Celestron 8 inch back focus with diagonal

[JohnJONeill]

Hi all

Im trying to use a C 8 Evolution with an ASI  294 mc pro. I have an inkling that there is too much back focus  as I cannot see any stars after checking ‘preview’ on the ASI app. 
Im using a baader 2 inch diagonal with Baader click locks at each end. The focal reducer is on the optical train first, then the rest goes on(see attached image. I would prefer the diagonal system as it allows OTA more freedom when slewing. 
Can anybody suggest a solution on how to achieve the 105mm spacing.

Thank you

 Jon

 

[Albir phil]

54 minutes ago, JohnJONeill said:

Hi all

Im trying to use a C 8 Evolution with an ASI  294 mc pro. I have an inkling that there is too much back focus  as I cannot see any stars after checking ‘preview’ on the ASI app. 
Im using a baader 2 inch diagonal with Baader click locks at each end. The focal reducer is on the optical train first, then the rest goes on(see attached image. I would prefer the diagonal system as it allows OTA more freedom when slewing. 
Can anybody suggest a solution on how to achieve the 105mm spacing.

Thank you

 Jon

 

You do not use the diagonal with the camera

[bosun21]

Are you are using the diagonal to clear the base of the mount at or near the zenith ? You can fit a longer Vixen rail allowing you to move the scope upwards relevant to the mount. You can remove the Clicklocks and attach the diagonal directly to the reducer and the same with the camera. You have to measure exactly how much backspace you need to reclaim.

[Elp]

Too much, I don't think you've got enough. Measure the distance from the bottom of the thread of the reducer (where an attached piece reaches its physical stop when connected) to the mirror in the diagonal, then up to the front of your camera (front face of the nose). Then add 6.5mm for the front of camera to sensor. I don't think youre near 105mm. A diagonal is a good way to eat up backspacing but likely you need some m42 extension rings. I usually don't use a diagonal and do it straight through instead, but this is more so you don't introduce tilt into the system.

[Elp]

Another way of doing it is instead of your camera, use an eyepiece in the diagonal to get a rough idea of where the focal plane is. Because you're using a reducer, the backspacing needs to be near enough accurate otherwise no amount of focusing will show stars.

[globular]

That looks like a baader #2956100 diagonal which has an optical path of 112mm.
Baader measure this from the shoulder of the nosepiece to the top of the clicklock.
Using it as is and you are over 105 before you’ve even started.

Removing the clicklock will save about 38mm, but you’ll need to add some sort of adapter to to connect the camera to the diagonal so that’ll add back 10mm or so at least.
Removing the other clicklock from the reducer, and the nosepiece from the diagonal, and instead directly connecting them with another adapter will add another 10mm or so.
With everything directly connected you have no rotation options, so you’ll want to add something that lets you rotate. (either yet another adapter and another 10mm or so, or make sure one of the diagonal adapters above gives you rotation (has a lock ring perhaps) and this’ll add 10mm or so too).

So you’ll probably end up something like 112-38+10+10+10=104.

A spacing ring or two and you’ll be at 105.

HOWEVER this sounds too close for comfort to me (you might spend a lot on adapters only to find you can’t make it work) and I think you’d be better off finding a straight through solution by mounting the ota as far forward as you can. The evolution mounts are good at coping with unbalanced configurations.

[JohnJONeill]

Thank you all. It looks like the straight optical train is more straight forward with the longer Celestron vixen bar. 
Kind regards 

John

[Elp]

You might find you need to counterweight the front end depending on what mount you're using (don't know how it affects the evolution alt az mount). I fashioned a vixen saddle that has threaded holes so I can screw in gimble counterweights into it, the saddle I then clamp under the front of the overhanging dovetail bar.