Merlin66's PST Mod "Bible"

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tschotland
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Merlin66's PST Mod "Bible"

Post by tschotland »

Hi,

I have gotten a bee in my bonnet to do a mod of my OG PST to a larger aperture in time to take my kid on a trip to south Texas to see the eclipse.

My understanding is Merlin66 has written the authoritative guide to doing these mods. I have sent him a PM requesting a copy, but I see he hasn't been on here in a while, so I am seeking alternate sources of the info in case he doesn't see my message in time. Is it gathered in one place anywhere else? A website or a single thread? I have done a lot of reading of individual threads, but people have been at this for a LONG time; there's quite a lot of info scattered all over.

If anyone is up for answering a handful of noob questions, here is my plan:

I bought my PST ages ago when they first came out. Mine has a blue coating (red on transmission) on the objective though; none of that rusty looking stuff I read that happened to some people. I haven't broken into the black box so I'm not sure what's in there yet.

My plan is to get an F10 4" or 5" refractor. I have my eye on the Explore Scientific First Light scopes as they are fairly inexpensive.

Question 1: Any reason not to use either of these scopes?

I don't know if 5" is overkill or not for my typical sky conditions; apart from the Texas trip I will be using it in Minnesota where I live.

Question 2: Should I bother with getting the 5"?

I considered getting a faster scope for portability and a Barlow to bring it to F10 but perhaps that is more trouble than it's worth.

Question 3: Is it worth trying to do this with a faster scope?

I will get an Altair ERF wide enough to cover my entire front objective. I'll machine my own cell to hold it. I don't want to mess with an internal ERF even though it would save some money.

Question 4: Is Altair a good brand? Will it do all the rejection of IR and UV I need for visual use?

My understanding is that some of the PSTs had an ERF coating and some did not. Those that did not have a 5mm blocking filter (BF5) inside. I don't know what I've got yet as I haven't broken into my black box. Maybe I have this whole story wrong as well. But the BF5 supposedly leads to a limited field of view, which is why people go out and buy a BF10. So my question:

Question 5: With a front ERF rejecting IR and UV, do I need an internal blocking filter at all?

And finally, inside the black box I think I will find a bandpass filter that selects the H-alpha line from the wavelengths passed by the etalon.

Question 6: Can I use the bandpass filter that came with the PST, or do I need to change something about it?

Thank you for reading!

Tom


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Re: Merlin66's PST Mod "Bible"

Post by marktownley »

Hi Tom! Welcome to the forum.

Quick question before answering all your questions. You do know with a 4/5" PST mod you are only going to see a very small part of the sun, not the full disk. What is your goal doing it? Personally, for the eclipse I would just use the PST as will give full disk view.

MArk


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Re: Merlin66's PST Mod "Bible"

Post by tschotland »

Hi Mark, thanks for your reply.

My main goal is to get a close look at prominences and fine details around the moon as it moves across the sun. With the PST as is, such close-up views are not very clear.

I don't intend to use it at all during totality, other than maybe for a quick peek. I only got to see 30 seconds of totality in 2017 before the clouds closed in again -- it had been raining for hours! If we are blessed with clear skies a scope is not where I will be directing my attention!

My goal has always been better close-up views. However I did not realize a full disk view was categorically impossible. Is the etalon aperture too small to take in the full disk image from a large objective?

Thanks,

Tom


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Re: Merlin66's PST Mod "Bible"

Post by tschotland »

Addendum: I took a closer look at the Altair ERF specs and I noticed that the 115mm H-alpha filter is rated OD5 out of band for UV/Vis/IR. The 130mm filter is OD4 for out-of-band Vis, and only OD3 for UV/IR. Why would that be, and do I need to do anything to compensate for the lower optical density of the 130mm?

Both of these filters are offered with holder cells. They turn out to add only a little to the cost, so I will not bother machining my own if I use an Altair ERF.


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Re: Merlin66's PST Mod "Bible"

Post by marktownley »

Hi tom

I think I would go with the 4" mod, the 5" is considerably larger to handle and mount. Altair is a good brand. You will need to use either the blocking filter and ITF from the PST, or purchase a BF10.

Mark


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Re: Merlin66's PST Mod "Bible"

Post by tschotland »

Thanks, Mark. I take it there is no single place where all this knowledge has been assembled apart from Merlin66's file. I am just worried I may be lacking some essential bit of knowledge; for example, why an additional blocking filter is needed if the ERF is itself an OD5 blocking filter outside of its 30nm passband (that's the spec for the 115mm Altair ERF). I guess I'll ask Altair.

Tom


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Re: Merlin66's PST Mod "Bible"

Post by marktownley »

There may well be information in the solar reference library.

Be interested to here what Altair say about the difference between the 2 ERFs and the relative differences in the OD of out of band blocking, i've never noticed that before.


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Re: Merlin66's PST Mod "Bible"

Post by tschotland »

Thanks, Mark. I don't know how I missed the existence of the reference library!

There is only one PST mod thread there, from Pedro Re'. It has a PDF file with a summary of how to do PST mods; for details, it references a file called "Modifying a Coronado PST Hα Solar Telescope" by one Ken Harrison, who I believe to be Merlin66. A quick google search on that title gave me the PDF. Am I allowed to post it, or the link, to the reference library, or does Merlin66 have to do that?

I dug a little deeper into the Altair OD5 thing; this appears to be noted as a new feature. Perhaps they introduced it on their smallest filters first. Given that white light observation film is OD5, I am thinking no energy blocking filter apart from the final line selection narrowband filter is needed. A BF10 filter ain't cheap! But I will get Altair's opinion on that and report back here.

I bought a used 4" F10 Celestron XLT and the Altair filter...I'm on my way!

Tom


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Re: Merlin66's PST Mod "Bible"

Post by marktownley »

If you have a link to it post away! :)


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Re: Merlin66's PST Mod "Bible"

Post by tschotland »

This is such an incredibly valuable resource that I am going to provide both the two links I found and the link to the web archive that was made of the first one:

https://markmcintyreastro.co.uk/wp-cont ... nR5e-1.pdf

https://web.archive.org/web/20220202123 ... nR5e-1.pdf

https://docplayer.net/201218910-Modifyi ... ev-5e.html

Please put this in the reference library. Thanks!


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Re: Merlin66's PST Mod "Bible"

Post by marktownley »

I will leave this here a while and then move the thread to the reference library.

Thanks for sharing the links :)


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Re: Merlin66's PST Mod "Bible"

Post by AndiesHandyHandies »

Hi

I have played with PST mods up to 180mm F10 and Quark Chromosphere set ups.

I have a 90mm F6.7 Triplet but a similar scope would do, if optically as good, LOMO territory.

I use the Quark Chromo first, includes blocking DS with a PST body and Swiss adapters.

Either after a 3x ES TE for full diskat F20 or adding a 2x ES TE for F40.

I get visual views very similar to Solar's images.

The PST Etalon does not show any strong sweet spot in the telecentric beam.

Bodging about I found the PST etalon is fine at F28.

Cheers. Andrew.


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