Verifying Internal Etalon Placement

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_RA_
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Verifying Internal Etalon Placement

Post by _RA_ »

Suppose we know exactly where our internal etalon needs to be placed within a build, to the exact millimeter.

Also suppose that, knowing that errors may be introduced despite our best efforts in the process of measuring and cutting, we plan some leeway for adjusting the etalon up and down the light cone -- not unlike focusing -- before finally locking it into position.

The question is, how can we know visually when it is exactly where it needs to be? Let's assume we know it's on band, having just tuned it in the context of a working system, and immediately moved it to our prototype.

What should we expect to see if it is too far toward the objective, or too far away? Or is it just a matter of moving it around until we're satisfied and then fix it in place? I'm guessing there will be a loss of FOV in the one instance, and vignetting in the other, but anything else?

Related, roughly how many millimeters tolerance is there for error on either side of the proper placement?

Thanks,

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Re: Verifying Internal Etalon Placement

Post by marktownley »

It will be where the sweet spot is the largest. Depending on how accurate you are able to determine this will give the tolerance.


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Re: Verifying Internal Etalon Placement

Post by MalVeauX »

Hi,

As Mark pointed out, you would notice the jacquinot spot change in diameter as well as the fall off from on-band tolerance to off-band gradient. The contrast within the jacquinot spot would change and the area around the jacquinot spot would look brighter (photosphere) with a darker on band chromosphere view within the jacquinot spot. If your FOV is wider than the jacquinot spot, it's very obvious. It would be harder if your FOV was smaller than the jacquinot spot because then you would just see gradients and grey smear and could only rely on it coming in and out of focus to know placement was appropriate.

I don't have a good basis for the next comment other than experimentation that I've done, but it seems about 6mm is a safe tolerance for placement of nearly any of this stuff.

Here's a real world look at what a jacquinot spot looks like when placement is correct and you see the on-band chromosphere in high contrast and darker with the much brighter photosphere showing in the off-band outer diameter with a small gradient between the two regions. Note, this is what happens with collimator systems, not telecentric systems.


The live view in FireCapture, you could see this visually at the eyepiece quite easily just like this:

PST_4xTele_IMX174_Reducer_01.jpg
PST_4xTele_IMX174_Reducer_01.jpg (121.12 KiB) Viewed 2261 times


Unprocessed live view vs processed a little:

PSTEtalon_SingleStack_SO4xTele_IMX174_Reduced_09.jpg
PSTEtalon_SingleStack_SO4xTele_IMX174_Reduced_09.jpg (61.96 KiB) Viewed 2261 times

Very best,


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Re: Verifying Internal Etalon Placement

Post by _RA_ »

Super helpful!

I'm proceeding very slowly with acquiring the parts I need to mate my LS50PT with my 102ED (documentation on hold in another thread). I'm using off-the-shelf components and aim to do everything non-destructively and hot-swappable. It won't be as neat as Lunt's modulars, but it will be a *lot* cheaper.

Huge Thanks!

~ RA


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Re: Verifying Internal Etalon Placement

Post by MalVeauX »

_RA_ wrote: Sat Jun 11, 2022 6:29 pm Super helpful!

I'm proceeding very slowly with acquiring the parts I need to mate my LS50PT with my 102ED (documentation on hold in another thread). I'm using off-the-shelf components and aim to do everything non-destructively and hot-swappable. It won't be as neat as Lunt's modulars, but it will be a *lot* cheaper.

Huge Thanks!

~ RA
Yup,

I've done it.

viewtopic.php?t=34331

You can use a Lunt 60 and Lunt 50, both work well for this. The key is just measuring back-focus location of the first collimator lens. You don't have to do any guess work.

Very best,


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Re: Verifying Internal Etalon Placement

Post by Bob Yoesle »

I have found that the placement is not too critical, and a +/- 1-2 cm shift from the optimal location has very little discernible effect on the image (at a 1200 mm EFL, collimator FL - 770 mm). YMMV based on objective and collimator parameters.

Imgp3930 sm arrowed.jpg
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Re: Verifying Internal Etalon Placement

Post by Rusted »

Hi Bob,

There are some subtle but fascinating details to your equipment.
Remote, mechanical tilter rod for the front etalon?
Adjustable positioning and tilt of the internal D-ERF?
Stiffening rods & carrying handle, between the tube rings?
All beautifully constructed to an amazing standard of presentation.
These all show your dedication to achieving the highest quality images. :bow

Regards
Chris


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Re: Verifying Internal Etalon Placement

Post by Bob Yoesle »

Hi Chris,

Thank your for the kind words :>)

Yes, there is a long rod with a fine-thread bolt for adjusting the front etalon tilt-tuning while at the eyepiece.
There is an internal SM90 etalon assembly, whose position which is adjustable. Unlike the Lunt internal pressure-tuned etalons with a blue-shifted CWL, and being originally designed as a front etalon and having a CWL on the long side of Ha, the etalon is negative pressure tuned by applying a slight vacuum to the etalon cavity. This partial vacuum decreases the gap refractive index, which lowers the CWL and brings the etalon on-band.
The front SM140 etalon's Coronado RG-ERF was replaced with a Baader 160 mm DERF; the internal etalon is provided with a Lunt 105 mm RG-DERF (for single stack use) and a 110 mm B+W Kaesemann circular polarizer to suppress any remaining retro-reflections. The Coronado BF30's ITF was replaced with a BelOptik UV/IR block on KG3 to improve final transmission image brightness.
There is just the single rod/carrying handle for the tube rings, which I have since revised by employing a slightly shorter D plate between the tube rings.

The views are amazing - but it was quite an adventure and learning experience putting it all together. You can learn more about this telescope in Chapter 9 of Solar Astronomy, as well as here.

Thanks again.

Bob


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Re: Verifying Internal Etalon Placement

Post by Rusted »

Thanks Bob. :bow


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H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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