Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
I used 350mm for the converging lens distance from the camera sensor.
This is the approximate distance to place the lens safely in front of the Lunt 60 Pressure Tuning etalon.
The calculator suggests I find a lens with a 1200mm focal length.
That's a rather long lens for a [say] 50mm aperture refractor. f/24!
Any constructive thoughts?
Thanks.
wilmslow astro formula refractor telecompressor rsz 500.jpg (39.98 KiB) Viewed 2062 times
Not sure that the right answer for your purposes, but if it is, that what happens when theory meets reality. There will likely be few if any f24 COTS lens to be found. It's the same situation with large negative collimation lenses as well.
But on the other hand you could find a larger lens with the same focal length and position it properly and you'd be good to go. There are a few ~ 1200 mm FL refractors out there, and the objective can be stopped down - although that might not be necessary.
Diagonally parked in a parallel universe.
Curiosity is the father of knowledge; uncertainty is the mother of wisdom.
What are you trying to accomplish? I'm confused by the focal reducer bit as its not needed.
You don't need F10, F7, etc. The focal-ratio largely doesn't matter. The collimator lenses just need to be sitting at a specific amount of back focus from the plane of focus and it scales with that value being constant. So for example, the latest Lunt modular pressure tuned 60 etalon chamber (the scope is 70mm F6, the internal etalon masks it to 60mm F7) doesn't operate at F7 in terms of transplanting it into another scope. The collimator operates at -264mm and doesn't care what the aperture and focal-ratio is, just that the collimator lens is at -264mm of back focus. I have measured this and scaled it from 60mm to 150mm and the only constant that it needs is the -264mm of back focus. The older Lunt 60 non-modular uses different collimators and has a different back focus value. A PST collimator lenses need to sit at -200mm of back focus from the focal-plane, also not really caring what aperture or focal-ratio is on the donor scope, just that it is at -200mm of back focus.
When using collimators, you don't need to calc anything, you just need to know what the collimator lens needs for back focus placement. That's unique to the collimator lens. Once you know that value, you can use it on any system, doesn't matter what the aperture & focal-ratio are, there are going to be optimal ones and sub-optimal ones, but the collimator itself only cares about its back focus placement relative to the focal plane. You can measure this by just focusing the disc and measuring from the focal plane to the collimator location and subtract that from the focal-length and you have the back focus distance. This value is constant and will scale to any other instrument if you can get it into that same back focus position (not focal-ratio based).
Using telecentric is different, but also uses back focus distance for the telecentric, and then placement within the telecentric (post telecentric) beam has an operating distance. Example, the TZ4 needs a little back focus for placement (around -93mm) from the focal plane, and then has nearly an 8 inch operating range after the telecentric where you can place etalons, blocking filters, etc (no collimator, no re-focus lenses, etc) and then the camera sensor (or eyepiece). These proper telecentric amps will have documentation for their placement requirements. Nothing to calculate.
It seems I have been worrying needlessly about the f-ratio requirement of the Lunt 60 PS etalon.
I have tried moving the etalon away from the optimum 264mm position without any visible gain in image quality.
As to a 1200mm lens there are lots of 150mm f/8 achromats about. I have one myself.
I rejected that idea as it was so oversized. Though easily hidden within the fabric of my OTA's oversized, main tube.
Rusted wrote: ↑Fri Feb 03, 2023 2:51 pm
Thank you Bob and Marty.
It seems I have been worrying needlessly about the f-ratio requirement of the Lunt 60 PS etalon.
I have tried moving the etalon away from the optimum 264mm position without any visible gain in image quality.
As to a 1200mm lens there are lots of 150mm f/8 achromats about. I have one myself.
I rejected that idea as it was so oversized. Though easily hidden within the fabric of my OTA's oversized, main tube.
I have done it and documented the procedure and results here:
The modular lunt 60 etalon's collimator is ideal at -264mm and you can scale it to any aperture as long as you place it at -264mm of back focus. I did it with a 150mm F8 (1200mm focal length) and the results are as you can see in my thread. It works well and is non-destructive to the Lunt with no custom parts.
That said, these days is simpler to just get a front mounted tilt tuned etalon like a Lunt 40 because its cheap $595 and put it behind a Baader TZ4 telecentric and it will operate at -93mm back focus so its non destructive and doesn't require chopping the tube or anything.
For the most part Marty is correct, especially if the objectives focal ratio is equal to or larger than the collimator lens. However, if the objective's focal ratio is smaller than the collimators, the effective aperture of the objective will be reduced to effectively what it would be for the collimators focal ratio.
In the diagram below, we can see the focal point of the objective and collimator lens are made coincident in all cases. This means the collimator lens is always best at this fixed distance to result in the best possible collimated light rays passing through the etalon.
A longer focal ratio objective will just use a smaller portion of the etalon, which generally will be a good situation as the etalon uniformity is likely to be better over a smaller proportion of its diameter. On the other hand, a smaller focal ratio objective will have a portion of its diameter eliminated from image formation, and effectively reduces the aperture of the telescope, which reduces the systems angular resolution.
This simplified conceptual diagram should help to visualize the differences:
Internal etalon geometry.png (100.95 KiB) Viewed 1997 times
Bob
Diagonally parked in a parallel universe.
Curiosity is the father of knowledge; uncertainty is the mother of wisdom.
Thank you Marty. I followed your build with great interest.
It inspired me to obtain a Lunt MT 60 for the donor etalon.
Then I read somewhere that the etalon must be matched to the F7 beam.
Having retained the primary focuser on my 6" f/10 I could easily adjust the spacing.
This did not lead to any improvement.
Then I tried some crude measures to fix the serious, tail end sag.
Because I began to struggle with an uneven sweet spot. Hence my renewed concerns about the f-ratio.
The Lunt 60 MT etalon provides a huge image improvement over my PST mod.
Even allowing me to use a 2X Shorty Barlow as standard to hold the camera.
With the PST that would have been impossible. I could only ever use weaker GPCs on the camera nose-piece.
Thank you again Bob. For the confirmation and illustration of the geometric problems associated with different focal ratios.
