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6107798 - 09/29/13 04:18 PM

Hi:

Looks like you have been making good progress! I have not yet reinforced my focuser, but I have shimmed the outer perimeter of my mirror in the mirror cell. I used some thin pieces of cork to fill the gap between the mirror cell and mirror taking care not to stress the mirror. My locking screw arrangement is stock - since I am happy with the stability of my PM (once locked).

I have a 2" Moonlite focuser that has the ability to introduce tip/tilt adjustments via four small set screws at the perimeter of the base. This feature has helped but there is still flex since I have not yet added the support plate you have incorporated.

Getting this all to work is a major task. I suppose it is the price you pay for an 8" astrograph that is affordable.

Peter


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AndreyYa
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Reged: 12/05/06

Loc: Sunnyvale, CA
Re: ASA 2" 0.73X Keller reducer on AT8IN new [Re: orlyandico]
      #6107921 - 09/29/13 05:34 PM

Quote:

Surprisingly, I think I have the problem licked! the only things I've done are

- re-did the collimation, still by eyeball using a Kodak film canister, since my laser is still a week or two out;

- I reinforced the focuser with an aluminum plate underneath

- I relocated the locking knobs for the main mirror (drilled and tapped new holes right next to the collimation knobs) so that tightening the locking knobs doesn't throw the collimation off as much as it used to

Here's a single 5-minute unguided sub of M27:
https://dl.dropboxusercontent.com/u/63497702/M27_300s_Ha_001.fit

Stars are "round enough" over most of the field. What this tells me is, the 2" Keller reducer's performance drops off fairly rapidly once you get beyond the 22mm image circle.

ASA has always said the imaging circle is 22mm, but my sensor is APS-C (24x16mm, QHY8) and it's obvious that the stars streak out in the extreme corners.

but Powernewts (which use the same corrector) claim to cover APS-C. It's obvious to me that they don't, at least not to the very corners.



Orly, thank you for interesting thread. I looked closely to ASA correctors, and I already know they are pretty picky regarding alignment and focusing.
Looks like your last shot has signs of not pretty precise focus. FWHMs are about 4 pixels. From my experience, bayer sensors should show a kind of 2 pixels FWHM after de-bayerization.
BTW, CCDInspector helps a lot in Newtons alignment, especially with critical issue of sensor plane not aligned with focal plane.
As I know, ASA 2" 0.73x correctors provide pretty good results with at least KAF8300 size sensors. For example, see this links from russian astro forums (hope, Google translate could help):
http://www.astronomy.ru/forum/index.php/topic,16429.msg1867501.html#msg1867501
http://www.astronomy.ru/forum/index.php/topic,30252.msg1843806.html#msg1843806
Comparison is not 100% fair - it had f/5 main mirror, but...

Edited by AndreyYa (09/29/13 05:36 PM)


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orlyandico
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Loc: Singapore
Re: ASA 2" 0.73X Keller reducer on AT8IN [Re: AndreyYa]
      #6108211 - 09/29/13 09:09 PM

Thanks AndreyYa, I know there are quite a number of issues there. I didn't autofocus either, just the Mark 1 eyeball and Nebulosity fine focus. Seeing was pretty bad as well. And yes, 8300 series sensors would be ideal..

Peter, yes you are right. Hassle versus money tradeoff. in an ideal world we'd all be using Tak Epsilons or similar.

I find it.. interesting that one of the Powernewt guys now uses an Veloce 200 Riccardi-Honders instead of their titular product.


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