Thread on etalon Lunt 50

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Ivan
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Thread on etalon Lunt 50

Post by Ivan »

Hi everyone, sorry for the stupid question, but what thread does etalon Lanta 50 have?

I took a photo of Gingerfish from this topic:
viewtopic.php?f=9&t=30724
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Re: Thread on etalon Lunt 50

Post by GreatAttractor »

It's M68x1.


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Re: Thread on etalon Lunt 50

Post by Ivan »

GreatAttractor wrote: Wed Apr 28, 2021 10:31 am It's M68x1.
Thank you, I have read your report on the modification of the Lunt 50 many times, there is even a drawing where the thread M68x1 is indicated, but there it is external and it confused me a little.

I'm trying to make my first mod. Could you answer a few questions that are probably also stupid?

1. I want to use WO megrez 90. It is a short refractor with a focal length of 621mm. F=6.9. This is pretty close to f7, but still not quite what you need. Tell me, do you need to aperture it to get f7, or can you leave it as it is?

2. I want to calculate the length of the adapter between the red cylinder Lunt 50 and megrez 90.
I read in your report that you first need to focus using OTA Lunt 50. (I also used a camera and a laptop.)
Then you need to unscrew the white tube with the lens. And the red cylinder with the camera (instead of blocking filter, I used a prism), without touching the Lunt 50 focuser, bring it to the new telescope.
Holding this design in your hands, you need to focus again and measure the distance to the new telescope from the red cylinder.
This will be the length of the adapter.

I forgot to write that I focused on infinity using objects on the horizon for this purpose.

I repeated all these steps I got that using the eyepiece I was missing 53mm, using the qhy 5r ll c camera was missing 55mm, using the qhy img0s was missing 57mm.

Of course, holding all this in the hands of accuracy is not achieved, but
the question is whether I did everything correctly and why are the results so different?

3. If you defocus the OTA Lunt 50, remove its lens, and repeat all the steps with the new megrez 90 lens, then I got 55-56mm.
It turns out that the focus of the OTA Lunt 50 is not so important before determining the length of the adapter?

Maybe I'm doing something wrong?

Thanks


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Re: Thread on etalon Lunt 50

Post by GreatAttractor »

1. It's fine. If the F-number is slightly smaller, the input beam is a bit too wide and gets truncated, therefore the effective aperture is reduced; but 6.9 vs. 7 doesn't make much difference.

2. I suspect the difference between cameras is down to measurement error. If you're sure it's real and you get it consistently, then maybe there's some additional effect I'm not seeing.

The difference when using eyepiece is probably due to eye accommodation. I.e., when you observe with the Lunt OTA, your eyes were maybe fully relaxed and focused at infinity, but with the Megrez - focused slightly closer, or vice versa. Hard to judge it consciously, that's why a camera is preferred.

3. When you're out of focus with the Lunt OTA, and then (without touching the Lunt focuser) you refocus by sliding relative to the Megrez, I'd think the converging beams from the objective are no longer fully collimated inside the etalon assembly - this would be detrimental to image quality in Hα. But since you're saying the end result is close, apparently something cancels out here; I'd have to do some drawing/manual raytracing to see why.

Anyway, I recommend the Lunt-OTA-in-focus approach; even if something's a little off with it, the end results are fine, as you can see from my mod.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Thread on etalon Lunt 50

Post by Ivan »

GreatAttractor wrote: Wed Apr 28, 2021 8:34 pm 1. It's fine. If the F-number is slightly smaller, the input beam is a bit too wide and gets truncated, therefore the effective aperture is reduced; but 6.9 vs. 7 doesn't make much difference.

2. I suspect the difference between cameras is down to measurement error. If you're sure it's real and you get it consistently, then maybe there's some additional effect I'm not seeing.

The difference when using eyepiece is probably due to eye accommodation. I.e., when you observe with the Lunt OTA, your eyes were maybe fully relaxed and focused at infinity, but with the Megrez - focused slightly closer, or vice versa. Hard to judge it consciously, that's why a camera is preferred.

3. When you're out of focus with the Lunt OTA, and then (without touching the Lunt focuser) you refocus by sliding relative to the Megrez, I'd think the converging beams from the objective are no longer fully collimated inside the etalon assembly - this would be detrimental to image quality in Hα. But since you're saying the end result is close, apparently something cancels out here; I'd have to do some drawing/manual raytracing to see why.

Anyway, I recommend the Lunt-OTA-in-focus approach; even if something's a little off with it, the end results are fine, as you can see from my mod.
Thank you very much! You've been very helpful.


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