Lunt 40mm etalon FSR and CWL

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Oak
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Lunt 40mm etalon FSR and CWL

Post by Oak »

Hi All,

I have been lurking here for a bit less than a year, and this is my first post on the board. I’ve recently acquired a Lunt 40mm scope and have done some basic measurements of FSR and CWL offset using a hydrogen lamp. Maybe someone here will find it interesting or useful. Much thanks to Christian Viladrich who was kind enough to share his calculation sheets. The basic results are these:

FSR: 11.47A (12A manufacturer specified)
CWL delta: 0.53A
Equivalent tilt 0.73 degrees
Etalon gap: 0.1878mm

FWHM and Finesse were also calculated, but I don’t believe those results are very reliable due to the so-so quality of my camera lenses and slight uncertainty in focusing. (FWHM and Finesse were not consistent between 2 cameras, whereas FSR and CWL were reasonably close). They are below in photo form, but please take them with a large grain of salt.

Best,
Derek



The interference patterns and fit in fityk (taken with a 51mm lens).

Image

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The results:

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Re: Lunt 40mm etalon FSR and CWL

Post by Oak »

Here is the setup:

Image


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Re: Lunt 40mm etalon FSR and CWL

Post by marktownley »

Useful information, thanks Derek. What is the thread size on the etalon please?


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Re: Lunt 40mm etalon FSR and CWL

Post by Oak »

M60x1. Full measurements below. Taken before I had a custom adapter machined to place the Lunt40 after my quark. That works very well, although first light was this weekend so no images taken with this setup yet.

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Re: Lunt 40mm etalon FSR and CWL

Post by minhlead »

I remember reading somewhere there is a mods that turn a tilt turned etalon into a pressure turned etalon by using a slim ring bearing.


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Re: Lunt 40mm etalon FSR and CWL

Post by tyler »

yes, it's here :
viewtopic.php?t=33217

I think I will give it a try with my LS35.


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Re: Lunt 40mm etalon FSR and CWL

Post by christian viladrich »

Excellent Derek !
For same reasons, I have not seen you post before.
Just wondering how much difference you have for FWHM with different lenses ?
Have you tried to stop down the lens to check the impact ?

Using the data you gave, and if we assume that the quality of the lens is better near the optical axis, then we can restric the calculation to the first two first rings (i.e. 0.000251). Then we find FWHM = 0.82 A (instead of 0.94 A).


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Re: Lunt 40mm etalon FSR and CWL

Post by Oak »

Thanks Christian. With a Cannon zoom kit lens I got a FWHM of 1.22A compared to the Sony zoom kit lens used above (.94 or better). Unfortunately, I don't have any short fixed focal length lenses that I can use for this - I wonder if that is one of the issues. Haven't tried stopping down the lens yet but that it a good idea.

-Derek


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