WO GT-81 Field Flattener spacing help

[Z3roCool]

Hi there, I have just purchased a WO GT-81 and the Flat6A FF to go with. I will be using with the ZWO Filter wheel and ZWO 1600 MM Pro camera. 

Looking at the manual that comes with the camera it states the following:

SCOPE - FF - M42 to M48 16.5mm Extender - T2 Extender 11mm - M42 to M42 (Male to Male) adapter - EFW - CAMERA

I have found info that states the FF needs to be set to 7.1mm for the GT-81. 

I am a bit confused as have found different information to this on the web and not really sure what I should be looking at. Any help would be much appreciated as this is my first telescope (Only used lenses before).

Many thanks,

 

Tom.

[ratskrad]

Hi, I just received my WO GT81 IV last week along with the focal reducer. The two numbers to remember are 7.1mm and 60.2 mm. The 7.1mm is the setting for the focal reducer. Spin the pieces until you see the 7mm mark and turn the locking ring. The second number 60.2mm is the distance for the back focus/spacing to the image plane of the camera. There are no instructions for the WO 6AIII but on the webpage for the WO 6AII FR it at least shows the two surfaces that require the 60.2mm spacing. Scroll down and there is a good schematic of the 6AII showing details.  Here are the links to the WO pages for both the 6AIII and the 6AII. Hope this helps.

https://williamoptics.com/products/accessories/barlows-flatteners/2019-all-new-adjustable-flat6aiii-t-mount-not-included

https://williamoptics.com/new-adjustable-flat6a-t-mount-not-included

Edited January 22, 2021 by ratskrad

[david_taurus83]

18 minutes ago, ratskrad said:

Hi, I just received my WO GT81 IV last week along with the focal reducer. The two numbers to remember are 7.1mm and 60.2 mm. The 7.1mm is the setting for the focal reducer. Spin the pieces until you see the 7mm mark and turn the locking ring. The second number 60.2mm is the distance for the back focus/spacing to the image plane of the camera. There are no instructions for the WO 6AIII but on the webpage for the WO 6AII FR it at least shows the two surfaces that require the 60.2mm spacing. Scroll down and there is a good schematic of the 6AII showing details.  Here are the links to the WO pages for both the 6AIII and the 6AII. Hope this helps.

https://williamoptics.com/products/accessories/barlows-flatteners/2019-all-new-adjustable-flat6aiii-t-mount-not-included

https://williamoptics.com/new-adjustable-flat6a-t-mount-not-included

That's not quite right. The total back focus for the GT81 is 62.1mm. So if you used a DSLR and t ring (44mm + 11mm = 55mm) then you would set the adjustment to 7.1mm to make up the distance to 62.1mm.

ZWO cameras usually come with the spacers to make 55mm as well so in this instance I would recommend setting the the ring to 7.1mm and go from there. You will probably find that you need to adjust either way unless your extremely lucky and have pin point stars corner to corner!

[ratskrad]

Did you click on the WO 6AII link showing the numbers and technical diagram I was basing what I posted off of. I understand the 55mm as I have been shooting with a DSLR for 8 years but the instructions at WO says otherwise and that different scopes that the flattener uses has different setbacks. The numbers do add up for all of the different scopes that use the 6AIII as far as setback plus 55 equals the totals backfocus. I have not yet had the chance to use the 6AIII as it is snowing here and will be for the next few days or more. I was an engineer in a previous life and am able to read the technical diagrams lol and I will add that the Made in China or in this case Made in Taiwan instruction manuals are either nonexistent or are known to have major issues. 

I will be reaching out to WO and asking them as I don't know if you have to compensate for the 6AIII offset by adding the the 7.1mm to the 55mm to get to the 62.1, maybe something in the design of the FF/FR and if not the case then like I implied the instruction or lack thereof is a problem in our hobby.  

Thanks for the reply.

Edited January 22, 2021 by ratskrad

[david_taurus83]

I used to have the 6AII. The measurements quoted all assume 55mm taken up by the camera from the shoulder. The 6AII has been discontinued for nearly 2 years now so I'm assuming the OP has purchased the 6AIII.

[newbie alert]

Measure the spacing distance,  as a 10mm spacer isn't always 10mm.. digital vernier are really helpful 

[ratskrad]

So I did some digging and went to the William Optics website and found the instructions for some of the other FR/FF. Now it appears from those pictures and diagrams that you lock the 6AIII at the 5.2mm mark and measure from the edge towards the front of the scope including the gap from the 5.2mm spacing and that is where the 62.5mm line to the image plane of your camera starts. When I measured to the edge of the 6AIII I get 40mm and that means 22.5mm of back focus needs to taken up. That is what you have to work with. I know on my two ccd cameras the set back is 17.5mm. I added a 5mm thick x 46mm wide spacer to get to 45mm and adding the 17.5mm for the setback gets me to 62.5mm back focus for the ASI2600MC-P and for the Orion G26 OSC I have a 5mm thick x 46mm male wide with a 42mm female thread on the other side that fits the G26. Makes a lot more sense than what I thought I was seeing, only human here lol. That's why I like to sometimes interact with others in this hobby/obsession that for now occupies way too much of my time, not complaining. This all helps me and I hope others figure out how some of this stuff works. 

 

Brown Dwarf thanks again for getting me to dig into this to better understand as the instructions were lacking as in none and others were confusing lol.  

 

 

[Gan]

On 22/01/2021 at 17:42, ratskrad said:

Hi, I just received my WO GT81 IV last week along with the focal reducer. The two numbers to remember are 7.1mm and 60.2 mm. The 7.1mm is the setting for the focal reducer. Spin the pieces until you see the 7mm mark and turn the locking ring. The second number 60.2mm is the distance for the back focus/spacing to the image plane of the camera. There are no instructions for the WO 6AIII but on the webpage for the WO 6AII FR it at least shows the two surfaces that require the 60.2mm spacing. Scroll down and there is a good schematic of the 6AII showing details.  Here are the links to the WO pages for both the 6AIII and the 6AII. Hope this helps.

https://williamoptics.com/products/accessories/barlows-flatteners/2019-all-new-adjustable-flat6aiii-t-mount-not-included

https://williamoptics.com/new-adjustable-flat6a-t-mount-not-included

very helpful information, Thanks to ratskrad. In regards to GT81 and 6AIII, a small correction if I may: the second number ( Backfocus) for GT81 is 62.1. ( from the same link as above) I haven't used this measurement in practice as yet, but I should try because last night I got very elongated stars in the periphery. I am still struggling with the full kit.