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Do we think about corrector backfocus the wrong way around?


pipnina

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Having seen many posts about the troubles of achieving proper backfocus spacing, and having my own issues with the topic myself, I have begun wondering if equipment manufacturers, as well as ourselves, are thinking about the business of corrector spacing all backwards.

It seems pretty expected that we buy a corrector, and then add things between it and the camera, be they female-male spacers, shims, off axis guiders, tilt adapters etc etc. But because of how accurate we need to be for most correctors to the ideal spacing, we end up outside in the cold constantly unscrewing our cameras or opening our filter wheels to add and remove and change our camera train to get that spacing just right, needing an autofocus routine every time we make any changes. It's slow, tedious, and exposes our kit to the elements and increases the risk of dropping things. My filters were nearly pristine when I first sealed them in the filter wheel, but I have had to spend so much time fiddling with backfocus this year they are starting to get a bit coated with the sticky kind of dust that doesn't just blow off! I am not keen to try and clean them but the dirt I see in my subs does annoy me.

And so I wonder, why don't we move the corrector instead? If correctors were fitted with a kind of helical focuser, that could move the lenses inside up/down the barrel, we could add our bits together in the camera train to achieve a rough spacing from the corrector lenses to the camera, but when it comes to fine-tuning it in the field, we'd just need to undo the locking knob on the corrector and shift the corrector lenses up-down a bit until we get to perfect spacing that way.

No finnicky disassembly, no dirty filters or camera sensor windows, no ice cold fingers trying to handle and tell apart 0.5mm shims from 0.8mm shins. Just turn the helical focuser of the corrector by 1/8th a turn and take another photo...

How has this not been done yet?! Do we suffer all of this because of the upfront cost of building the correctors into their own mini-focuser, only to spend that cost on shims and spacing rings (and our sanity) instead?

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12 minutes ago, raadoo said:

You could have a go using one of the variable adapters that you see from TS / Altair / Optec? They promise to allow you to fine tune the backspacing without taking things off the imaging train.

I have no personal experience with these adapters, but perhaps SGL members who do can chime in.

The big problem with adapters like these is they consume a lot of backfocus. This one starts at 20+mm, which is as thick as my filter wheel! They could be suitable for some of the fancy refractor, or typical cassegraine correctors, but where most correctors ask for a 55mm~ spacing, you'd likely have to give up an important bit of kit to accomodate one of those. I think that adaptor also would rotate the camera as you perform an adjustment? I can't quite tell.

It's an option I've considered but they don't seem like they'd be helpful for anyone using a 55mm spaced corrector, unless they only need the camera and an OAG perhaps.

It also moves the camera so definitely affects focal position, I don't know how much moving the corrector only affects the focal plane position but it could cause less motion and allow easier "real time" view of the corner star shapes changing.

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On 15/12/2022 at 13:42, pipnina said:

The big problem with adapters like these is they consume a lot of backfocus. This one starts at 20+mm, which is as thick as my filter wheel! They could be suitable for some of the fancy refractor, or typical cassegraine correctors, but where most correctors ask for a 55mm~ spacing, you'd likely have to give up an important bit of kit to accomodate one of those. I think that adaptor also would rotate the camera as you perform an adjustment? I can't quite tell.

It's an option I've considered but they don't seem like they'd be helpful for anyone using a 55mm spaced corrector, unless they only need the camera and an OAG perhaps.

It also moves the camera so definitely affects focal position, I don't know how much moving the corrector only affects the focal plane position but it could cause less motion and allow easier "real time" view of the corner star shapes changing.

It seems someone at TS has been listening. They just released an adjustable corrector: https://www.teleskop-express.de/shop/product_info.php/info/p16155_TS-Optics-Full-Format-0-8x-Corrector-for-APO-Refractors---M92x1-Connection---ADJUSTABLE.html 

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1 hour ago, raadoo said:

I like the idea but it would be perfect if the lens assembly itself moved independently inside the housing. That way you should stay relatively focused while you adjusted in live view or camera looping subs quickly.

Or as Adam points out above, set it to factory specs and cheat with BlurX!

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