Rasa11 and back focus issue?

[b1ues]

Just purchased a demo Rasa and tried to use for the 1st time last night.

 

i have a ZWO EAF , using ASIAIR plus and at the front, the camera adapter connected to a ZWO filter draw and then ASI 533mc Pro camera. I coul not get focus, just these rings. Any ideas where I need to start correcting?

Edited March 15, 2023 by b1ues

[fwm891]

The distance from the camera mounting plate to the focal plane is critical on a RASA consult the RASA White paper on the Celestron website for the technical specs on this distance. You will need to take the thickness of your filter drawer and the 533 back focus from the RASA 11's back focus for the dimensions of any required spacers. See attached speccy bits towards the back pages.

RASA_White_Paper_2020_Web(3).pdf

[b1ues]

excellent! Thanks for your response

 

[michael8554]

And 0.5sec exposure is a bit too fast.

Michael

[tomato]

There is a thread on Cloudy Nights on this topic:

https://www.cloudynights.com/topic/689275-rasa-11-back-focus-takes-no-prisoners/page-2
 

The RASA11 back focus is 72.8mm and it appears you need to be very close to this (+/- 0.5 mm) to get good star shapes right across the field.

[powerlord]

Wow - if you are just beginning and have started with a RASA11, you really are in at the deep end! Hope you sort your problems out, and looking forward to seeing your first images. 👍

stu

[fwm891]

20 minutes ago, michael8554 said:

And 0.5sec exposure is a bit too fast.

Michael

Not with a RASA....

[b1ues]

 

That white paper is great..but now the maths headache! 

If I read this correctly, the adapter plate is 3.5mm above the cuff. The ZWO filter draw is 21 mm, the 2" filter 0.7mm and the 533 cuff and sensor plain is 16mm =41.2mm

Does that mean I need 31.6mm spacers to get to 72.8mm?

or! the back focus on the camera is 55mm. so, do I use the spacers with the camera, + the adapter plate and filter = 59.2 mm deduct from 72.8mm leaving 13.6 mm. and that's the gap I need to cover?

or are both of these methods the wrong way of getting BF with the kit I have?

 

Edited March 15, 2023 by b1ues

[michael8554]

"or! the back focus on the camera is 55mm"

The ASI 533MC Pro camera has a 15mm Flange Distance with the M42 adapter screwed  on.

Many Focal Reducers and Coma Correctors have 55mm Back Focus.

Michael