PST DS etalon choice

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EricF
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PST DS etalon choice

Post by EricF »

Hi all,

I used a PST for the first time last weekend (normally use a friend's 90DS). It's an older PST with the tilt-only external etalon. SS image was pretty good, better than I was expecting. DS showed much more contrast on the surface. But definitely had a sweet spot that wasn't obvious with SS.


Question: would a PST with a rich-view external etalon be any better? Would that help with the sweet spot?


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Re: PST DS etalon choice

Post by marktownley »

Hi Eric

Simple answer, no, not necessarily.

Was the sweet spot circular or banded?

Mark


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Re: PST DS etalon choice

Post by Bob Yoesle »

"It's an older PST with the tilt-only external etalon. SS image was pretty good, better than I was expecting. DS showed much more contrast on the surface. But definitely had a sweet spot that wasn't obvious with SS."

1. This seems strange, as to the best of my recollection the PST was always an internal etalon telescope using about a 20 mm etalon within a collimator lens group, and double stacked with an external 40 mm etalon/TMax screwed onto the objective cell.

The MaxScope 40 telescope was an external 40 mm etalon only scope, and could be double stacked with a second front-mounted etalon:

SM40.jpg
SM40.jpg (472.53 KiB) Viewed 852 times

2. The size of the Jacquinot spot will only be about a degree in diameter with a single stack air-spaced 0.7 A front etalon, and is reduced in size when the band-pass is reduced. For a typical DS with a second 0.7 A front filter, the band-pass becomes about 0.5 A, and the JS is reduced to about 0.8 degree - still large enough to get the entire solar disc in the spot with room to spare. An internal etalon will generally make the JS a bit smaller due to field angle magnification. The JS is generally circular, and will be compounded as Mark notes by banding if sufficient tilt is applied to the etalon(s).

3. What etalon did you use to DS the original external etalon with? If it's a MEADE Coronado 40 mm etalon, it generally will have a high off-band center wavelength (CWL), and requires significant tilt to get on-band, and you're likely to get banding - especially when double stacking. The RichView etalons generally require even more tilt, and the RV tuning widens the bandpass as well due to only applying pressure to the center of the etalon.


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Re: PST DS etalon choice

Post by EricF »

Yes, it was definitely a 'standard PST' - internal etalon. This was not a SM40.

The external DS etalon was tilt only, and it was all black. It's possible the gold cap had been removed. Referring to the top left picture in your attachment, all of the housing on the DS etalon was black, except the Coronado lettering (white).

With the DS etalon installed, there was a band across the sun with good detail. From memory, the band was about 2/3 the width of the sun.


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Re: PST DS etalon choice

Post by Bob Yoesle »

OK, I misread your statement regarding single vs. double stacked.

Black Coronado etalons came mostly from the Isle of Man production:

Coronado IOM 90 & 40.jpg
Coronado IOM 90 & 40.jpg (91.96 KiB) Viewed 828 times

These should be very good and require little tilt to get on-band. The only way to know for sure what's going on would be to carefully examine what the external etalon does single stacked, and/or use a hydrogen spectrum tube to examine the interference pattern as shown here.


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Re: PST DS etalon choice

Post by EricF »

Yup. That's what it looked like.

Didn't take much - eyeballed it to get the tilt to zero (just looked at the gap), and then adjusted tilt while looking through the scope. Other than not seeing the entire sun, it was quite good.

So in the end, a new PST with external DS (t-max + rich-view) should be expected to have similar image quality ? (neglecting any wild QC issues)


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