I need some advice with backfocus - Forums

Topic starter

Apr 30, 2022

Hi everyone!

I own an Esprit 80ED refractor (super apo triplet) 400mm f/5. I shoot with a non modified DSLR and I want to update to a cooled dedicated camera (an ASI 533 MC Pro). So, I was investigating about backfocus for my fielt flattener (not reductor). The flattener came with the Esprit and the manual explains that the backfocus distance between the flattener and the camera sensor needs to be 66mm (2,4 inch).

Then, I searched the different accesories I use now and the threads schema is that one:

Ok, the flattener has an M66 outlet and therefore I have to put its thread from M66 to M48
This already takes 6 mm, it is what I have underlined in yellow. I understand that this is the distance I have to look at and not the 5.9mm . I don't know if anyone can guide me better and tell me if I'm wrong.
So from the 66mm backfocus I already have 6 mm less and there are 60mm left.

Then we must take into account the depth of the sensor from the ASI casing and the extension that it already has.

So those 11 + 6.5 mm together with the other 6 mm are already 23.5 mm and to cover the 66 backfocus I still need 42.5 mm
I look at the parts drawing of the ASI533 that appears in its manual and it is:

All right, the extenders are 16.5mm and 21mm but if I add them to the 23.5mm that they already had, I get 61mm. Where do I get the 5 mm that I still need? Of those rings that they call "spacer" I think that they are of different thickness in mm but I have not seen in the manual where it says how much each one is to know if with those or with some of them I already cover those 5 mm. Can someone shed some light for me please?

Many thanks!

Daniel

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Andy Wray

Apr 30, 2022

I think you have answered your own question. You need another 5mm to get to a 66mm backfocus distance. These are readily available on Amazon or telescope sites. Either look for M42x0.75 or T2 5mm extension tubes.

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Daniel Arenas

Topic starter

Apr 30, 2022

Andy Wray:
I think you have answered your own question. You need another 5mm to get to a 66mm backfocus distance. These are readily available on Amazon or telescope sites. Either look for M42x0.75 or T2 5mm extension tubes.

Thank you Andy,

Maybe I haven't expressed myself in a correct way.
What I wanted to know is whether my thoughts were correct or not. I have read a lot for this camera and the backfocus and this afternoon I calculated what I need only in my case looking at manuals etc

jewzaam

May 1, 2022

You are correct, you need 5mm extension as pointed out already. The spacers provided for ZWO cameras are to be placed between components. They are thin: 0.1mm, 0.2mm, 0.5mm and maybe 1mm (I can't remember). They're used to tweak backfocus if you need little adjustments.

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Daniel Arenas

Topic starter

May 1, 2022

You are correct, you need 5mm extension as pointed out already. The spacers provided for ZWO cameras are to be placed between components. They are thin: 0.1mm, 0.2mm, 0.5mm and maybe 1mm (I can't remember). They're used to tweak backfocus if you need little adjustments.

Thanks jewzaam 🤗

Steven

May 1, 2022 · Edited: May 1, 2022

The Esprit 80 manuals have been a disaster of errors and mistakes ever since it came out.
It doesn't help that there are 2 different Esprits out there, old and new.

- The old requires 61mm back focus and came with a 6mm adapter ring. (to allow for 55mm)
- The new requires 66mm back focus and comes with a 11mm adapter ring. (to allow for 55mm)

But, even though they changed that, they only changed a part of the manual. You're looking at a manual that is a mix of both.
Telling you to get the 66mm of the new Esprits, but it is still showing the 6mm ring of the old Esprits

the M66 to M48 adapter came with your Esprit, that you screw onto the field flattener, is 11mm.. Not 6mm.
There is your missing 5mm that you're looking for, you already have it, it's just an error in the manual.

To get 66mm, you just need that adapter ring + 55mm, so this should work if you screw this onto the M66-M48 adapter that comes with your Esprit. (You might need finetune and maybe add a 1mm spacer somewhere if you're using a filter)

Helpful

Daniel Arenas

Topic starter

May 1, 2022

The Esprit 80 manuals have been a disaster of errors and mistakes ever since it came out.
It doesn't help that there are 2 different Esprits out there, old and new.

- The old requires 61mm back focus and came with a 6mm adapter ring. (to allow for 55mm)
- The new requires 66mm back focus and comes with a 11mm adapter ring. (to allow for 55mm)

But, even though they changed that, they only changed a part of the manual. You're looking at a manual that is a mix of both.
Telling you to get the 66mm of the new Esprits, but it is still showing the 6mm ring of the old Esprits

the M66 to M48 adapter came with your Esprit, that you screw onto the field flattener, is 11mm.. Not 6mm.
There is your missing 5mm that you're looking for, you already have it, it's just an error in the manual.

To get 66mm, you just need that adapter ring + 55mm, so this should work if you screw this onto the M66-M48 adapter that comes with your Esprit. (You might need finetune and maybe add a 1mm spacer somewhere if you're using a filter)

Oh! That’s a great information. I will measure that adapter ring to be sure that it’s 11mm.

Thousands and thousands of thanks.

Daniel Arenas

Topic starter

May 2, 2022 · Edited: May 2, 2022

@Steven de Vet

I've measured the M66 to M48 ring and that's the point. It's an 11mm!

Thanks a lot dude!

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