12 replies to this topic

[Etien]

Hello, I don't seem to find a workable backfocus / light path solution using the full Zwo Monty ( caa efw and Oag-l ) once I include a reducer.

 

Some experts are pointing to Askar FRA500 telescopes with their 158 mm maximum working distance from drawtube with all adapters for photography and observation, but I don’t seem to find a workable setup once I include the Askar f/3.9 Full-Frame Reducer for FRA500 Flatfield Astrograph even overloading the reducer with an askar M54 special adapter for precise backfocus adjustment, which is supposed to give an extra 2mm.

 

Any suggestion would be welcome ( it doesn’t have to be askar based )

cheers, etienne

Edited by Etien, 15 May 2025 - 12:11 AM.

[archiebald]

I'm not a ZWO guy, however, as I am finding in my own research the same principle applies to almost any image train.

 

For a "conventional" scope requiring the typical 55mm backspacing after the flattener / reducer, then with an OAG and an EFW, there is no space for a rotator.

 

Your options are;

1. Switch to a guide scope, use the space for the rotator.
2. Look for an astrogaph (Petzvel etc) that has at least 70-75mm of range of available focus,
3. With a conventional scope, place your rotator in front of the flattener / reducer, however I'm not sure if the ZWO CAA is large enough to allow this setup.

 

This video from Wanderer explains it a little  https://www.youtube....h?v=1wW3czy6WIc

Edited by archiebald, 15 May 2025 - 02:27 AM.

[Chrisofweden]

I have the same challenge (FRA600 and 130PHQ).
I asked the team at First Light Optics about a solution and they were very helpful but could only confirm that putting the CAA in the train, together with the reducer, OAG and EFW is not doable.
Without the reducer, then back spacing is not an issue and there are several solutions to available to match threads etc.
This has led me to hold off on getting the CAA. I was planning on getting two of them - one for each scope - but now I’ll either carry on using manual rotation or perhaps get only one. I don’t have the reducer for my 130PHQ and don’t feel I need one either since at 1000mm FL I use it for smaller, brighter objects (galaxies, PN and the like). This means that whilst the CAA can be mounted, it’s not needed since the objects almost always fit in the FoV anyway…

The FRA600 is a great scope, with or without the reducer, and I would love to be able to put a CAA in there when using it on nebulae and other larger targets but that would mean I can’t use the reducer and still have interchangeability for my mono train… problems problems!

I do have an old OSC camera (294MC Pro) and could mount that instead, but would still want at least a Filter Drawer any way…
Could go back to using a guide scope instead of the OAG on this shorter length scope, but again that would mean that interchangeability between scopes and imaging trains would be more or less ”wiped out”.

At the end of the day I think the real issue is the fact that the EFW and camera (2600MM Pro)are bolted together, making it a veritable PITA to quickly switch things around. I do though understand why bolting them together is a good thing…

As you can hear, I’m kinda stuck between several less than optimal choices. Most likely won’t be a CAA in my set-ups any time soon. ZWO or any other, for that matter..

Please do post here, if you find a solution!

/Chris

Edited by Chrisofweden, 15 May 2025 - 03:07 AM.

[MichaelACG]

Hi Chris.

 

I decided to use a DUO camera.
--> No space needed for OAG
--> Easier handling

 

Downside is that I cannot use the very thin narrowband filters. "All" say, that 3 oder 4nm is too little light for the guiding camera.
--> I use the 6nm filters from Altair Astro, and guiding works flawless, even without reducer, so f/5.6.

--> With reducer, it's f/3.9 ... so even more photons for the guiding camera.

 

Best regards.

Michael 

[klangwolke]

Coould you post a photo of your optical train? I have a FRA600 and have run in to the same issues. I haven’t found a good solution. Am I just not adding things up correctly? I wanted to find a combination using some (sub)set of the efw, caa, a 2600 duo, back focus adjuster.

[Digitaliz.se]

Easy fix. Skip the OAG and get a guidescope.

 

/Stefan

[Chrisofweden]

Easy fix. Skip the OAG and get a guidescope.

/Stefan

Certainly one alternative, for shorter focal length scopes like my FRA600, but my 130PHQ at 1000mm is borderline for that approach. Indeed, I did do it that way when I first acquired the 130, but quickly got into trouble with guiding - it deteriorated to between 0.8 and 1.0 total rms which is certainly not good enough for an image scale of 0.79. I don’t know what caused the deterioration, since one would think it would be unchanged from when guiding the 600. My only theory is that the extra weight and length of the bigger scope was just too much inertia for the set up?

Moving to an OAG and ASI174mm mini fixed that immediately…

Just my experience…

/Chris

Edited by Chrisofweden, 15 May 2025 - 05:29 AM.

[Digitaliz.se]

I have 800mm Focal Length and have 30mm guidescope on it, I don't have problems with guiding.

/Stefan

Certainly one alternative, for shorter focal length scopes like my FRA600, but my 130PHQ at 1000mm is borderline for that approach. Indeed, I did do it that way when I first acquired the 130, but quickly got into trouble with guiding - it deteriorated to between 0.8 and 1.0 total rms which is certainly not good enough for an image scale of 0.79.
Moving to an OAG and ASI174mm mini fixed that immediately…

Just my experience…

/Chris

[Brian Carter]

 

it is a Pegasus rotator, not a ZWO. I screwed the rotator to the focuser and put a Baader click lock on the other side. The camera assembly is ASI2600 - EFW - OAG-L, Starizona reducer. 
 

they key to this set up is to have enough focus distance for your focuser. I have a moonlite and it has plenty of back focus. 

Edited by Brian Carter, 15 May 2025 - 06:13 AM.

[Chrisofweden]

I have 800mm Focal Length and have 30mm guidescope on it, I don't have problems with guiding.

/Stefan

As I wrote, this was just my own experience and obviously there are no ”whole truths” in this hobby. At the time, I needed to create two independent rigs, with interchangeability, so a second guiding set up was any way on the cards. I chose OAG then, unaware of the looming ”issue” with back spacing if/when I wanted to add in a CAA on a mono setup…

It’s not the end of the world and would only affect me when I want to use the FRA600 together with its reducer. Other scenarios work on both scopes, since they are not back focus restricted.

/Chris

Edited by Chrisofweden, 15 May 2025 - 05:39 AM.

[Etien]

thanks everyone for your answers.

 

 

 

it is a Pegasus rotator, not a ZWO. I screwed the rotator to the focuser and put a Baader click lock on the other side. The camera assembly is ASI2600 - EFW - OAG-L, Starizona reducer. 
 

they key to this set up is to have enough focus distance for your focuser. I have a moonlite and it has plenty of back focus. 

 

 Going to look is something similar is doable

 

Does a reducer dans be put any place in the chain?

Edited by Etien, 15 May 2025 - 09:45 AM.

[Chrisofweden]

I can only speak for the Askar FRA and PHQ scopes, which relate to your original post. On these, the reducer is screwed right INTO the backend of the focuser, replacing the three conical spacers that are otherwise screwed ON TO the back end.
As explained, introducing the reducer creates the requirement of 55mm back focus from it. Without the reducer, you can play around with back focus distance all you want. For example, by removing one or more of the conical spacers…
To use a CAA on the unreduced FRA600, all that’s needed is to take away the last (from the scope view) of the spacers. This gives you the correct diameter and thread size for the CAA and only changes the total distance by a couple of millimeters, which doesn’t matter at all since it’s a “petzval”…

/Chris

Edited by Chrisofweden, 15 May 2025 - 10:40 AM.

[Brian Carter]

thanks everyone for your answers.

 Going to look is something similar is doable

 

Does a reducer dans be put any place in the chain?

The reducer is the hard part, mine has to be 56mm from the sensor, no more no less.  The ZWO rotator is the same width as the OAG and either would accommodate a 55mm back focus.  Adding a rotator between the reducer/OAG would create too much space.  Ironically, I sometimes image with just a 1x flattener which has a 108mm backfocus.  This could easily go after the rotator and I could have the entire thing as a single unit.  But my reducer is 56mm.

 

I bought the rotator initially to screw together an entire camera/filter/rotator unit that could be moved from scope to scope.  I just haven't been able to make that work, so the rotator stays on my primary imaging scope.  If I use any other scope, I have to rotate the camera manually, like a caveman.  I'm actually really happy with the setup now, it installed cleanly and works flawlessly, so I won't complain too much that my original plan didn't work.