From: Mark Gaffney <markgaffney@me.com>
Date: January 30, 2011 2:38:09 AM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] OT: Astro-Tech 8" RC Astrograph
Hi Milton,
Here`s Brian`s response to the question about the 0.75 reducer he has..
I know he also considered the AT8RC but decided against it for his own reasons (which I can`t recall exactly).
I have just asked him to re-iterate them..(!)
Mark,
I ordered the following for the QHY8:
CCD Telecompressor (CCDT67) <http://www.buytelescopes.com/Products/6709-Astro-Physics-ccd-telecompressor-67.aspx>
2"/T-2 Tapered Nosepiece for CCD Cameras (AT16T) <http://www.buytelescopes.com/Products/6906-Astro-Physics-2t-2-tapered-nosepiece-for-ccd-cameras.aspx>
I added the following item to get the DSLR as close as possible to the CCDT67 to give about 0.75x reduction. This reduction in F ratio just about cuts my exposure time in half. An F ratio of 0.7 would give me exactly half the required exposure. It works quite well; I’ve been shooting things at about 4-5 min that previously took 10 minutes to get the same results... And the 0.75x reduction is large enough not to produce any noticeable vignetting on an APS-C sized sensor which I did notice with the standard AT16T extension.
Remember, the CCDT67 is NOT a flattener. Don’t need flattening with the C8; therefore, you can vary the backspacing a fair bit to give a fairly large range of reduction w/o impacting edge stars, etc. Astro Physics calls the CCDT67 a 0.50-0.85x reducer (totally dependent upon the backspace and camera sensor).
The graph at http://tinyurl.com/bwaCCDT67 gives the correlation between backspace and reduction.
bwa
On 30/01/2011, at 7:53 PM, Milton Aupperle wrote:
Hi Mark;
On 30-Jan-11, at 12:51 AM, Mark Gaffney wrote:
> Hi Milton,
> One thing occurred to me reading your response & that`s the memory
> that my friend Brian Allan uses a 0.75x reducer on his Edge HD C8.
> I don`t know what kind exactly but I could ask if you like?
There are lots of Reducers out there, but none are designed
specifically for the Edge. I've read about people using a reducers
from Televue with them and other scopes.
> I was under the impression that the Edge scopes had lockable mirrors
> which would eliminate mirror flop at least when you`ve arrived at
> your imaging parameters..
Nope, it reduces not eliminates. The only way to eliminate it is to
have fixed mirrors. And you still get image shift when you focus,
thought I've heard the Edge are considerable better than my old C8 is.
> Brian does use a Q guider which has a guidescope of 50mm on ADM
> rails etc attached to the scope & an Antares Red dot finder (I have
> one of each of these too now myself & have used my Scorpion 20SOM on
> it, guiding with Astro IIDC-the problem was I think some stiction in
> Dec guiding with my CG-5).
Like I said before, unless you mount is working perfectly, it simply
isn't going to guide with any software.
> I have had the mount covers off trying to deal with electrical
> problems since we last talked about this. I ran into problems with
> the self tapping screws inside the plastic covers incidentally..
> Here`s one of Brian`s images with the 0.75 reducer & Canon 60D..
> http://picasaweb.google.com/116260312230579398213/MVREmissionReflectionNebulas#5565670325686523986
It would have been great to see it at actual resolution instead of
scaled down. You can hide a lot of tracking errors and distortion when
scaling down.
If you look closely the stars on the edges are elongated compared to
the ones near the center. It could be his focal reducer doesn't have a
flatner or this could be just the Edge HD. The larger the CCD size,
the more pronounced this becomes as the in focus section is curved.
Thats one of the issues I hope to eliminate with the ATRC8. If stars
are elongated on the edges and sharp at the center, you can't use them
accurately for Photometry.
>
> I did look into the AT8RC`s a while ago & was assured by people at
> the time I think they could be also used visually..
At low power the central obstruction begins to cause issues visually
with the ATRC8.
> I think you posted me a link to some custom curved spiders for them
> as you were concerned about the diffraction spikes they exhibit..
Yes I did. I would not try and replace them though in the ATRC8.
> Interesting what you`re saying about C-mount camera`s being easily
> adapted to Hyperstar units which can cut imaging time on DSO`s up to
> 25x I believe with minimal guiding..
All lower end mounts need guiding. Unless your using a CGE Pro or the
high end GE type mounts, your going to get a considerable amount of
Periodic error. After "Hyper Tuning" my HEQ5, I get around +/- 15 arc
seconds as the worm turns. Before replacing the main bearings, worm
bearing and returning the gearing I was getting in the +/- 40 arc
second range.
> Each scope has it`s advantages of course but if one of your prime
> goal is Astrometry or Photometry..(?)
And imaging. I've seen Lunar and Planetary images taken with them too.
TTYL..
Milton Aupperle