Please could you check this WO FLT 120 image train?

[JCIdaho]

Hi, I'm buying a new rig to send out to a remote site.

I'd really appreciate any second pair of eyes on the following image train. The backfocus for the 68III should be 55mm, I hope this will be close enough..

	Aperture In	Aperture Out	Length
WOFTL120		M92F
68III Field Flattener	M92M	M48M	17.5
M48F to M68M	M48F	M68M	1.4
Pegasus Falcon Rotator v2	M68F	M68F	15.5
M48M to M68M	M68M	M48M	1.4
M54M-M48F-2	M48F	M54M	2
Zwo Filter Wheel	M54F	M48	20
ZWO Tilt Plate	M48		5
ZWO asi2600mm			12.5
Total			57.8

 

thanks!

[tomato]

Are you sending the kit direct to the remote site from the supplier? I would recommend setting up in your backyard and trying it out before sending it off if you can.

To try and answer your question I use Esprit 150s with the matching SW FF which calls for a 55 mm backfocus. I get to around 1 mm of this and have no issues with an APS-C sensor, but I do routinely use BXT in my processing workflow.

 

[globular]

30 minutes ago, JCIdaho said:

Instead of these two adapters, pegasus do an M68M to M54M at 1.4mm .... so you save 2mm and end up at 55.8 instead of 57.8.

https://www.firstlightoptics.com/pegasus-astro/pegasus-adapter-m68-male-to-m54-male.html

 

[JCIdaho]

@globular thank you - will do!

[JCIdaho]

44 minutes ago, tomato said:

Are you sending the kit direct to the remote site from the supplier? I would recommend setting up in your backyard and trying it out before sending it off if you can.

 

thanks, I'll get to test a couple of nights in advance - so not much of a window. Am flying out to the remote site prior to handing over. I've ordered some spacers etc just in case!

[Laurin Dave]

Can't you put the the flattener on the camera side of the rotator thereby permitting the correct backfocus ?   This will also mean that your flats are invariant to camera rotation.

Dave 

[JCIdaho]

Hi @Laurin Dave - I've considered this, but I can't find an adapter that goes from M68 - the rotator, to the flattener (M94M)....

[Laurin Dave]

7 hours ago, JCIdaho said:

Hi @Laurin Dave - I've considered this, but I can't find an adapter that goes from M68 - the rotator, to the flattener (M94M)....

Have you considered having one made?   Teleskop-Express: TS-Optics custom-made Adapter-Sonder125    

Dave

[JCIdaho]

@Laurin Dave if the rotator goes before the flattener, the light cone is trimmed to ~68mm, so a more shorter cone going through the flattener. Presumably no concern with that? Thanks 🙂

[Laurin Dave]

31 minutes ago, JCIdaho said:

@Laurin Dave if the rotator goes before the flattener, the light cone is trimmed to ~68mm, so a more shorter cone going through the flattener. Presumably no concern with that? Thanks 🙂

Can't imagine so as APSC diagonal is 28mm

[JCIdaho]

Thanks @Laurin Dave I've ordered the bespoke part. If it works, would be good for the flats, and maybe to add an OAG down the line.

[globular]

23 hours ago, JCIdaho said:

The backfocus for the 68III should be 55mm

Where did you read this @JCIdaho?

According to the WO website....

https://williamoptics.com/products/flat68iii

... the 68iii in combination with the FLT 120 needs...

The 'Adjustment Distance' is done with the 68iii itself; like this.....

And the back focus of 65.7 is then from the flattener to the image plane.

And I suspect that all this is optimised for the flattener directly connected to the telescope thread.  If you put something between the telescope and the flattener then it won't be optimal at these distances - and may not be optimisable at all.

HTH

[JCIdaho]

Hi @globular - Thanks! I have to say, it's extremely confusing. Looking at what you posted, I would agree the backfocus from the end of the flattener to the sensor should be 65.7mm.

But, have a read of this: https://www.cloudynights.com/topic/833579-william-optics-flat-68iii/.

That convinces me the backfocus should be 65.7 - 10.7mm = 55mm.

If this is indeed the case, then their website/specification is v poor and confusing?

As for what you say "And I suspect that all this is optimised for the flattener directly connected to the telescope thread..."

Agree - this is my hunch too. I'm tempted to test it anyway, and gamble the cost of the custom adapter if it doesn't work. The adapter may one day (?) come in handy if I don't use!

 

 

Edited August 22 by JCIdaho

[JCIdaho]

On balance, I think it would work with the rotator in between the flattener and the scope, but then I'd need to reduce the 10,7 accordingly. I'll certainly report back!

[globular]

4 minutes ago, JCIdaho said:

this is my hunch too. I'm tempted to test it anyway, and gamble the cost of the custom adapter if it doesn't work. The adapter may one day (?) come in handy if I don't use!

Agreed. Experimenting yourself is the way to go - if you can afford a few mistakes.