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RC8 Focal Reducer - position and spacing


halli

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Hi All

I have recently been using a GSO RC8 for imaging and am pleased with the results.  

Main issue being the amount of noise I am getting with my DSLR - latest attempt at M81 attached.  

I have now bought an Astro Physics FR and am in the process of fitting it.  

The Astro Physics web site recommends a distance from the FR to the focal plane of 101mm for the maximum reduction of 0.67.  The FR itself contributes 16mm according to the web site.  I have an OAG fitted with a nose piece which screws into the FR and then the SLR fits diretly into the OAG assembly. I am assuming that the order of connection is FR (this slides into the 2" telescope eyepiece holder), OAG nose piece, OAG, SLR.

 Although I can physically measure the distance from the FR to the camera body which around 60mm giving a total of 76mm with the FR included -  Im not sure how much to add from the SLR body interface (without the T2 camera adaptor included) to the sensor which represents the focal plane.

 Can anyone help with this distance for a 1100D or point me in the right direction  please? Or am I getting confused ? Also the image circle at the sensor is 29mm  - is this ok for a DSLR?

Any help appreciated and thanks very much in anticipation

Ian

post-36401-0-92477800-1443961919_thumb.j

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Thanks Chris that seems about the right value given the depth of the camera.  

The chip size has a diagonal of 26.6 mm so the image circle needs to be greater than this I guess.  A reduction factor of 0.6 appears to be able to achieve this from the website.  The spacing to  achieve this is around 120mm from their  graph.  I have found by removing the OAG nose piece I can achieve around 100mm if the camera sensor distance is 55mm with just the reducer and OAG.  This coincidentally gives around 0.67 which is the quoted reduction value for a 29mm image circle ie if I have calculated correctly !!

I will try this configuration out and see what the result is .  However the weather forecast is not to good !

Cheers

Ian

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Hi - if its the 0.67 AP reducer the distance is not critical with  an 8inch RCT - for 067 you are talking 16mm for the reducer and the rest to make  about 85mm in total but apart from slightly greater vignetting the distance is very flexible in my experience with two RCTs!-best wishes Tony

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Thanks for the info Kirkster.

Coincidentally I have around 100mm as well with just the FR, OAG and DSLR ! ie if the DSLR equates to 55mm.

I have measured the FOV using astrometry.net by submitting images for plate solving taken with and without the FR and the result was:

Without  FR  47.5 x 31.6 arc min

With FR  65.4 x 43.5 arc min

The ratio of these is around 0.73 ie 47.5/65.4.  

Im not sure that the ratio of the FOVs mathematically equates to the focal length reduction but intuitively  I feel it may do -  if so its not far off 0.67!

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