Inward Focus Problem

[Jm1973]

Hi everyone. I need to ask for some advice.

I recently bought an Evostar 80ED DS Pro, with the x0.85 dedicated flattener/reducer. It is fitted with a Moonlite CF focuser and Moonlite stepper motor.

I haven't had chance to use it yet due to weather, but I have taken it into the garden during the day and tested focus with ny DSLR attached, and I can achieve focus at about 2300 steps out from 0. 

So far so good.

 

However I also just bought one of the dedicated Evostar 80ED rotators from FLO. I fitted it and tested again during the day, and now I cannot get enough inward travel on the focuser to achieve focus.

Do I have any options to enable me to get focus with the rotator? I don't really want to be cutting down the OTA or anything.

 

Or do I have any other options for rotating the DSLR? I suppose I could turn the OTA in the tube rings to assist in framing, but I was hoping for something more accurate and less of a faff.

 

If anyone had any experience with this, or any ideas generally I would be very grateful.

 

[scitmon]

https://www.firstlightoptics.com/adapters/astro-essentials-adapter-for-sky-watcher-focal-reducers.html might work?  I successfully use this with the stock focuser, reducer and a dslr.

[Jm1973]

30 minutes ago, scitmon said:

https://www.firstlightoptics.com/adapters/astro-essentials-adapter-for-sky-watcher-focal-reducers.html might work?  I successfully use this with the stock focuser, reducer and a dslr.

Aha! This looks interesting. So this fits in place of the stock connector on the 0.85 reducer? And enables rotation?

 

[SamAndrew]

What are you focusing on during the day? the focus position should be further out when focusing at infinity so you might be ok.

[Jm1973]

9 minutes ago, SamAndrew said:

What are you focusing on during the day? the focus position should be further out when focusing at infinity so you might be ok.

I'm focusing on the roof of a building about half a mile away. I thought it was the other way around though? ie. it's further in when focusing to infinity. I am probably wrong.

[scitmon]

23 minutes ago, Jm1973 said:

Aha! This looks interesting. So this fits in place of the stock connector on the 0.85 reducer? And enables rotation?

 

Yes, where you would normally screw your reducer onto the draw tube, you screw this adapter on instead.  Then the t nose slides into the  focuser like an EP would.

[John]

8 minutes ago, Jm1973 said:

I'm focusing on the roof of a building about half a mile away. I thought it was the other way around though? ie. it's further in when focusing to infinity. I am probably wrong.

You are correct - when focusing on closer targets, the focuser needs to move further outwards.

 

[Jm1973]

6 minutes ago, John said:

You are correct - when focusing on closer targets, the focuser needs to move further outwards.

 

Thanks. Thought I was going mad for a minute then.

[Jm1973]

7 minutes ago, scitmon said:

Yes, where you would normally screw your reducer onto the draw tube, you screw this adapter on instead.  Then the t nose slides into the  focuser like an EP would.

Great. That sounds like it will do the trick.

/puts hacksaw away

[scitmon]

3 hours ago, Jm1973 said:

Great. That sounds like it will do the trick.

/puts hacksaw away

Before you do go and buy one, I would double check you can't get focus with a star or the moon.  I don't know how much your moonlite focuser has effected the spacing, but I find it strange that just adding this rotator would throw it off so much. 

[Jm1973]

So I've been doing a bit of research and I think I've found out what's going on...

Firstly it's not just that the focal point is quite far forward, meaning that the addition of the rotator effectively stops me from being able to achieve focus. Also there is a lot of coma and other distortion, which there shouldn't be as I am using the dedicated 0.85 reducer.

I have discovered that the sensor of the camera should be 55mm back from the reducer. I measured and mine is something like 100mm back. When I bought this it came with an adaptor which connected the reducer to my Canon 450D. However, it is not the M48 to EOS adaptor that is sold by FLO, it is one of these:

Obviously this adds way too much backward spacing for the reducer to work, and also pushes the camera further back meaning it has to come further forward to achieve focus, if I am correct. This would seem to be the source of both my issues - focus and field distortion.

I am going to purchase the Astro Essentials M48 to Canon EOs adaptor and see if that sorts things out.

Sadly I sold the rotator when I thought I couldn;t use it, and will probably have to buy another one now. Ah well.

 

Edited February 12, 2021 by Jm1973