Tal 100RS & PST mod

[hunterknox]

Hi all,

I'm considering buying a Tal 100RS for a PST mod. Is there anyone who has done this before? I'd like to know if the tube would need cutting and, if so, whether the focuser could still be retained if I redrilled the tube.

I've also got a good idea for the ERF cell but I could do to know the outer diameter of the dew shield and tube. Would anyone be willing to do some measurements for me?

Thanks,

Tom

[Astro_Baby]

Nope...not done it but have considered it. If you pull it off do post up.

[Peter Drew]

The scope at F10 will be an excellent donor for a PST conversion. You will probably have to shorten the tube slightly as the PST etalon has to be 200mm inside the prime focus. You will need a 100mm dia ERF to exploit the full aperture and the 5mm blocking filter will mean that you will not achieve a full solar disc image, a BF10 would be the minimum.

[Merlin66]

I have a detailed write-up available on PST mods.

Drop me a PM with your email address and I send a copy.

Peter has outlined the basic requirements very well.

[Whippy]

If it's ok Ken, can you send a copy of the write-ups my way too?

Tony..

[Merlin66]

It's a 6.5Mb file

I need an email address to send it...pm me details

[Whippy]

Cheers Ken .

Tony..

[hunterknox]

Thanks guys. Looks like I'm going to have to be the pioneer on this one.

Ken: I already have your excellent guide on the mod, ta very much.

My original plan was to use a Celestron 100ED that I have already, cut it down and change the focuser from the screw on version, however when I've come to do the measurements the replacement focuser I've got is about a millimetre too wide for the tube. I know there's a guy on another forum who has the same setup I was aiming for already but I'm guessing there might be a slight variation in the tubes, so I'll probably sell the Celestron to fund a new Tal.

Couple of other oddities I've found with my PST: my pentaprism is slightly misaligned but glued onto the holder bits that way position, which I'm not sure is standard, and I've also got a red filter under the blocking filter where some people have just got an extension tube. I'm guessing that might be part of the energy rejection system on my blue lens PST.

It's a world of revelations this PST modding business!

[Merlin66]

Yes,

the misaligned prism is not uncommon. It usually need to be ripped of the support and re-glued back into the correct position - top face parallel to the inside of the black box.

When Coronado changed to the blue objective they added a UV-IR filter in front of the blocking filter ( to give some additional protection to the Blocking filter coatings...Hmmm)

[hunterknox]

P.S. Can anyone let me know the outer diameter of the Tal's dew shield?

Thanks in advance...

Tom

[hunterknox]

Yes,

the misaligned prism is not uncommon. It usually need to be ripped of the support and re-glued back into the correct position - top face parallel to the inside of the black box.

When Coronado changed to the blue objective they added a UV-IR filter in front of the blocking filter ( to give some additional protection to the Blocking filter coatings...Hmmm)

Thanks again Merlin - I owe you more than one. Reckon I'll have to be brave and rip the prism out then.

I'm guessing that with the Baader ERF installed it should be OK to remove the extra filter.

Cheers,

Tom

[Merlin66]

With the ERF in place it really doesn't matter....

[nightfisher]

Tom, i just measured the dew shield on my 100rs....its aprox 130mm at the lens cap end and aprox 115mm at the end where it meets the ota, this was measured with an engineering rule across the outer edges, and is just a guide

[hunterknox]

Thanks nightfisher, that's perfect. I think I'll have to take the plunge then. Will report back after a bit of hacksawing...

[Whippy]

I'll look forward to hearing about your results. I'm mulling over doing a PST mod and a TAL100 seems an ideal candidate for a donor scope.

Tony..

[Peter Drew]

The easiest way to mod the original PST is to cut the "gold" tube a few inches ahead of the knurled tuning ring. The resulting back end can then be used as a Ha "eyepiece", retaining the spacing integrity and the focusing system. On mine, I have also made a reconnection sleeve so that the original can still be used as a grab and go or for a quick view of the Sun to see if it is worth rigging up a larger unit. You will have to sleeve or tape up the "gold" tube as it is 50mm diameter.

[step_hen]

The easiest way to mod the original PST is to cut the "gold" tube a few inches ahead of the knurled tuning ring. The resulting back end can then be used as a Ha "eyepiece", retaining the spacing integrity and the focusing system. On mine, I have also made a reconnection sleeve so that the original can still be used as a grab and go or for a quick view of the Sun to see if it is worth rigging up a larger unit. You will have to sleeve or tape up the "gold" tube as it is 50mm diameter.

Hi Peter- have you any pictures of this mod? what kind of length needs to be hacked off the gold tube? Does the donor OTA need modified too?

Stephen

[Merlin66]

For this mod you just have to leave about 50mm of the gold tube infront of the etalon. Yes, you will still have to modify the donor tube to get the etalon EXACTLY 200mm inside the prime focus.

(IMHO I don't like this solution - yes it will work but the opportunity of reselling the PST when you want to replace the etalon you initially use with an "even better" one -the holy grail of mods - is lost. Paying 25 gbp for a 2" PST nosepiece is a small price to pay.)

Just my 0.02euro