Back focus issue

[Davor]

Hi there,

 

I have a problem that might not be to others but I van not solve it. I started doing some planetary images with SW Mak 180 Pro and using ASI 120 MC-S camera from ZWO. I made a simple train  , i.e 1,25" to 2" reducer, and extention 1,25" delivered with camera giving together 42,5 mm (from the last machined part of the telescope i.e from the Cryford focuser that I added to my Mak). Together with camera back focus of 12,5 mm it is exactly 55 mm. It is working relatively fine so far.  I have also tried to put the same train (for moon imagining) on the SW Evostar 72 /420 mm refractor, (as on the picture) but there is no way I could even come close to some focus. Just for info, I have bought thet Evostar for DS ( to work with my canon and ASI 120 MC-S as guder) when my T ring and flttener will arrive.  I do not ubnderstand that for testing I could not reach focus with evostar. It has its own great focus as well as camera when used with its wide angle lens.

Thank you in advance

[Lonestar70]

Hi Davor,

I think you will need and extra 2" or 3" long extension between the scope and the camera arrangement to compensate for the missing diagonal that would normally be fitted to the scope for visual use.

I have the ED 80 version of the Evostar and I need to add the extra extension, especially when I don't use the Field Flattener/focal reducer which serves to provide the extra length.

The 55 mm backfocus is related to the field flattener/focal reducer.

I hope this helps.

[Davor]

Thank you Lonestar 70. I will try to make that tonight. When flattener arrives I will tham make another train for Canon DSLR

Regards
Davor

[fozzybear]

Davor have a look at this image might help you so for your camera you need 134.4mm +/- from the focuser to sensor

Edited April 25, 2021 by fozzybear
added text

[Davor]

Thank you, Sub Dwarf, clear 

 

[fozzybear]

1 minute ago, Davor said:

Thank you, Sub Dwarf, clear 

 

this is for the 72ED scope so when you have the reducer and your DSLR no extensions required 

[fozzybear]

3 minutes ago, Davor said:

Thank you, Sub Dwarf, clear 

 

btw it's @fozzybear but never mind