From: Alan Friedman <alan@greatarrow.com>

Date: August 12, 2009 7:46:35 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] matching scope and camera

Hi Willie,

The subject is different for planetary imaging vs. deep sky. Locations that have 3-4 arc second seeing in long deep exposures will still have brief periods of much lower turbulence which can be gleaned with DMK and other streaming cameras for bright planetary subjects. The deep sky definitions of oversampling do not apply here. For the moon and planets, F30 is a good general number to target if you are looking to achieve the maximum resolution from your imaging set-up. Most seem to work somewhere in the range of .1 - .2 arcseconds/pixel. 

best,

Alan

On Aug 12, 2009, at 1:32 AM, Willie Strickland wrote:

On Aug 11, 2009, at 11:41 AM, Milton Aupperle wrote:

> followed choice of focal length (too long for seeing conditions, too
> short for image scale resolution

I dont remember seeing this discussion in the manual.

I know that there is a maximum and minimum resolution for any scope
and it makes sense to me that the same is true for imaging.
Astroplanner calculates that our scope has a min of 81x and a max of
968x. I could never get it focused, even when the seeing seemed quite
good while using an eyepiece.

I calculate that I had the equivalent of somewhere in the neighborhood
of 1500x magnification when using the 21AF04 in our 5490, F9 scope.
That is the reason that I used the FR the other night. It worked out
much better with the .5 FR.

I have read some on this subject. My problem is remembering the
details and understanding the application.

Our average seeing at the observatory is between 3 & 4 arcsec with 2
being sometimes available. The software we use for White Dwarf
imaging, calculates the seeing for us and I have actually recorded
less than 2 arcsec seeing on a few occasions.

From memory (unfortunately not very reliable these days) I believe
one should try to plan a plate scale of 1/2 the average seeing. So,
with 4 arcsec seeing on average, I should use a plate scale of about 2
arcsecs/pixel with that scope. Anything less than that is
oversampling or wasted resolution. Am I getting that correct?

I calculate that the 21AF04 on the 5490 FL scope produces a plate
scale of .21 arcsec/pixel.

206 x 5.6 micron pixel size of ÷ 5490 FL

That camera is almost perfect for my ED80 with its 600 FL at 4 arcsec
seeing. I havent tried it on that scope yet though.

Am I getting this correct? Am I confusing unrelated issues? Sorry
for the diversion.

Willie Strickland
cwskas@earthlink.net