Backfocus help!

[Weaver209]

I've been using a StellaLyra 8"RC for a few months now, and with my Canon 60D it worked perfectly. Now I've got an ASI533-MC Pro, things are much more complex! I'm struggling to get stars in focus or round enough for DeepSkyStacker to stack the images properly, and other than the possibility of dew on the camera, I can't see why this might be. 

Using a Bahtinov mask I'm getting perfect focus on bright stars, but the smaller stars in images of M51 recently seemed a bit naff. I therefore wanted to make sure I had my backfocus right!

According to the GSO online manual, the backfocus should be 254mm to the camera sensor from the rear cell, and I think I've managed this. 

If using the Astro-essentials 0.75x reducer, I need 80mm backfocus to the sensor, but where does the reducer itself go in the train? do I need to put it in the train with the same spacers on before the focuser? 

 

Sorry this is all a big learning curve!

[Doversole]

Have you resolved the issue? Not that I could help myself but I am interested in the same equipment!

[Clarkey]

Generally with reducers you can go slightly closer than the stated distance and it will still be OK. I think you may find the Astro Essentials reducer has a fairly small image size so you will get some vignetting. I changed to the TS Optics CCD47 which gives up to 0.67 reduction, but I have it slightly closer so it gives 0.73 reduction and a larger imaging circle.

In terms of where it goes in the imaging train, I have mine inserted into the draw tube so I can change from reduced to native FL without removing the focuser (giving the 254mm distance). From what I remember the same set up worked with the AE 0.75. The image shows the reducer and whole lot is pushed into the draw tube up to the filter wheel. The stars should be flat across the field without a reducer, if they aren't it is either tilt or a collimation problem.