Looking for guidance in double-stacking an SM90 from inside a TV102i

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danpro1
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Looking for guidance in double-stacking an SM90 from inside a TV102i

Post by danpro1 » Sat Apr 14, 2018 11:37 pm

Good day to the Solarchat community!

I have a Coronado SM90 etalon and its Coronado BF combo mounted on a Televue TV102i refractor. It has given me great views for 15 years :-) . I am now looking to double stack this setup by adding an internal etalon. I believe that the second etalon could be mounted internally on the front end of the focuser. I have looked into this forum to find the best way to do this, but have not been successful, although I am sure it is somewhere to be found! Could someone please point me to the right topic or provide guidance on this modification? For instance, should I use a PST etalon, an SM60 etalon or a SM40 etalon? Will I have to send it to a company for special tuning?

Thanking you beforehand,

Daniel

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Bob Yoesle
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Re: Looking for guidance in double-stacking an SM90 from inside a TV102i

Post by Bob Yoesle » Sun Apr 15, 2018 5:40 am

Hi Daniel,

Your best solution is to double stack with a second 90 mm front etalon.

Adding an internal etalon will require optimizing a set of collimating/refcusing lenses - a negative elelment directly in front of the etalon, and a positive element behind the etalon. The larger the internal etalon, the better, as this will reduce field angle magnification which improves contrast uniformity. These elements will be incorporated into an etalon "module," which will need to be considerably father up inside the OTA than where the focuser lies - approximately 2/3 rds the focal length towards the objective for the 60 mm etalon. Next, you'l have to devise some method to tune the etalon, as they are usually a bit on the red side of the H alpha line, and require either tilting, mechanical pressure ("rich view" tuning), or a sealed module in which to apply an air pressure change (partial vacuum) to be brought on-line.

While all this can be done, in the end it will be a lot easier and much cheaper just to add a second front etalon - trust me - things are often way more difficult than they at first seem in the world of narrow H alpha filter implementation.

A second front 90 mm etalon will cost more initially than a small one to incorporate internally. But all the necessary additional work described above will end up costing you way more in the end. And finally, the front etalon will usually perform better that an internal etalon when it comes to contrast uniformity. (See: https://www.cloudynights.com/topic/4809 ... on-basics/ )
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danpro1
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Re: Looking for guidance in double-stacking an SM90 from inside a TV102i

Post by danpro1 » Sun Apr 15, 2018 7:33 pm

Hi Bob and thank you for taking the time to provide a very valuable piece of advice and the reference to an excellent Cloudynights post.
I understand that a second front-mounted etalon would provide the best DS setup. I do have another question then: I have read of people using regular refractors with a front-mounted ERF and a PST etalon with blocking filters as the back end; no other lenses seem to be required and the results are very good. I thought that using such a set up and adding my SM90 in the front would constitute a improved setup compared with what I already have, without additional tuning. Does that make sense?
Thanks,
Daniel

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Re: Looking for guidance in double-stacking an SM90 from inside a TV102i

Post by Bob Yoesle » Mon Apr 16, 2018 1:17 pm

Hi Daniel,

Yes, this can be done, and you will find lots of examples here and elsewhere on the internet by googling "PST mod."

The PST "Stage 1" mod involves using the PST etalon with its collimating and refocusing lenses attached to the "PST box." You basically are removing the PST's gold tube with the 40 mm objective and replacing it with a larger objective with an ERF (energy rejection filter) somewhere before the PST eltalon/box - usually a full aperture ERF is placed over the objective, but some use a sub-aperture ERF inside the OTA (which typically wouldn't be needed if you double stack with your SM90, as it has the ERF). The front of the etalon (the collimator lens) needs to be placed 200 mm inside (towards the objective) of the objectives focal point. This may or may not be possible with your TV102. Most people seem to use a home-brew OTA due to needing the right spacing, or hack off a portion of the OTA of an existing scope. Not sure you'd want to do that with your TV102.

With the Stage 1 mod you are stuck with the PST's pentaprism focuser and small blocking filter.

The PST "Stage 2" mod involves removing both the gold tube and PST box from the etalon asembly with the collimating and refocusing lenses still in place. This again requires that the etalon's collimator lens is placed 200 mm inside the objectives focal point, and an ERF for the new OTA, but allows the use of a better focuser and larger blocking filter than the PST's. Etalon implementation requires a bit more work in assembling the OTA.

For both of these mods the new larger objective should have as close to a f10 focal ratio as possible (same as the PST's).

The downside of these mods is the increased field angle magnification for the etalon. The PST has an objective FL of 400 mm, and a collimator FL of -200 mm. The field angle magnification is a low 2 x (400 / 200), which is typically what you find for most internal etalon telescopes. With the TV 102 this field angle magnification will increase to (880 / 200) = 4.4 x, and therefore the jacquinot spot (sweet spot) will be more significantly reduced in diameter, and the scope will typically not work as well for full-disc viewing (and another reason the second front etalon is better - when double stacking, the jacquinot spot is actually even smaller than for a single filter due to the decreased FWHM (bandpass) of the double stacked filter system.) PST mod scopes typically work well for closer in higher magnification views and imaging.

So there are some significant trade offs in going with these mods, which might or might not be to your expectations. If you are going to use the scope double stacked with the SM90 on the objective, I'm not sure the mods are worth the effort - they usually are for getting more aperture than the native PST offers, which you already have. If you are wanting more resolution (for close up viewing or imaging), then doing the PST mod can be implemented with much larger objectives, such as larger refractors or even 8 to 11 inch SCTs with appropriate ERF's. In that case your SM90/TV102 system is ideal for better full-disc and moderate scale viewing and imaging.

You could also try double stacking your SM90/TV102 with a DayStar Quark, although the EFL will be increased to 3696 mm due to the built-in 4.2 x telecentric, and you'll again have poor full disc views unless you have very long FL eyepieces at your disposal. The image will usually be quite dim per the people who have tried it. A Combo Quark with a Baader TZ3 3x telecentric might work better. And don't forget the limits of your local daytime seeing for these increased magnification views, which may prove less than ideal for your local conditions.

Again, for the SM90/TV 102, adding a second SM90 is a better double stacking solution from my perspective, and in the end will be less trouble.
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danpro1
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Re: Looking for guidance in double-stacking an SM90 from inside a TV102i

Post by danpro1 » Sat Apr 21, 2018 2:30 pm

Thank you Bob for describing the different configurations. You are so generous. I'll keep that post as a reference. The solution is now clear: 2 x SM90!
Daniel

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