From: Milton Aupperle <milton@outcastsoft.com>

Date: March 12, 2009 4:57:46 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] FOVs Celestron radial Guider..


Mark;


On 12-Mar-09, at 4:15 PM, Mark Gaffney wrote:

Hi Milton, Some time ago you gave the range of distance in arc minutes  

from the centre of FOV for the CRG as 20- 35 arc minutes.


That 20 to 35 arc minute estimate was using a Celestron 0.63x Focal Reducer with a C8. My C8 has a true focal length of about 2100 mm. I also have a LPR (Light Pollution Reducing) filter and the 5 slot filter wheel behind the OAG, which pushes the camera further back. The real focal length comes out 1080 mm, which makes the true focal reduction of 0.515x. I've boughten a zero length adapter which shortens the distance to the camera by about 30 mm. I'm expecting the new reduction to be around 0.55x to 0.60x, which means I should get less coma at the edges of the fiel than I did before.


I guess  

that`d be for your C8. (would things be remarkably different for my  

C9.25?


And yes the C9.25 has a longer focal length, so your FOV will be smaller than my C8 by about 12%.


This seems to place the FOV for a 0.3 Mogg reducer/extender  

outside this estimate (based on The Sky 6`s FOVI window). Would this  

work, having a FOV for the imaging camera larger than the guide  

cameras` or would the prism get in the way perhaps?


I don't see how your going to do that. The Focal reducer should go in front of the OAG, otherwise the Guide Camera and the Imaging camera will never be in focus at the same time. The slightest tilt / twist of the OAG mirror for the guide camera or change in the Cameras focus will cause one or the other camera image to go out of focus.


You should really look at how I have my or other peoples rigs set up:


here is my current rig:


http://www.outcastsoft.com/AstroImages/Scope01.jpg


and here is how I did it in the past (in the Other Folder of the Files section of the Astro IIDc Yahoo Group):


http://groups.yahoo.com/group/Astro_IIDC/files/Other/OAG_Visual_Representation.png



I guess there`s  

only one way for me to check & I`ve found I need a Mogg 1.25-T yet!  

Mark.



Okay, from your last message that just arrived your going to use a 0.63x and a 0.33x  adapters at the same time? Why would you make your life so much more complicated?? I really suggest you skip using two focal reducers for now.


First off, your going to get really severe brightness drop off at the edges of the field of view for the imaging camera, which means your usable FOV is even smaller than you think it is. Secondly your going to get really bad color coma and stars won't be stars at the edges FOV, they will be rods or elongated multi color blobs. Thirdly your going to have awful time getting both the imaging and guider cameras in focus at the same time, assuming you have enough back focus with the C9.25" to get the imaging camera in focus too.



HTH..



Milton Aupperle