Help please with Filter Wheel + OAG choice

[michaelmorris]

I’m starting to put together my new imaging set up.

My first purchases are planned to be a new filter wheel and off-axis guider.  These will need to fit between a Starlight Xpress Trius SX-694 Mono CCD camera and either a Skywatcher .85x Reducer/Flattener for ED80 (when attached to my SW 80ED) or a Celestron f6.3 focal reducer (when attached to a Celestron 9.25” SCT). 

A crucial factor when deciding which combination of filter wheel and OAG to go for seems to be the gap needed between the focal plane of the camera and the back of the focal reducer.  As I understand it, for the 80ED set ups, this gap is between 55 and 56 mm.  For the Celestron it’s 105mm. 

Fitting an off –axis guider and filter wheel  between the camera and focal reducer for the Celestron set up should be easy.     However, for the 80ED setup this is potentially problematic.  With the front flange of the SX694 camera being 17.5 mm in front of the focal plane, this means I have to get a filter wheel and off-axis guider + any necessary adaptors into just 39 mm.  That is not a lot of room! 

At first glance a seemingly obvious choice for a 7/8 position filter wheel and off-axis guider would be Starlight Xpress’s own combination. However this takes up 42mm, meaning that the only way to get the set up to achieve the optimal distance is to remove the cameras tilt adjustment plate.

https://www.firstlightoptics.com/user/starlight-xpress-filter-wheel-review.pdf

I’ve being doing some research and have come to the conclusions that my bests bet might be to go for the combination of a Xagyl FW8125 8-position wheel with a Skywatcher off-axis guider.

 

https://www.firstlightoptics.com/blog/blog-53.html

 The Xagyl filter wheel seems to add just 19mm to the optical path and the Skywatcher off-axis guider adds just 12mm more because it will thread straight into the SW focal reducer.   By my calculation this combination would add just 19mm + 12 mm = 31 mm to the focal path, meaning this combination should work without having to remove the camera’s tilt adjuster. 

 17.5mm (camera) + 19mm (Filter wheel) + 12mm (OAG) = 48.5mm. This is well inside the 55/56mm needed for optimal use of the SW focal reducer.

Is my logic, maths or information flawed?  Have I missed anything?

 

[Demonperformer]

Michael,

I don't actually know the equipment you have listed above, but I do have one of these. The body is genuinely 9mm from front to back and so would gain you an extra 3mm over the SW OAG - if you don't need an extra adapter to fit it to whatever you have attached on each side of it.

HTH