Am I fudged?

[CKemu]

So I have the Skywatcher 100ED Field Flattener ("FF" from now on) and the following imaging train:

FF > M48 to M54 Adapter (2mm)* > OAG (24mm) > Filter Wheel (22mm) > Baader Filters (0.7mm) > ATIK 490EX (13mm)

*this is on order from FLO, so currently unable to test this setup.

61.7mm total
Required back focus is 55mm ...fuuuuuuu-dge.

6.7mm which I can't really shave off, but I have an imaging circle of 40mm and the chip diagonal is 15.97mm, do you think I will get away with it? I see posts with people sweating over 1mm, but they're often DSLR users! Perhaps this is something that can't easily be answered, gosh I wish there was a tool to simulate this m'larky!

[newbie alert]

What oag are you using?

[tomato]

That does seem a lot of mm for your OAG, I use a ZWO item, 16.5mm thickness, which would just about make it.

[CKemu]

59 minutes ago, newbie alert said:

What oag are you using?

 

33 minutes ago, tomato said:

That does seem a lot of mm for your OAG, I use a ZWO item, 16.5mm thickness, which would just about make it.

I'm using the ATIK OAG for their EFW2.

[tomato]

I guess you could screw it all together then see just how much of the sensor FOV is unusable (if any).

[The Lazy Astronomer]

FYI, don't count the filter thickness in your backfocus, instead add 1/3 the filter thickness on to your required backfocus. 

Doesn't really help much though as you'll still be ~5.8mm out. But worst case is you may need to look for a thinner OAG, or perhaps a Starlight Express filter wheel with a built in OAG. Frustratingly for you, it may require more £££ to be spent, but not a horrific outcome (your title had me thinking something had gone badly wrong!)

[CKemu]

11 hours ago, tomato said:

I guess you could screw it all together then see just how much of the sensor FOV is unusable (if any).

Think that's the plan, take a few shots and check out the stars afterwards - for a hobby that requires patience, I am terrible at having patience and hate wasting clear nights though! ha
 

10 hours ago, The Lazy Astronomer said:

FYI, don't count the filter thickness in your backfocus, instead add 1/3 the filter thickness on to your required backfocus. 

Doesn't really help much though as you'll still be ~5.8mm out. But worst case is you may need to look for a thinner OAG, or perhaps a Starlight Express filter wheel with a built in OAG. Frustratingly for you, it may require more £££ to be spent, but not a horrific outcome (your title had me thinking something had gone badly wrong!)

The ZWO OAG is quite thin, no idea if it would attach to my ATIK EFW2 - will have to research, but that would save a good chunk of space. More £££, well I can cope with a few more if it gets me in to a position where I can use the full sensor width - but I shall test this once the part comes and "waste" a clear night.

The title stems from frustration more than actual disaster!

[CKemu]

It is never easy eh...........manufacturing companies need to create a "BASIC" set of mechanical conventions and tolerances,.....because DEAR FUDING LORD my patience is being worn thin right now.