Back spacing query

I’ve received the Vixen Reducer and Flattener for my SD81s. I’m trying to find out how much back spacing is required. Judging by the attached diagram I think I need 63.5mm if I’m using the reducer and 129.5mm if I’m just using the flattener. Am I reading this right? 129.5mm seems a lot.

Also, the reducer is used in conjunction with the flattener but it’s not placed 129.5mm from the flattener from what I’ve seen.

Any guidance gratefully received!

According to the diagrams which you attached you would need 137.5 +/- 1 mm of backfocus with just the flattener (which extends slightly the focal length) and if you use the flattener+spacer ring+reducer you'd need 54/56 +/- 1 mm of backfocus. Now, it is not that you really "need" anything since Vixen will give you all the required hardware except the camera if you buy the suggested accessories. Considering this, your backfocus requirement is that of the T-ring (usually 10 mm but also between 9 and 12) and the backfocus of the Canon cameras, which is 44 mm, ergo 54/55 mm plus or minus 1 mm. Consider buying spacers in 0.2mm increments.

Thanks. I have an ASI533 camera which comes with 17.5mm of back spacing (I think) before anything else attaches to it so it’s just a question of maths after that? The camera comes with everything to give 55mm of spacing so that sounds easy.

I guess with a small sensor it’s not that critical anyway?

Piers Palmer:
I guess with a small sensor it’s not that critical anyway?

It depends on how picky you are about star shapes. But, yes, I expect that at those focal ratios once you're within 1/2 mm of the best position it won't make much of a difference.

Thanks - I'm not picky!

May I just be needy for a minute and ask how you arrived at the distances you recommend as I can't see anything in the diagram which results in those distances. I'm sure you're right…I'd just like to understand for my own future reference!

If you are planning to use filters with this set up, please don't forget to take their thickness into account. So many people do and They make a difference with your backfocus calculation.

Thanks, yes. You add one-third the filter's physical width I think?

Piers Palmer:
Thanks - I'm not picky!

May I just be needy for a minute and ask how you arrived at the distances you recommend as I can't see anything in the diagram which results in those distances. I'm sure you're right...I'd just like to understand for my own future reference!

I'm sure you're familiar with the arrangements shown above the picture you posted. So, whatever your choice is (flattener or flattener+reducer) you'll end up with all the require hardware sans the camera and camera T-ring adapter. They use a Canon EF as illustration point and its lens flange to sensor distance is 44 mm. The thickness of the T-ring coupler varies, but the original one (for the Nikon F flange to sensor distance of 46 mm ) had 9 or 10 mm optical thickness which ended up giving a flange to sensor distance between 55 and 56 mm (which was mostly adopted for the Canon T-thread adapters), which is why nowadays everyone and his/her mate in the industry is using 55 mm as "typical" back-focus requirement for astro-cameras.

I think this is where I'm making it much harder than it is. The diagram above, for the reducer, has a distance marked fB = 63.5. If that was the required back focus distance, then if I take the 17.5mm built-in to my ZWO camera, leaves me needing to provide 46mm….which I can do quite easily, but you're saying I need to end up with 55mm (or the 17.5mm plus 37.5mm I guess) which, again, is easy for me to do.

I used to find apostrophes really really tricky, then all of a sudden I got how they worked. I think back spacing is my astro-imaging equivalent of apostrophes!

I just ignored those numbers and instead focused on the last bit, the one you need to provide yourself: the camera and the T-ring adapter (this is regardless on whether you are using a DSLR or not). All the other bits should be included in whatever package you want to buy, inclusive of this so-called "wide photo adapter", which I believe is where the difference in numbers comes from.

andrea tasselli:
According to the diagrams which you attached you would need 137.5 +/- 1 mm of backfocus with just the flattener (which extends slightly the focal length) and if you use the flattener+spacer ring+reducer you'd need 54/56 +/- 1 mm of backfocus. Now, it is not that you really "need" anything since Vixen will give you all the required hardware except the camera if you buy the suggested accessories. Considering this, your backfocus requirement is that of the T-ring (usually 10 mm but also between 9 and 12) and the backfocus of the Canon cameras, which is 44 mm, ergo 54/55 mm plus or minus 1 mm. Consider buying spacers in 0.2mm increments.

Just out of interest, where do you come up with your numbers from based on that diagram? I can't see anything on there that would result in 137.5mm or 55mm backfocus requirement. The flattener alone says 129.5mm and the reducer says 63.5mm. I presume the spacer would go in between them.

Am I missing something?

It looks to me like the back focus for the flattener alone is 129.5 mm. The back focus for the reducer alone is 63.5 mm.

If you use the flattener and the reducer together, the spacer ring that connects them puts the flattener and the reducer at the correct distance to each other. Then the resulting back focus of both together would still be the 63.5 mm of the reducer since it would be the nearest element to your sensor..

The back focus of your camera is 17.5 mm. What we don't know is will you be using a filter wheel, OAG, or other filters/accessories that you will have to take into account? Then just subtract the BF of your camera, all accessories, and the adapters that connects them from the stated BF of the nearest element (either reducer, or corrector)

Steve Cooper:
It looks to me like the back focus for the flattener alone is 129.5 mm. The back focus for the reducer alone is 63.5 mm.

If you use the flattener and the reducer together, the spacer ring that connects them puts the flattener and the reducer at the correct distance to each other. Then the resulting back focus of both together would still be the 63.5 mm of the reducer since it would be the nearest element to your sensor..

The back focus of your camera is 17.5 mm. What we don't know is will you be using a filter wheel, OAG, or other filters/accessories that you will have to take into account? Then just subtract the BF of your camera, all accessories, and the adapters that connects them from the stated BF of the nearest element (either reducer, or corrector)

This is exactly what I was thinking also. If you had OAG (16.5mm), Filter Wheel (20mm) and Camera Backfocus (17.5mm); add 1mm for filter effect then you would need a 10.5mm extension tube to reach backfocus.

Thanks - that's how I was reading the diagram so is what I'll work with. I have also asked Vixen directly so hopefully they can confirm.

What I find odd (and the main reason I asked here) was this information was only tucked away on one page of one document on the global Vixen website. It's not mentioned anywhere in the manual that comes with the hardware and it's not marked on the items anywhere. I would have thought for something critical to the products' use, they'd make it very clear.

As for achieving the spacing, the ZWO camera comes with the extension tubes to give 55mm of spacing, but I have a variety of additional extension tubes ranging from 0.3mm to 9mm, so I'll manage something! My filter holder is 21mm so I think it is just down to maths now. Do I measure from the bottom of the thread, rather than the glass element? That's how I read the diagram.

I wonder why I need to use an additional spacer with my SD81, but if I had the 103 or 115, I wouldn't need to. It doesn't matter, just curious!

Thanks everyone. If Vixen get back to me, I'll let you know.

Vixen have confirmed the reducer requires 63.5mm of back spacing. They didn’t understand my question regarding the back spacing requirement for the flattener, but I’ll go with the distance in the diagram.

thanks everyone!

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