23 replies to this topic

[lionelg]

It seems I've been spoiled by my RedCat 51 since back focus was never an issue. I now have a shiny new William Optics FLT 91 and matching 0.8 flattener (6A III) and haven't managed to get it to focus. Here's where I am:

• I removed the "Rotolock" from the FLT 91 and screwed in the 6AIII flattener/reducer.
• I set the (adjustable) flattener to 5.4mm using the scribed marks. I didn't find this number on the WO website anywhere, but forum posts I've read seem to all be around 5.1 - 5.5mm, so I started there. If I understand things correctly, that *should* mean that the back focus from the back of the flattener is now 55mm. (Note; there are no filters installed at the moment)
• Behind that I have an ASI 533MC with a 55mm back focus setup (i.e. 21mm spacer + 16.5mm spacer + 11mm spacer, when when added to the 6.5mm distance the sensor it set into the camera body adds up to 55mm).

But I can't pull focus. The closest I get is with the focuser all the way in the tube, but it's way out of focus. I'm not sure what to do next, other than try random things. I figured the collective wisdom of the forum was a better way to approach the problem. 

 

Can anybody tell me what I'm doing wrong?

 

I've attached a picture of my setup.

 

Edited by lionelg, 23 September 2021 - 11:38 AM.

[lionelg]

I figured out what I was doing wrong and figured I'd post the answer here in case it helps someone else. (Sure would help if WO produced a manual of some kind for this scope).  Anyway, as shipped, the FLT 91 has a very large extension space attached between the scope and the "rotolock." To attach a camera, you need to remove the spacer, not just the rotolock mechanism. The 6AIII then attached to the actual field rotator on the back of the scope using the M92>M63 Adapter is comes with. Attached pics may help.

 

 

[licho52]

Seems to be a common trap to fall in with these, there's another person in the 91FLT thread who had the same problem.

[Sp@rky]

I figured out what I was doing wrong and figured I'd post the answer here in case it helps someone else. (Sure would help if WO produced a manual of some kind for this scope).  Anyway, as shipped, the FLT 91 has a very large extension space attached between the scope and the "rotolock." To attach a camera, you need to remove the spacer, not just the rotolock mechanism. The 6AIII then attached to the actual field rotator on the back of the scope using the M92>M63 Adapter is comes with. Attached pics may help.

 

2021-09-23_09-27-33-sq.jpg

 

2021-09-23_09-28-10-sq.jpg

Thanks just came across this as i was wondering if that spacer is what they call the visual tube. 

[MAT_Blue]

I just received my new FLT91 with the Flat6AIII.  I have an ASI294MC-Pro camera to attach to it.  The camera has the same 6.5mm of backfocus as your ASI533MC does.  It came with the same 11mm, 16.5mm and 21mm spacers that you have for your ASI533MC.  I plan to remove the "Visual Extension Tube" from the OTA and adjust the flattener to 5.4mm between the silver knurled ring and the body of the flattener (the black part that has the FLAT 6AIII on it).

 

Are you getting good results with this setup?  Flat fields and 0.8x focal reduction?

 

Thanks.

[mphotog]

 

I just received my new FLT91 with the Flat6AIII.  I have an ASI294MC-Pro camera to attach to it.  The camera has the same 6.5mm of backfocus as your ASI533MC does.  It came with the same 11mm, 16.5mm and 21mm spacers that you have for your ASI533MC.  I plan to remove the "Visual Extension Tube" from the OTA and adjust the flattener to 5.4mm between the silver knurled ring and the body of the flattener (the black part that has the FLAT 6AIII on it).

 

 

Are you getting good results with this setup?  Flat fields and 0.8x focal reduction?

 

Thanks.

 

I Believe you are wrong. You will notice on the flattener there’s a gauge on one side. That is where are you adjust the 5.4 mm. Not after the gray ring. That is how I have it adjusted on my FLT 91 and it works perfectly.

Edited by mphotog, 10 March 2022 - 01:13 AM.

[mphotog]

This is how all of William optics flattener/reducer are adjusted

Attached Thumbnails

• 

[MAT_Blue]

I see the gauge now.   I hadn’t unscrewed the adjuster far enough to see it.  So, I set the 5.4 value and leave it there, regardless of what I have behind it?  Or do I try to maintain the same distance from the back of the flattener to the camera for every setup?  I don’t see how the 5.4 value is determined. 

Edited by MAT_Blue, 10 March 2022 - 11:36 AM.

[mphotog]

No, Typically ZWO cameras have a 55 mm backfocus , that flattener needs an additional 5.4 mm adjustment. So in your case the total backfocus will be 60.4 mm

[OldManSky]

I see the gauge now.   I hadn’t unscrewed the adjuster far enough to see it.  So, I set the 5.4 value and leave it there, regardless of what I have behind it?  Or do I try to maintain the same distance from the back of the flattener to the camera for every setup?  I don’t see how the 5.4 value is determined. 

The Flat6AIII works for several WO scopes.  The 5.4 value is set for your scope, such that once that's set you have 55mm of backfocus from the rear of the flattener.  You can use the "standard" adapters that came with your camera to get 55mm spacing in front of the camera.

The adjustability is nice to fine-tune the flattener spacing without having to resort to 1mm rings or such.  But start at the recommended 5.4mm value, get 55mm on your camera, and then test/adjust from there.

[MAT_Blue]

Thanks, everybody.  It all helped.  Now, if the clouds will just go away!

[mphotog]

The Flat6AIII works for several WO scopes.  The 5.4 value is set for your scope, such that once that's set you have 55mm of backfocus from the rear of the flattener.  You can use the "standard" adapters that came with your camera to get 55mm spacing in front of the camera.

The adjustability is nice to fine-tune the flattener spacing without having to resort to 1mm rings or such.  But start at the recommended 5.4mm value, get 55mm on your camera, and then test/adjust from there.

Yeah, do what old man said. He stated it better than I did.

[StevenBellavia]

I am posting to this thread in case others stumble upon it, trying to adjust the Flat 6AIII for their FLT-91

 

I might suggest 3-D printing a spacer ring, and not using the threaded lock ring.

The lock ring I received was not the same thickness all around and was causing tilt, so I 3-D printed my own that fills the entire gap.

 

See posts #9 and 13 on this thread:

 

https://www.cloudyni...-apo-refractor/

 

I have since designed a newer version, that is now 15.4mm thick, which should place the Flat 6AIII close to the 5.4mm mark, which is what is recommended for the FT-91, but of course, start there and adjust as needed. (The threaded lock ring is about 10.0mm thick, hence why the printed rings needs to be 15.4mm thick.  My original print was an arbitrary 14.0mm thick, which is what is shown below, and I made up the space on the camera side).

 

STP and STL files for the new 15.4mm ring are here:

 

https://drive.google...gzK?usp=sharing

 

Steve

 

P.S.  I also purchased a GT-71, with its Flat 6AIII, and that lock ring is exactly the same thickness everywhere.  But I am still 3-D printing a spacer ring for that too, which will be 19.3mm thick, to achieve the recommended 9.3mm number for that scope.  Not needed, but I am still doing it.

Attached Thumbnails

• 

Edited by StevenBellavia, 12 June 2022 - 06:48 PM.

[aae991]

Old thread, but it just saved my butt with my new FLT91 and FLAT 6AIII. I’m trying to get setup quickly to test it tonight. If I had seen this thread I would have been a very frustrated camper!

Thanks for all the invaluable information!

[astropaulb]

Here's another example setup showing what worked well for me after fiddling with it for a bit, used a 6.5mm adjustment on the 6AIII:

 

 

And the fairly decent resulting stars:

 

 

 

[DavesView]

Here's another example setup showing what worked well for me after fiddling with it for a bit, used a 6.5mm adjustment on the 6AIII:

 

 

 

And the fairly decent resulting stars:

 

 

I'm running very close to your numbers. My total back focus for my FLT91 is 60.4 using the 6AIII and results in round stars all the way out to the edges. I'm set at the WO recommended 5.4 mm on the FF.

[DrDispatch]

just bought an FLT91 and the 6AIII.

i have it setup for 53mm back focus (am waiting for 3mm spacer).

as i image with it I notice the stars are good except left side, is this tilt or result of needing adustment?

 

 

 

[StevenBellavia]

just bought an FLT91 and the 6AIII.

i have it setup for 53mm back focus (am waiting for 3mm spacer).

as i image with it I notice the stars are good except left side, is this tilt or result of needing adustment?

 

stars_left.png

One-sided, and side-to-sde aberrations are generally caused by tilt.

 

A 3mm spacer?  I would recommend not using that.  Something that short can cause more issues.  That isn't even enough room for threads?

Can you get to the "correct" spacing another way?  I use the ZWO spacers, and some Baader Delrin shim-spacers to get to 55.7mm, as follows:

16.5 + 12 + 0.5 (Delrin) + 20 (filter wheel) + 0.2 (Delrin) + 6.5 (camera) = 55.7mm

 

Also make sure you are on the correct "number" on the Flat 6AIII

 

Note that my Flat 6A III had a non-uniform locking ring causing tilt, so I 3-D printed a spacer and eliminated it completely, as stated in post #13 above.

 

Steve

Edited by StevenBellavia, 24 April 2023 - 12:31 PM.

[DrDispatch]

One-sided, and side-to-sde aberrations are generally caused by tilt.

 

A 3mm spacer?  I would recommend not using that.  Something that short can cause more issues.  That isn't even enough room for threads?

Can you get to the "correct" spacing another way?  I use the ZWO spacers, and some Baader Delrin shim-spacers to get to 55.7mm, as follows:

16.5 + 12 + 0.5 (Delrin) + 20 (filter wheel) + 0.2 (Delrin) + 6.5 (camera) = 55.7mm

 

Also make sure you are on the correct "number" on the Flat 6AIII

 

Note that my Flat 6A III had a non-uniform locking ring causing tilt, so I 3-D printed a spacer and eliminated it completely, as stated in post #13 above.

 

Steve

Steve,

 

I actually read your posts on the 3d spacer, i have Prusa mini and may have to try that. 

i will look at some shims, i have a few.

 

I also have a ZWO tilt adapter for solar and may see if I can get the left side fixed that way

Edited by DrDispatch, 24 April 2023 - 12:48 PM.

[madtrout]

I have a WO FLT91 with the 6AIII reducer/flattener and a Nikon Z6II full frame camera. The flattener is set to 5.4mm and I'm getting distorted stars with tails pointing inwards at the edges, both sides. I've tried different flattener settings, but still have the distortion. Other posters commenting on a GT81 with the same flattener say it is impossible to correct, and that WO admits it can't correct to the edges fo a full frame sensor. Any advice? Thanks, Mark

[Christine_Z]

I have a WO FLT91 with the 6AIII reducer/flattener and a Nikon Z6II full frame camera. The flattener is set to 5.4mm and I'm getting distorted stars with tails pointing inwards at the edges, both sides. I've tried different flattener settings, but still have the distortion. Other posters commenting on a GT81 with the same flattener say it is impossible to correct, and that WO admits it can't correct to the edges fo a full frame sensor. Any advice? Thanks, Mark

Crop the image...The 6AIII isn't flat across an APS-C sensor with the FLT91. Simply it is not designed for a full frame sensor.

The much larger and more expensive 68III flattener can handle a larger sensor, although not sure about a full frame.

Edited by Christine_Z, 26 May 2024 - 04:32 PM.

[StevenBellavia]

I have a WO FLT91 with the 6AIII reducer/flattener and a Nikon Z6II full frame camera. The flattener is set to 5.4mm and I'm getting distorted stars with tails pointing inwards at the edges, both sides. I've tried different flattener settings, but still have the distortion. Other posters commenting on a GT81 with the same flattener say it is impossible to correct, and that WO admits it can't correct to the edges fo a full frame sensor. Any advice? Thanks, Mark

Russel Croman's BlurXterminator.

You can set the stellar and non-stellar sharpening values to 0, and it still corrects distorted stars very nicely.

 

https://www.rc-astro.com/

 

Note: I have no affiliation with RC-Astro.  I am just a satisfied customer.

 

(:

 

Steve

[madtrout]

Thanks for that suggestion, but it seems to only work with PixInsight, and I'm using Affinity Photo. I'll investigate the 68III, or just live with the crop. 

Edited by madtrout, 26 May 2024 - 07:22 PM.

[Djennings]

Hey Everyone, I was conflicted on this flattener/focal reducer setting so I took both rings down so I could see the gauge on the focuser itself. I then took my micro meter gauge and set it to 5.4 mm. Set one point on the zero and the other was just south of the 5 mark, not two marks over as each small mark is 1 mm. As you will notice that it goes from 0 to 5, 5 to 10, etc.. so each mark is 1 mm thick. So for those looking to get a 5.4 mm setting, just go past the 5 mm mark by just a hair, if that and tighten it down. Then set you 55 mm length for your camera. 

 

Hope this helps..

 

Thanks,