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Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Mon Sep 30, 2019 10:47 am

MAURITS wrote:
Sun Sep 29, 2019 1:05 pm
What a great review Marty.
In the first place you need for sure the knowledge to make such a pst etalon, looks on the first sight difficult.
The results are awesome.
Thankfully fine folk here on S.C. lend their wisdom, as this stuff isn't really intuitive unless you know what each component is and is doing and that's not really published by the manufacturer.

Next I want to mod the new 10mm blocking filter to also be straight through to eliminate that mirror being in the imaging train.

Very best,

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by george9 » Wed Oct 02, 2019 1:07 am

For the 10mm blocking filter, will you skip whatever the mirror is accomplishing? I am guessing it is redundant with your red CCD filter. E.g., in current Lunts, it passes IR out the system instead of reflecting it up into the eyepiece. Not sure about pre-Meade Coronado, but it is the same team. Not sure exactly what my early pre-Meade blocking filter (Prom15T) mirror does.

George

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by Merlin66 » Wed Oct 02, 2019 1:22 am

I tested the Coronado BF15 mirror with the spectrograph some time ago and found as you say it's a IR reject mirror.
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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Thu Oct 03, 2019 1:11 am

Merlin66 wrote:
Wed Oct 02, 2019 1:22 am
I tested the Coronado BF15 mirror with the spectrograph some time ago and found as you say it's a IR reject mirror.
george9 wrote:
Wed Oct 02, 2019 1:07 am

For the 10mm blocking filter, will you skip whatever the mirror is accomplishing? I am guessing it is redundant with your red CCD filter. E.g., in current Lunts, it passes IR out the system instead of reflecting it up into the eyepiece. Not sure about pre-Meade Coronado, but it is the same team. Not sure exactly what my early pre-Meade blocking filter (Prom15T) mirror does.

George
Good points, so if I already have a filter that is rejecting long IR wavelength, it accomplishes the same thing then?

Very best,

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by george9 » Thu Oct 03, 2019 7:24 pm

For imaging I expect it is fine. The red-CCD should cover what the camera should be sensitive to.

For visual, I am just not that well versed in near-, intermediate, and far-IR. The Lunts are very redundant for safety I know (so that, for example, forgetting the blocking filter doesn't blind you). What I understand is:

KG3 covers intermediate and far IR
UV/IR coated KG3 covers all IR
Baader red-CCD covers near and intermediate IR to 1200 but not longer
Baader 35nm H-alpha covers near and intermediate IR to 1200 but not longer

(By the way, I noticed Baader has a disclaimer never to use their 35nm as an internal ERF, not that they say it will fail.)

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Mon Oct 07, 2019 2:03 am

george9 wrote:
Thu Oct 03, 2019 7:24 pm

(By the way, I noticed Baader has a disclaimer never to use their 35nm as an internal ERF, not that they say it will fail.)
I think, with 99.9% certainty, this is to prevent someone from pointing this filter, at focus, at the sun as a stand alone "Ha" filter thinking it's meant for that and not just a night-time filter. Many use this filter in the light beam, but not at focus, as an ERF and it's the ideal one actually.

Very best,

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by Merlin66 » Mon Oct 07, 2019 6:51 am

Yes,
I saw that .....
As we keep saying solar mods are non standard and should be treated with extreme caution.
We are stretching the envelope.....always opt for the safest option.
I personally think all ERF and sub ERF for H alpha should be Baader D-ERF filters.
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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by Rusted » Wed Oct 09, 2019 2:05 pm

Marty,

Thanks for your interesting post on the mechanics of PST modification.
If only it were that easy! Then we would all be able to match your exquisite results with the same "kit of parts." ;)

I see you use an aperture stop on your 6" f/8 to provide a true 120mm @ f/10.
My [rather limited] understanding is that a faster [f/8] telescope is self-limiting to f/10 by means of geometry alone.
This being due to the f/10 correction optics of the PST etalon lens group.
They won't allow an angle of incidence larger than an f/10 light cone to pass.

Do you consider is there is any particular advantage to stopping down the objective?
Reduced internal heating perhaps? Stray light reduction?

An internal D-ERF can/should only be placed where the F/8 light cone falls within its clear aperture.
Too near the objective and the light cone will heat the D-ERF supporting baffle and cause vignetting due to annular overlap.

Perhaps the 120mm aperture stop will allow a more forward placing of the D-ERF?
Less intensely focused heat, nearer the objective, due to the larger illuminated area?

Thanks
Chris

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Thu Oct 10, 2019 2:56 pm

Rusted wrote:
Wed Oct 09, 2019 2:05 pm
Do you consider is there is any particular advantage to stopping down the objective?
Reduced internal heating perhaps? Stray light reduction?

An internal D-ERF can/should only be placed where the F/8 light cone falls within its clear aperture.
Too near the objective and the light cone will heat the D-ERF supporting baffle and cause vignetting due to annular overlap.

Perhaps the 120mm aperture stop will allow a more forward placing of the D-ERF?
Less intensely focused heat, nearer the objective, due to the larger illuminated area?

Thanks
Chris
Hey Chris,

The thermal load alone is a consideration. If I left it 150mm F8 and just reduced to F10 at the collimating lens, the thermal load is that of a 150mm still in the tube at least (not the same at focus of course). I think it also matters for focus, but only by a little bit, and the collimating lens is rather specific, my understanding is that the angle of the rays of light are different at F8 than at F10 in the same -200mm position, but I'm not well versed enough to really speak to it beyond that, or I think everyone would avoid cutting their OTA tubes and just let the collimating lens do the F10 thing, but I don't think its that simple.

Very best,

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by marktownley » Thu Oct 10, 2019 7:38 pm

Rusted wrote:
Wed Oct 09, 2019 2:05 pm

Do you consider is there is any particular advantage to stopping down the objective?
Stopping down an objective also reduces spherical aberration. There will be less Spherical Aberration with a 150/8 stopped down to 120, than with a 120/10 operating full aperture.
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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by Rusted » Fri Oct 11, 2019 4:18 am

Thank you both for your responses. I shall try stopping down my 150/8 H-a 'scope.

Though an umbrella might be a better mod given our present run of wet weather. :roll:

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by bart1805 » Sun Oct 13, 2019 8:15 pm

MalVeauX wrote:
Mon Sep 30, 2019 10:47 am
Next I want to mod the new 10mm blocking filter to also be straight through to eliminate that mirror being in the imaging train.
With the Coronado BF's, the part with the valuable BF is not threaded, it is just glued to the bottom of the eyepiece holder. First remove the BF itself. Then use heat to loosen the glue and you will be able to pop the piece out of the holder.
With Lunt BF's it works basically the same. But the Lunt piece has a T2 thread you can use.

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Re: Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Mon Oct 14, 2019 1:22 am

bart1805 wrote:
Sun Oct 13, 2019 8:15 pm
MalVeauX wrote:
Mon Sep 30, 2019 10:47 am
Next I want to mod the new 10mm blocking filter to also be straight through to eliminate that mirror being in the imaging train.
With the Coronado BF's, the part with the valuable BF is not threaded, it is just glued to the bottom of the eyepiece holder. First remove the BF itself. Then use heat to loosen the glue and you will be able to pop the piece out of the holder.
With Lunt BF's it works basically the same. But the Lunt piece has a T2 thread you can use.
Thanks!

Prior to this I had attempted to gently unthread it, but it didn't budge so I didn't force it until I found more information. I will have to try this soon.

Very best,

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