Struggling with correct spacing - FF/FR and Camera

[Kodo65]

Hi Guys!

I have a TS CF90 APO with the stock flattener/reducer. According to TS the back focus for the FF/FR is 55mm from FF/FR thread flange. Attached I have an Atik Horizon II with a back focus of 13mm from the camera thread flange according to Atik. Now, I thought a simple arithmetic operation would yield an approximate distance of 55 - 13 = 42mm. I've added a Baader UV/IR filter and added 1mm for that glass in the image train. However, I still seem to be "way off" from being able to have the entire FoV in focus. All four corners show elongated stars pointing "inwards" indicating that the camera is too close. The measured distance between the flange of the FF/FR and the sensor position of the camera (visible in the picture) is 43mm - give or take 0.5mm.

I'm beginning to feel rather stupid! Is my reasoning totally nuts or...? I'd really appreciate your thoughts on this as waaay more knowledgeable than me on this subject.

Many thanks in advance.

[Stuart1971]

You say the measured distance between the sensors and rear flange of the reducer is 43mm, but should it not be 55mm…🤔

Edited February 20, 2023 by Stuart1971

[Elp]

Have you tried adding 0.5-1mm increments to it and taking an image?

[Adreneline]

2 hours ago, Kodo65 said:

Is my reasoning totally nuts

Absolutely not! I know it's a different scope etc. but the built-in R/F on an SS13028HNT I am using is 55mm and when used with my ZWO camera (6.5 mm back focus), as @Elp implies, the smallest changes make a significant difference. I notice you also have a filter in the imaging train (looks like a Baader CLS or UV/IR cut?) and that will contain 2 mm thick glass requiring an extra 0.66 mm to be added to the back-focus distance. I would set it up at as close as you can to 42.66 and then as @Elp suggests introduce incremental changes and look to see what you get. I have a stock of 0.1 mm shims - very handy! Good luck! Adrian

[Stuart1971]

Am I missing something here, the OP has stated that he only has 43mm between the flange of the FF/FR and his camera sensor. Surely that is the issue as it should be 55mm….also stated by the OP…..🤔🤔

[Elp]

They've accounted for this via the 13mm front of camera to sensor which has been subtracted from the 55mm total, leaving 42+1mm (thereabouts) remaining to make up with spacers.

[Kodo65]

Hi guys!

The distance between the FF/FR flange and the flange of the camera is 42mm + the additional 13mm to the sensor adds up to 55mm which I hope is correctly thought out. I guess I'll just have to keep adding spacer(s) until I find the optimal distance. I've understood that there will be some back and forth until the optimal distance is found. I guess this explains why Petzvals are so nice

Many thanks to all of you for your answers and suggestions!

Cheers

[StevieDvd]

There is a good picture of the impact of the sensor being too close or too far here and your camera has a marker for the sensor plane (as seen in your image) so easy to measure the backfocus setting with a ruler.

Your mention of the sensor being too close matched the diagrams shown on the above link.  So allowing for the filter (0.66) and the tolerance +/- 0.5 I'd try a total of 56mm or 57mm and see if that shows as being too far. Then you know the distance is between your 42mm spacers (should read 55mm using the sensor plane line) and 57mm. using the sensor plane line).

 

 

[Stuart1971]

3 hours ago, Elp said:

They've accounted for this via the 13mm front of camera to sensor which has been subtracted from the 55mm total, leaving 42+1mm (thereabouts) remaining to make up with spacers.

Ah well he said it was from sensor to flange, so there was the problem….👍🏻