GT81 and Flat6aiii backfocus

[Wyvernp]

Morning,

I'd love to get some advice from anyone who uses the GT81 and Flat6aiii, what are you using as your backfocus distance? I've read online anywhere between 55, 60.1 and 62.1 and am struggling to get my corner stars shapes to be anything but streaks.

any advice would be appreciated.

Wyvern

[Wyvernp]

Additionally I'm sure I remember seeing a picture somewhere where it indicated that depending on which direction the stars streaks were pointing i.e. from the corner to the centre or 90 degrees from that. That the backfocus distance needed to be increased/decreased? an I just making that up?

[michael8554]

Sorry, I've only got a sketch of that chart.

If it's too close, the stars point to the corners.

If it's too far, the stars form "star trails", as if Polaris was in the centre of the image.

But a past discussion implied the Too Far and Too Close labels were reversed !

Flatteners for mirrored scopes and for refractor scopes  require optics with opposite correction ?

Try a gross offset to check which is true for your setup.

Michael

[Wyvernp]

Thanks, I've already done a little analysis with various backfocus distances but I'm still too much of a beginner to be able to know what they mean.

61mm

73mm

Hopefully someone can point me in the right direction.

 

[david_taurus83]

37 minutes ago, michael8554 said:

Sorry, I've only got a sketch of that chart.

If it's too close, the stars point to the corners.

If it's too far, the stars form "star trails", as if Polaris was in the centre of the image.

But a past discussion implied the Too Far and Too Close labels were reversed !

Flatteners for mirrored scopes and for refractor scopes  require optics with opposite correction ?

Try a gross offset to check which is true for your setup.

Michael

I had a thread about this and I may have said this. In fact, I have seen both kind of distortion either side of optimum distance. I think at the time I was investigating whether filters in the image train needed to add or minus an amount of back focus.

I found the quoted distance on my old WOGT71 and Flat6A II to be as best as I could get.

Your centre stars look out of focus BTW and this can exaggerate the effect at the corners.

[david_taurus83]

2 hours ago, Wyvernp said:

Morning,

I'd love to get some advice from anyone who uses the GT81 and Flat6aiii, what are you using as your backfocus distance? I've read online anywhere between 55, 60.1 and 62.1 and am struggling to get my corner stars shapes to be anything but streaks.

any advice would be appreciated.

Wyvern

According to this you should be somewhere between 5.1 to 7.1mm on the adjustment, assuming you have 55mm from the shoulder. Pretty much what you've said above. I spent many an hour trying to "perfect" my spacing. Try too close at 55mm with the ring closed and work out from there. My corners were never as bad as yours though.

 

[Laurin Dave]

Having just bought a GT71 and Flat 6A-III I'm hoping that the info on the attached will help....

https://www.firstlightoptics.com/reducersflatteners/william-optics-adjustable-flat6a-iii-08x-reducer-flattener.html

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

However on the FLO page there seems to be a discrepancy between the "Adjustable Distance" (7.1mm for the GT81) and the backfocus distance quoted of 60.1mm which to my mind would give an "Adjustable Distance" of 5.1mm.. (ie 60.1 -55) whereas on the William Optics page the "Adjustable Distance is quoted as 7.1mm which is consistent with the backfocus quoted there of 62.1mm.   

I'd assume that the WO page is more up to date and therefore suggest you try 62.1mm with the flattener adjustable distance set to 7.1mm and see how this works, hopefully pretty close..  Mine GT71 is set to 9.1mm and I shall be testing soon ..  

HTH

Dave

[Wyvernp]

I'll give it another go tonight and see what I can do.

9 minutes ago, david_taurus83 said:

Your centre stars look out of focus BTW and this can exaggerate the effect at the corners.

Yes, in addition to the backfocus I'm also fighting backlash issues on my autofocuser.

I think my confusion comes from different backfocu distances being stated for the flat6aiii depending on which site you look at. i.e.

https://www.cyclopsoptics.com/flattener-reducer/william-optics-flat6a-iii-0-8x-adjustable-reducer-flattener/

and

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

[Wyvernp]

5 minutes ago, Laurin Dave said:

I'd assume that the WO page is more up to date and therefore suggest you try 62.1mm with the flattener adjustable distance set to 7.1mm and see how this works, hopefully pretty close..  Mine GT71 is set to 9.1mm and I shall be testing soon ..  

Yes, this is exactly the discrepancy I saw, I am 7.6mm tonight as I'm .5mm short of 55mm.

Thank you all for your help.

 

edit. I may even have to go a bit further to compensate for filter thickness.

Edited June 25, 2020 by Wyvernp

[The Admiral]

This is a timely post as I've just ordered this reducer/flattener for use with my Altair 102mm, with a fl of 710mm (the same as the Z103)! Pity that the light skies will make setting up a little problematic. I very much like the idea of having a built in adjuster, it should save a lot of messing with spacers.

The current info on the WO page is at least consistent

 

2 hours ago, michael8554 said:

If it's too close, the stars point to the corners.

If it's too far, the stars form "star trails", as if Polaris was in the centre of the image.

But a past discussion implied the Too Far and Too Close labels were reversed !

Flatteners for mirrored scopes and for refractor scopes  require optics with opposite correction ?

That's my understanding too, but you've thrown in a bit of doubt with the statement about the labels being reversed! I suppose it'll come down to sucking it and seeing, but I'd be interested to read something on the basic principles at play here.

Ian

[Wyvernp]

I'll definitely keep this topic updated on my progress.

[Wyvernp]

well I've had a bit of a play and things are looking better but still needs some work

[hadyn42]

Hello Guys

Just going to mention that was a notice on here a couple of days ago mentioning that there are a faulty batch of WO Flatteners out there.

Best regards,

Hadyn - IOM

 

[The Admiral]

5 hours ago, hadyn42 said:

Hello Guys

Just going to mention that was a notice on here a couple of days ago mentioning that there are a faulty batch of WO Flatteners out there.

Best regards,

Hadyn - IOM

 

Can you provide  link please?

I've found a link:

Ian

 

Edited June 26, 2020 by The Admiral

[Wyvernp]

I remember now I promised to update this topic, I now have my backfocus set at 60.4mm and all star shapes all the way to the edges are now round.

It's not 100% perfect on close inspection but the difference between this and the previous mosaic is chalk and cheese...and I love cheese.

Edited July 15, 2020 by Wyvernp