Coronado SM60 1st gen... What is this?

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alpenglow
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Coronado SM60 1st gen... What is this?

Post by alpenglow »

I recently landed a first generation scope with the front mounted etalon, single stack, and the contrast is remarkably good -- double stack like! Prominences were highly evident, filaments were dark and plage was quite bright. This is with my BF15 which has the Maier replacement ITF. Compared to my other Coronado etalons, this one ranks near the top.

Nothing was thread locked so I had look inside and to my surprise there were some internal optics, unlike my first generation SM90 (Meade made). Is this what's making all the difference? I tried the SM60 on a Pronto and the view wasn't as good. I also tried double stacking with an SM60II and this resulted in a very dim image. Notice the internal optic has a central obstruction too.

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Re: Coronado SM60 1st gen... What is this?

Post by marktownley »

It's an etalon.

It looks double stacked because it is double stacked ;)


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

Thanks Mark. I guess if it walks and talks like a duck, I should have guessed. I'm surprised there's nothing on the label to distinguish it as a double stack.

I wonder if it would be safe to use the scope without the front etalon? The coatings of the front objective are not the same as my SM60II's objective. Unfortunately I haven't been able to track down a manual for the first generation scope. It would be interesting to visually test the performance of the internal etalon by itself.

I just tried holding it up to a hydrogen spectrum tube and it's slightly off band, although today's barometric pressure is a bit high. It appears the internal etalon is pressure tuned and can actually be adjusted with a double pointed lens wrench.


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

I found answers from this CN thread:

https://www.cloudynights.com/topic/5491 ... lar-scope/

1) An ERF is needed to use the internal etalon in single mode;
2) You can still download the original Tuscon AZ manual here: http://gallery.orpington-astronomy.org. ... ual_06.pdf
3) It's possible to tune the internal etalon but it's not advisable.


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Re: Coronado SM60 1st gen... What is this?

Post by DeepSolar64 »

I remember Stephen I think stating the objective of the SolarMax II series has an ERF coating enabling it to be used in single stack. I suppose this is true for the single stacked Lunt scopes too. I guess the first generation David Lunt Coronados lacked the coating on the objective making the doublestacked one the only one with an ERF. The ERF on the SMII stacking etalon is red.

Anyway...


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

After humming and hawing for weeks, I decided to retune the internal etalon (aka internal high resolution module). It was a simple matter of heating the center setscrew with a soldering iron for 5 seconds to loosen the thread locker and then tightening it about 10 degrees until I just saw a solid center dot with a hydrogen spectrum tube. It didn't feel like the added tension was much at all (Caution: over tensioning could lead to a cracked optic).

Image

Unfortunately I can't test the internal etalon on its own until I order an RG630 filter (I assume) as an ERF. However, I mounted a pre-Meade SM40 front etalon which I know is practically, if not perfectly tuned to 656.3nm and the contrast was superb. The SM40 only required the slightest tuning/tilting to displace the reflection; tilting diminished contrast. Bright plage across the disk, medium dark filaments and bright prominences were observed, almost as good as today's Solar Picture of the Day:

https://rainerehlert.com/00_SCHForum/20 ... _9Q_RE.jpg


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

I also have a Meade Coronado Solarmax 60 II with an internal tunable etalon. The Meade front objective has different coatings (blue-violet reflective) than the pre-Meade one and I made the leap of faith that the coatings of the front objective act as an ERF. Still wanting to test the SM60I's internal etalon on its own and encouraged by yesterday's results, I removed the front objective of the Meade with the help of a heat gun. I used a pyrometer to monitor the temperature to avoid overcooking the scope. Got it to about 60°C and was able to unscrew the front assembly from the tube without the need for strap wrenches. Meade used minimal thread locker; OTOH Coronado used none on the SM60I. This made it easy to swap objectives.

I briefly tested the Franken-SM60I and indeed the retuned internal etalon on its own provides very good contrast, superior to the SM60II.

Naturally this is not a permanent arrangement and I swapped the objectives back. I've ordered an RG630 filter from AliExpress for the SM60I and it will take a month to arrive.


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Re: Coronado SM60 1st gen... What is this?

Post by DeepSolar64 »

I think it was Stephen in one of his reviews of the SMII60 that he mentioned the objective having an ERF coating. This is true for the SMII90 as well. I think he worked closely with Meade Coronado for a while since they were a sponsor of CBSAP for quite some time. Orion ended that when they bought Meade. Hopefully in time we can get them back as a sponsor. I don't think any major solar brand backs us. Not Coronado or even Lunt. And we advertize their products everyday!!

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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

DeepSolar64 wrote: Fri Nov 04, 2022 5:21 pm I think it was Stephen in one of his reviews of the SMII60 that he mentioned the objective having an ERF coating. This is true for the SMII90 as well.
Thanks James, I forgot that you mentioned this in your earlier post. I'd love to own or have access to a spectrophotometer to check these kinds of things, including the RG-630 that I've purchased. Even though you'll find transmission curves for some filters, they are generic and not accurate to the unit one has purchased-- much to the surprise of many narrowband astrophotographers.


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

It occurred to me that this internal etalon assembly could be incorporated into a larger diameter refractor or an SM90, similar to a stage 2 PST mod.

The clear aperture is 33.97mm, a little bigger than the PST's 30mm. However mine is presumably designed for f/6.7 as opposed to the PST's f/10.

I measured the etalon's screw thread to be M58x0.8, which is great news for adaptation since M58-M48 adapters (and M48 extension tubes) are widely available. How do I determine the optimal placement of the etalon in the light path of an f/8 or f/9 scope?


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Re: Coronado SM60 1st gen... What is this?

Post by marktownley »

alpenglow wrote: Wed Nov 09, 2022 2:57 pm a little bigger than the PST's 30mm. However mine is presumably designed for f/6.7 as opposed to the PST's f/10.
PST clear aperture is 20mm

alpenglow wrote: Wed Nov 09, 2022 2:57 pm How do I determine the optimal placement of the etalon in the light path of an f/8 or f/9 scope?
33mm x 8 = 264mm ahead of the focal point. This would be to the face of the collimating lens ahead of the etalon. It is designed to work with the faster scope, and this is what I would recommend doing.


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Re: Coronado SM60 1st gen... What is this?

Post by alpenglow »

Thanks Mark.

It still says on Meade UK's Website:
The PST features a 40mm diameter dedicated Ha optical system with 30mm internal etalon primary blocking filter, incorporated focuser, bandpass fine adjustment controls and built-in solar viewfinder.
There's some discussion of it here.


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Re: Coronado SM60 1st gen... What is this?

Post by marktownley »

alpenglow wrote: Thu Nov 10, 2022 7:52 pm Thanks Mark.

It still says on Meade UK's Website:
The PST features a 40mm diameter dedicated Ha optical system with 30mm internal etalon primary blocking filter, incorporated focuser, bandpass fine adjustment controls and built-in solar viewfinder.
There's some discussion of it here.
Yup, I know.

It's wrong on the meade website.


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