Newbie setting up back focus on Skywatcher Evoguide ED50 with T7C astrocam

[uk_friendly_fire]

Got my Pi4b Stellarmate setup with Evoguide ED50 and T7C astrocam but focusing is an issue each night out. The setup is easily transportable and dismantled frequently so I think the answer is a fixed train setup. Although I contacted the T7C supplier about the focal plane measurement I never got an answer. 

Does anyone know what it is?

Has anyone setup the train and how was it done?

Thanks for any help

[happy-kat]

unless you are not using the 50ED for something else why not leave the astro cam connected then your focus will be retained

[uk_friendly_fire]

Thanks - It's an idea but sadly the requirements are the kit can be ultra transportable.

My setup needs to be dismantled to pack away small enough to go caravanning (or hiking?) and enjoy better skies than I get here in greater London.

I also wish to learn the principals and apply it to my other scopes if the need arises (Celestron 5se, Skywatcher ED72, 114P, & Heritage 130P

[happy-kat]

Are you using any of the 3 available flatteners for the 50ED?

[uk_friendly_fire]

No flattener. The scopes focal length is 242mm so by my reckoning I just take off the T7C focal plane measurement as measured from behind the ED50's objective lens. Is this correct?

[happy-kat]

When I used an imaging camera (not DSLR) I needed to use the extra part that came in the box (I was not using a flattener as was having a go at the Moon)

[uk_friendly_fire]

Yep using that "extra" tube, but the camera, which is a shotgun cartridge in shape, is free to slide in and out of it changing focus until clamped by the thumb screws at the correct distance - 242mm behind the objective lens taking into account the position of the camera sensor in the camera body. It's this forward and backward movement that needs setting to provide a perfect focus at the sensor e.g. 242mm minus the recess depth of the sensor from the forward edge of the camera casing (I think that's correct?).

Problem is I don't know its correct depth or how to accurately repeat this setting for each night's use. It must have a simple solution but I'm not there yet - perhaps I looking to over engineer this?

[Peter_D]

Would a parfocal ring help? 

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p581_Baader-FR-4---Parfocal-Ring-for-1-25--eyepieces-and-adapters.html

If so, FLO will definitely have one too. 

HTH

[happy-kat]

A parfocal ring came in the box with the 50ED

[TerraC]

I had a play around with my 50ED and a Bresser Microkular camera from my sons microscope a while back.  I had a go in daylight on some distant trees to find out the focal measurement myself.

I found even with the extension tube on the 50ED that the camera needed to be almost hanging right out the back to get focus.   It was possible but I didnt come up with a method of fixing it in place.  It was just musings as I was thinking of an ultra cheap way of utilising the Bresser as a guide camera and didnt take it any further. 

Maybe give it a try in daylight and see if you can find focus that way?  Start of with something like a 25mm eyepiece to get focus and replace with the camera, then you can look towards fixing with spacers or extension tubes.

 

Edited December 21, 2021 by TerraC

[The Lazy Astronomer]

Be aware the flattener is an absolute necessity for all but the smallest of sensors (or the very centre of a larger sensor) due to strong field curvature with this scope.

[uk_friendly_fire]

Thank you all for your replies. A Parfocal ring might be part of the solution but I think you would have to wind the ED50's focus ring out a number of times before achieving focus so still no neat pop-in-camera-and-go solution yet..

[happy-kat]

This skywatcher USA gives an example of using the provided parfocal ring to reset focus when using a camera like your with it's long neck, around 14 minutes in

https://www.youtube.com/watch?v=CeCwuxSbeRo&t=1487s