From: Willie Strickland <cwskas@earthlink.net>

Date: August 11, 2009 11:32:07 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: matching scope and camera

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