Back focus with Askar telescopes? - Forums

Topic starter

Jan 29, 2023

Hi or hey all,

Thank you very much for sharing your beautiful amazing wonderful images from this brand, i have one although i didn't use it yet more than once i am still happy to have one from them, Askar is definitely doing a great job to produce great scopes, and it is getting better and better every time they make new one.

So my question is about using their reducers, do you get the back focus right or still people struggle to have that mysterious back focus done properly? I only tested the scope once without a reducer as i don't want to miss up with back focus calculations, but i have the reducer to be used, so i must face and deal with the back focus, and i still don't know which main camera i will use as i have mixed of cameras anyway, and if i use a color camera where i only use 1 filter basically should i add that filter thickness into calculations without using a filter wheel or drawer?

Michael Ring

Jan 29, 2023

You have to add one third of the filter thickness to the calculation.
I saw you are using Sequence Generator Pro, perhaps you could in addition install NINA on your PC and the HocusFocus Plugin, there you have a very nice tool that exactly tells you if you need more or less spacers and when the amount of spacers is good enough.

This post should help with details:
https://www.astrobin.com/forum/c/astrophotography/equipment/fixing-the-dreaded-tilt-and-backspacing-error-in-optical-systems-with-objective-analysis/

Helpful

Stephen Jones

Sep 11, 2024

I use the Askar FRA500 with a reducer. My boys purchased it for a surprise. I notice on the Asiair that it is at 348mm rather than 350mm and pixinsight header says 347.8. Astap shows minimal tilt and you can see the 2mm difference…if Astap is correct. But I really like it. Not a guru by all means but the subs are really nice. I cant see 2mm doing anything to my photos; Im not that pedantic as the stars are fine to me. Sort of beats the purpose of a petval though.

Daniel Petzen

Sep 11, 2024

I just got the Askar 103 APO with the 0.8 reducer / flattener. I only managed to coble together a back focus of 54mm, but I think I got away with it, as I'm getting what looks like a good flat field:

I want to work a bit more with filters, so I still decided to get the back focus adjuster from Askar: M54/M48 Backfocus Adjuster

It can be fitted without changing the existing back focus on the 103 APO (when set to 0 of course) and have a +/-2mm adjustment. I've not researched how it fits with the other Askar scopes.

I just ordered it, so I've not had a chance to try it out yet.

Helpful

Stephen Jones

Sep 11, 2024

Dan, Looks good mate. Id be happy with that also. My small amount of tilt is via my OAG-L. I banged my WO50MM guide scope on and all good with the tilt. I reckon these Askars are quite good. I like that 103 and your photos.

Daniel Petzen

Sep 11, 2024

Hi Stephen

That is really good to know, thanks! I'm completely new to refractors, so even spotting issues with incorrect back focus can be tricky for me.

I had a terrible OAG shadow on my first night, but managed to adjust for it. I've had so many problems with my OAG (I even gave altogether on my C9.25), that I too fitted a guide scope to see if I can improve the guiding. This is the setup I'm going to try tonight:

Yes, OK, I'm waiting for some parts! I've ordered a 300mm Losmandy bar and a correctly sized dew heater (that one is from my C9.25! LOL!).

...and thanks for the comments regarding the photos. I feel I owe it to the telescope, though! I get razor sharp subframes just flowing in, even when I'm at 1" RMS error. It's like cheating! :-)

Respectful Supportive

Stephen Jones

Sep 11, 2024

Dan
I really liked your Lagoon…good stuff. Good looking piece of gear there! I have to rotate my scope so that the EAF is at top as it will hit the azimuth bolt om my AZEQ6. The 300mm losmandy is a good size. Have fun with that scope.