WO GT81 (2019 version) and FLAT 6AIII Spacing Advice

[IanL]

I'm working on setting up my new WO GT81 (the current 2019 version) with the WO FLAT6AIII flattener/reducer. I'm struggling a bit to understand the correct spacing as the diagrams and drawings provided by WO are pretty unclear and also inconsistent. I've got slightly elongated stars in one corner and edge which tells me  that the spacing is not right and possibly a slight tilt somewhere in the train.

So the setup is:

GT81 -> FLAT6AIII (screwed directly the the M63 thread on focuser) -> OAG/EFW2  (screwed to M48 thread) -> ASI1600MM (screwed to M42 thread)

The diagram below suggests that there should be 55mm between the back face of the 6AIII body and the sensor which is 6.5mm inside the front face of the 1600MM. I also need to allow a further 0.35mm to account for the 1mm thick filter (approximately) so the distance is actually 55.35mm. To get there I thus a spacing between the back face of the 6AIII and the front face of the 1600MM of 48.85mm (55mm - 6.5mm + 0.35mm). So having done that I have as close to the 55.35mm 6AIII to sensor spacing as I can get.

I know it should be good since it's a straight swap of train from the M48 thread on the back of my SW80ED + 0.85x reducer to the M48 thread on the GT81 and FLAT6AIII M48 thread. The rear of flattener to sensor spacing for both setups is 55mm or so, and the ED80 setup was giving good results.

Next I need to set the 6AIII adjustment per the table in the above diagram.

The diagram below shows how to set the adjustment:

This uses the Z81 (not GT81) as the example, but if you subtract the 55mm spacing from the 58.2mm spacing for the Z81 (both in first diagram above) you get 3.2mm (per the second diagram). So for the GT81 following the same maths you take the 60.1mm 'Backfoucus' value, subtract 55mm and thus set the adjustment to 5.1mm.

All good, except there is a second diagram floating about showing different values:

This diagram says the back focus for the GT71 is 64.1mm (not 62.1mm per first diagram), and for the GT81 it is 62.1mm (not 60.1mm). So in both cases it says another 2mm of adjustment is required. This is backed up by the 'adjust distance values of 9.1mm and 7.1mm respectively.

I'm just wondering if anyone knows which of these two values (62.1mm or 60.1mm) is the correct one. I've currently gone with 62.1mm which isn't giving a great result (worse than my SW80ED and 0.85x reducer). Of course I can keep adjusting until I get it right, but due to the fact you need to screw the 6AIII in or out to adjust, it is painful as I need to unplug all the cables and remove the extension tube from my guide scope for each attempt, then reconnect it all and take a new sample image. I've got limited time and even more limited clear skies so was hoping for a bit of advice to try and get this nearer to right second time around?

Thanks for any help you can offer.

Edited January 2, 2020 by IanL

[IanL]

Didn't get any response on this, maybe being too specific

Anyway for reference if anyone else has the same issue and having had time to do some trial and error I can confirm that the reducer needs to be set to at least the 9.1mn mark on the scale for a GT81, the 7.1mm setting in the second diagram is definitely too close. The extra 2mm outwards adjustment goes from visibly egg-shaped stars in the corner to visually acceptable ones. If anything I might try going a bit further out as measurements using FWHMEccentricity script in PixInsight suggest the stars are still somewhat stretched in the corners.

Results with the original setting (7.1mm + 0.33mm to account for 1mm thick filter glass):

 

 

Results with the 9.43mm setting (9.1mm + 0.33mm to account for 1mm filter glass):

 

The stars are visibly rounder in the corners, measurements of eccentricity suggest a bit more spacing needed as ideally would want to be below 0.45 across the entire field.

Measuring using a digital caliper, assuming a 1mm thick filter, you're aiming for approximately 90mm between the rear face of the fixed part of the reducer and the front face of the camera body (assuming standard ASI 6.5mm sensor setback), there are a few models with a different value so do check. You'd need to add a further 0.33mm for each mm of filter thickness or reduce by 0.33mm if using an OSC with no filter :

Edited January 26, 2020 by IanL

[Laurin Dave]

Hi just read your posts,  I am in the process of setting up a GT71 Flat6AIII so they are very helpful.  (I also have the rotator which may be introducing tilt  )..  

Thanks 

Dave

 

[Astro Grandad]

Hi, I think the table you are referring to is only for DSLR cameras including mirrorless, (the one with the extra 2mm.), the first table with diagram is for dedicated astro cameras.

I have a WO GT81 with flattener/reducer quite a good scope but not amazing and your right with the confusion over focal distancing, I set mine to 5.2 + 1mm but still have minor trails but only on one side which suggests tilt not focus, I use a Altair Hypercam 269C. I don't get why your measurement is nearly 90mm though ! as you should be looking at 55mm from the face of the camera to the face of the flattener I think !!!, also why did WO make the flattener a reducer as well - no need.

 

Cheers  Neil

[Neo_uk]

On 08/02/2021 at 16:33, Astro Grandad said:

Hi, I think the table you are referring to is only for DSLR cameras including mirrorless, (the one with the extra 2mm.), the first table with diagram is for dedicated astro cameras.

I have a WO GT81 with flattener/reducer quite a good scope but not amazing and your right with the confusion over focal distancing, I set mine to 5.2 + 1mm but still have minor trails but only on one side which suggests tilt not focus, I use a Altair Hypercam 269C. I don't get why your measurement is nearly 90mm though ! as you should be looking at 55mm from the face of the camera to the face of the flattener I think !!!, also why did WO make the flattener a reducer as well - no need.

 

Cheers  Neil

Hi Neil...

Just wondering why you refer to the gt81 as "quite a good scope but not amazing" ? .

The reason I ask is I'm considering buying one and just thought I'd ask your opinion as you have this scope ..have you had any issues with it ?.

Many thanks Chris.

[Astro Grandad]

Hi Chris, don't get me wrong the scope is a work of art - absolute precision, my gripe is without a flattener/reducer the usable imaging area is small before you get elongated stars if you get me, so my question is why did they make a flattener/reducer and not just a flattener as in my opinion you don't need a reducer, just to give you some idea of the imaging area with the flattener/reducer you could fit three or four Orion nebulas and the running man, sometimes images look to small just something to bare in mind unless you like wide field images, at the moment I am imaging the M78 and the Witches head nebula and they no where fill the field of image, anyway I hope I haven't put you off.

Regards

 

Neil