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EdgeHD 8" and TV PowerMate 2x results in crazy backfocus

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#1 vriesk


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Posted 08 November 2019 - 10:19 AM


I have the TV PowerMate 2x, which I specifically want to use for Solar and mayhaps Lunar/Planetary photography. I have the TV T2 adapter attached to it.

The stated optimal backfocus for photography with the Edge HD 8" is 133.35mm. The TV website claims something about parfocality of the PowerMates, but it is unclear to me what exactly do they mean - are they parfocal to each other, or are they focally "invisible" when inserted to the train - and if so, whether they're invisible when using the T2 adapter (~31mm* stack length), the 2" eyepiece (~71mm stack) or 1.25" eyepiece with the supplied reduction (~81mm stack). In addition, it is not clear whether that statement is expected to accomodate for the added length of the 2" baffle of the PowerMate, which can't fit inside the threaded end in the scope, so needs to stick out those additional ~50mm or so. Add 6mm for a filter as well.

So I went to check that experimentally. I took the official Celestron T2 adapter, attached it to a camera and confirmed the whole stack sums up to those ~133mm. I have focused the scope and locked the clutches, making sure I don't touch the focus knob later on.

Then I attached the Powermate (with extra ~6mm or so space, not directly touching the OTA thread). And... hey, where did that go? The focus plane is now something like 200-250mm behind the TV T2 adapter (this is a very rough estimate, as it required me basically handholding the camera behind the OTA and trying to guess the focal plane with it). Now that's nothing I expected.

With something around 5-7x full focus knob turns, I'm able to bring the focal plane close to the TV T2 adapter - 80-100mm from it, which is something more along my expectations. What's going on?

What would be the optimal way to mount this? Adding spacers to cover the discovered 200mm+ backfocus will result in ridiculously long (and wobbly) imaging train. Should I somehow guesstimate compromise between train length and focus knob manipulation (say, go only 3 full turns)? Has anyone figured this out? Is there something fishy with my PowerMate piece?

To add clarity, the train schema is the following:

Edge HD 8" OTA -- Baader Clicklock 2" visual back** -- 2" Baader solar continuum filter (~6mm) -- PowerMate 2x -- TV T2 adapter -- ?? T2 spacers ??? -- Nikon F T2 adapter - Nikon DSLR

* The numbers above might be somewhat off here and there - but given the huge backfocus drift, I don't think it matters that much.
** The Clicklock visual back is not relevant for the calculation, as it's not adding any length to the train as the PowerMate with filter sits directly on the OTA's thread anyway.

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