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My PST stage one / two mod


Moonshane

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I didn't have the cheek to put this in the DIY thread as all I have done is bought the various parts I need with a view to putting them together in the near future. To give an idea of what you can buy at what approx. cost used I have also included the price paid.

The parts are as follows:

A somewhat 'rustic' Tal 100 refractor which has been cut down by the length required to allow me to position the PST etalon at the required distance from the focus point of the donor - -200mm. £60

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A standard PST - this came with the box and of interest to me had already been taken apart for a PST mod previously. This means that the sometimes troublesome Loctite in the joints has already been removed. The whole thing unscrews on a tool free basis now making the different formats (see below) much easier.  £325

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Munsen rings and dovetail bar - allows the PST or mini PST/white light/night-time scope (see below) to be mounted on any standard dovetail mount. £10

Helical focuser and various adapters - allows the PST stage two (PS2) mod to be completed along with other side benefits (see below). £125

PST adapter - fits into 2" focuser to allow the stage one mod (PS1) and PS2 with ease. £10

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90mm Baader DERF in a cell - restricts the aperture to 80mm but will be fitted inside the tube at a future date. £210

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All this fits together and dismantles extremely easily and allows different functions to be performed.

If you unscrew the black box and etalon as a complete unit you can then screw in the stage one mod adapter onto the end of the etalon. This then fits into the drawtube of the donor scope. With the addition of the DERF you have now got a PS1. All you leave behind is the plain glass objective and gold tube of the PST so you are not changing any of the internal workings of the PST.

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As mentioned I also bought the required focuser and adapters of the usually more involved PS2. Once the etalon has been removed you can then screw in this arrangement to the focuser and (I have found with a 35mm extension) use either a standard diagonal for night time or birding etc or a Lunt wedge for white light solar.

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A further benefit of this focuser arrangement is that if you remove the etalon this then screws back into the gold tube and the focuser screws onto the etalon effectively creating a small (40mm aperture) PS2. However you do still need a blocking filter and by luck, the eyepiece holder from the PST black box is the same thread as many Celestron 1.25" diagonals. Unscrew this complete and swap with the eyepiece holder from the diagonal and then insert this into a 2"-1.25" adapter and the helical focuser on the mini PS2. You now have a much more streamlined unit for travel use. 

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I may flock the inside of the PST gold tube as for white light in particular there is a fair amount of contrast loss due to off axis reflected light. I might make this removable.

For the larger PS2, simply remove the etalon, focuser and blocking filter diagonal from the gold tube, add the PST mod adapter and you insert the whole lot into the drawtube of the Tal plus of course the DERF on the front end and you have a ready to go PS2.

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Future mods include a paint job on the Tal tube - might even replace the tube for some thicker gauge ali, a BF10 filter  and reducing the size of the cell to allow insertion into the tube further down allowing me to make use of the full 100mm aperture.

If anyone has got this far, I wonder if they can answer a question regarding the diagonal I am using for now. It is a Celestron diagonal which uses a prism rather than a mirror. Are there any implications for this? I suspect there is no difference (other than quality concerns) but would appreciate confirmation.

Looking forward to my first views with the larger aperture. Another SGLer is kindly loaning me some guide scope rings to allow me to mount the OTA - after all the above I have spent up!

Any questions just ask. Any comments or corrections would be great too.

Cheers

Shane

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Its all coming together very nicely Shane, sorry i cant answer the question regarding the prism diagonal, i would not want to give any bad advice where Solar is concerned!

The pic of the TAL tube looks really kind of "naked" with no dew shield or finder and and rings.

At the mo i feel like lobbing my white light filters in the bin, i really wanted to image the huge sunspot but cloudy in fact i cant remember the last time it was clear at a weekend, and i can only do Solar at weekends, i guess this is to be expected living in an area that is so high above sea level

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Very interesting project Shane :smiley:

I've used the Celestron prism diagonals and the only issue that might be relevent is that they have a different length light path to a mirror diagonal, somewhat longer I seem to recall.

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cheers John - that may actually help (or of course hinder depending on how long the additional light path is. will see what happens when I start trying to focus.

I know what you mean Jules. I often take a scope to work and grab a half hour at lunchtime. t's worth it if you get the chance although the cloud gods are not fooled - it's amazing how often it is sunny either side of 12-1pm yet the cloud rolls in just about 12.05pm!

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cheers John - that may actually help (or of course hinder depending on how long the additional light path is. will see what happens when I start trying to focus.

I know what you mean Jules. I often take a scope to work and grab a half hour at lunchtime. t's worth it if you get the chance although the cloud gods are not fooled - it's amazing how often it is sunny either side of 12-1pm yet the cloud rolls in just about 12.05pm!

Shane, if its a problem with the diagonal i will swap you for my revelation dialectric, just remembered i got this from John, what goes around lol. I know only use it occasionally in the Mak

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cheers Jules, that's a kind offer. I have a couple of options available to me one is the prism one, the other is swap a mirror from another one or buy another cheapie celestron mirror one. the main thing is that the thread size of the Meade / Celestron 1.25" diagonals is the same as the one in the PST eyepiece holder. I'll see what happens and come back. If you ever do sell your Revelation though do let me know.

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Just let me know if you need it and we can sort something out, if i end up making my mind up about getting another newt it may require a trip over for secondary collimation! but with my dithering this may take a while lol, any way we are of topic

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Just sent you a pm Shane.

All looks excellent :-)

One question, I'm slightly confused by your reference to the clear objective of the PST. Is the front objective not a form of ERF? I guess that would still mean it can be used for WL as it's just the UV/IR which is blocked?

Stu

Sent from my iPhone using Tapatalk

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Thanks for taking the time to do this thread Shane. The whole PST modding thing seems to offer a lot more flexibility for improvement while just swapping out a couple parts offers some level of transportability for a solar and night sky observing setup. Stupid question but would I be right in saying that if you got another PST and stacked the etalons that this would be a double stack ???? so in effect you'd have a 100mm double stack Ha scope ??? for a fraction of the cost of a dedicated Ha scope ??

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Just sent you a pm Shane.

All looks excellent :-)

One question, I'm slightly confused by your reference to the clear objective of the PST. Is the front objective not a form of ERF? I guess that would still mean it can be used for WL as it's just the UV/IR which is blocked?

Stu

Sent from my iPhone using Tapatalk

The erf is 80mm in the cell so front mounted the 100RS becomes 80mm but fitted inside the 100RS along the light path he uses the full aperture of the 100RS. At least that's the impression I got ???

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The erf is 80mm in the cell so front mounted the 100RS becomes 80mm but fitted inside the 100RS along the light path he uses the full aperture of the 100RS. At least that's the impression I got ???

I'm talking about Shane's comment about using the 40mm PST objective for white light and Astro..... Not heard of that before, or perhaps I'm dazed and confused.... Again ;-)

Sent from my iPhone using Tapatalk

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I think the blue objectives are just multi-coated but the gold ones were an ERF. All I know is that it focused OK on distant trees.

I just read that the mini ERF can be removed if you use a front mounted ERF to improve clarity. Might do this too.

Yes, a future mod is to insert the DERF inside the tube in the cell to get use of 100mm rather than the 80mm front mounted option.

Re the double stacking I am pretty sure this is not possible with the PSTs but is possible with the 40mm Solarmax scopes.

for now I reckon I'll be happy with an 80mm and then a 100mm single stack for under £800.

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Re the double stacking I am pretty sure this is not possible with the PSTs but is possible with the 40mm Solarmax scopes.

for now I reckon I'll be happy with an 80mm and then a 100mm single stack for under £800.

You can double stack a standard PST, it does give a dimmer image though - stick with the PST frakenscope IMHO.

Another good thing about having the D-ERF inside the tube is that it is protected.

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A couple of comments....

The original front objective included an energy rejection coating (ERF), unfortunately the coating was not stable and failed prematurely. This is seen on the early PST's as a "rusty" appearance on the objective.

The later "blue" objective I believe is just AR coated, Coronado changed the design and included the "mini-erf" - an ITF filter in the eyepiece holder.

With the etalon and blocking filters removed, the PST becomes a 40mm f10 achromat.

The optical distance from the rear section of the etalon to the final focus is fixed at 200mm, due to the re-imaging lens used.

Yes, you can double stack PST etalons. I have a special M50 female/male adaptor I made to align, space and orientate two etalons......It did work, but I found the tuning to be so much of a fiddle I gave up. I can supply manufacturing drawings for the adaptor.

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Cheers for the comments Ken. Would it be ok to flock the inside of the gold tube? I presume there is not much heat in this section of the pst? I am thinking about leaving it in permanently for standard pst use as well as when converting to white light solar and night time use.

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WOW! seriously? I just had my first experience of a PST stage 2 mod. Even with a BF5 and cheap prism diagonal, insulation tape holding on the DERF temporarily and not 100% sure I had the focuser in the right position the views were absolutely staggering, really quite photographic. Detail in the proms was stunning, especially the large one at about 3 o'clock and the extremely high thin ones at 12 O'clock. surface detail was phenomenal and I now see what the sweet spot is and found that the most effective method was scrolling around the disk to see detail without touching the etalon (recommended method by Peter Drew who gave me the confidence to go ahead with this project). the bad news is that I am now convinced that a BF10 is necessary!

Furthermore, I have just remembered that this is at 80mm effective aperture! More detail to come!

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Great stuff Shane. Glad it is living up to expectations.

Some sun here today but not had a chance to get out for quite a while unfortunately.

Looking forward to more reports :-)

Sent from my iPhone using Tapatalk

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