Askar 600 f5.6 back focus - Forums

Topic starter

Jul 28, 2022

I received a new Askar 600 scope but I am confused about the back focus for Astro cameras.
The instructions list back focus as 160mm. The scope comes with a cone and 2mm filter drawer that adds up to 80mm.
I put on my Zwo asi533 with the standard 55mm adapters and I am 25mm short.
Where/How do you get the additional 25mm?
I set it up briefly and cannot get it to plate solve Polaris no matter the focus adjustment so I am missing something?
If anyone has this scope, I would appreciate some advice on the set up.
Thanks.

Dan Kearl

Engaging

Claudio Tenreiro

Jul 28, 2022 · Edited: Jul 28, 2022

Hi, I have that scope and no problem with the back focus, here you can see the configuration, after the cone I have the sapacers that come with the camera. In this case I am using the ZWO 294 MC-P

ASKAR FRA600 with ZWO294MC-P

Andy Wray

Jul 28, 2022 · Edited: Jul 28, 2022

Forgive me if I am wrong here, but as long as the focusser has more than that 25mm of travel you should be able to achieve focus at the desired 160mm ?

I found the following article on-line which explains things quite well: https://agenaastro.com/articles/miscellaneous/a-primer-on-back-focus-in-astronomy

Also: I would usually platesolve away from the celestial pole wherever possible. Or do you mean you can't get SharpCap to work for polar alignment?

Claudio Tenreiro

Jul 28, 2022 · Edited: Jul 28, 2022

Andy Wray:
Forgive me if I am wrong here, but as long as the focusser has more than that 25mm of travel you should be able to achieve focus at the desired 160mm ?

I found the following article on-line which explains things quite well: https://agenaastro.com/articles/miscellaneous/a-primer-on-back-focus-in-astronomy

Also: I would usually platesolve away from the celestial pole wherever possible. Or do you mean you can't get SharpCap to work for polar alignment?

Yes, I use 20mm from the focuser, which add all together to 160 mm. Regarding plate solving I am not sure what does Dan means, I have no problem with plate solving using NINA.

GalacticRAVE

Jul 28, 2022

Can you get it in focus? The ASKAR is designed such that when you are in focus you also have the right back focus (this only applies when you are not using the reducer), simply as the flattening elements sit ahead of the focuser. I have the FRA400, and if I recollect correctly, in focus the focuser draw tube is out 2 cm (but it is longer than 2cm - again, if I recollect correctly, I cannot check as I am away from my scope right now).

Matthias

Helpful

DanRossi

Jul 28, 2022

This scope doesn't require special backfocus spacing because it's a Petzval design, meaning it has a native flat-field right out of the box. You should be able to attach your camera and get a perfect flat-field as long as you hit the focus plane. If you need an additional 25mm, you'll need to add spacers to hit 25mm.

Well written Helpful Concise

Claudio Tenreiro

Jul 28, 2022 · Edited: Jul 28, 2022

Can you get it in focus? The ASKAR is designed such that when you are in focus you also have the right back focus (this only applies when you are not using the reducer), simply as the flattening elements sit ahead of the focuser. I have the FRA400, and if I recollect correctly, in focus the focuser draw tube is out 2 cm (but it is longer than 2cm - again, if I recollect correctly, I cannot check as I am away from my scope right now).

Matthias

Yes, it is the same with the FRA600, 20 mm using the focuser, From the manual it is stated the 160mm for the focal plane, from the field rotator.

Dan Kearl

Topic starter

Jul 29, 2022

Thanks everyone, I setup tonight, adjusted focus to 25mm, pointed at Deneb and took 5 sec, images adjusting focus (22mm actual focus adjustment).
Turned off and back on, pointed at Polaris and aligned.
Guided with just a 120mm scope and got very nice images.
I use an Asiair which I should have mentioned.
i took 5 and 10 minute images of good old NGC 7000 since it is so familiar and was very happy.
Plug and play, seems like a great scope
Thanks again.

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