PST mod - donor scope advice

[lukebl]

Hi all. I'm thinking of going for a Skywatcher Evostar 102 for my PST mod. The main reasons for this are a) convenient size, b.) it already has a 2" focuser so the PST etalon can fit in straight away without modification or buying a new focuser and c), it looks as though the focuser is simply fixed with 3 screws so all I have to do is shorten the OTA, drill 3 holes and replace the focuser.

Does this make sense?

[michael.h.f.wilkinson]

Sounds right. Peter Drew will no doubt be able to give better advice

[trev27]

That's exactly the same donor scope I used and it works a treat. I think I cut about 3 inches off and the optimum point for me is with the focuser racked out about an inch. The focuser isn't the best on that scope but you'll be using the one on the pst box for a stage 1. Don't usually have it mounted on that tripod - was just testing it out ;-)

The increase in resolution was quite dramatic.

Trev

[keithatrochdale]

Don't forget the TAL100 makes a great conversion too;

And if you make a carrier for part way down the tube, you can use a 75mm Baader DEFR which will save £100s, like this;

[lukebl]

Thanks for the input guys.

Much as I like the idea of using a TAL, I like the simplicity of a front-end filter and using the existing 2" focuser. I don't fancy all that TAL modification, and I'm nervous about having the filter behind the objective lens (I know that they work, are cheaper and are supposed to be safe, but I'm being cautious!).

[lukebl]

One last thing. Where can you get the filter holders which fit over the front of the dewshield? I can't find them anywhere.

[allcart]

Either will work Luke, although the Tal has the optimim focal ratio. don't forget that once modded the focuser is only used for fine tuning and then not used again, but with the pst removed, you can still use the scope normally again by extending the drawtube.

The smaller internal ERF will work and will be safe and will save you money.

The major expenses come with a stage 2 mod. Good luck Luke.

I have the Celestron Omni 120 XLT for my next mod, but am holding off until I iron out my problems with the Tal stage 2 mod.

[trev27]

The one I have is from TS http://www.teleskop-express.de/shop/product_info.php/info/p3251_Custom-cell-for-ERF-filters---glass-solar-filters-up-to-D-135mm.html. Not the cheapest but I wanted to play it safe and always found their service top-notch. It's the D-ERF in mine.

[Peter Drew]

Late on the case (as usual ) You will be more than pleased with the setup you describe. I mention the following on the grounds for consideration, specially as there seem to be a number of enthusiasts on the brink of doing a PST mod. The Tal 100 is probably the best donor telescope optically for the price. An internal Baader 75mm D- ERF is safe and economical, over £400 less than the 110mm diameter one necessary to retain full aperture if front mounted, there is also the expense of a suitable holder. Cutting a slot in the OTA is no more difficult than shortening it.The PST tube is nominally 50mm diameter so does not fit well in a 2" focuser, an adaptor to do this is sometimes available from Astroboot. The main expense involved in upgrading to a Stage2 is a larger blocking filter, this can be offset by the saving with the small ERF. It might be worth repeating that for a full aperture straightforward PST mod the donor telescope must be F10 or longer.

[ArmyAirForce]

I'm fairly new to solar imaging using the clubs Helios scope. Is the reason for doing PST mods to get a larger aperture for better detail resolution? Can someone explain the benefit?

[lukebl]

I'm fairly new to solar imaging using the clubs Helios scope. Is the reason for doing PST mods to get a larger aperture for better detail resolution? Can someone explain the benefit?

As far as I know, that's the reason. Well, that's why I'm doing it!

[trev27]

I'm fairly new to solar imaging using the clubs Helios scope. Is the reason for doing PST mods to get a larger aperture for better detail resolution? Can someone explain the benefit?

Yep, and at the larger aperture details like filaments towards the limb can take on a 3d like appearance. That + the increased image scale.

[Merlin66]

As has been said...increased image resolution.

The bandwidth of the etalon remains unchanged (at around 0.7A for a single stack) - the added resolution is the clarity of the Ha features both surface and with the proms.

[ArmyAirForce]

Thanks.

[Peter Drew]

For internal 75m ERF holders try www.thorlabs.co.uk/thorProduct.cfm?partNumber=LMR75/M

[Sqyff]

Forgive me for jumping in on this one.

Allen (Allcart) mentioned the Tal 100rs has the optimum focal ratio for a PST mod. Does this mean that the tube does not have to be cut so the scope can be used for normal stargazing if required?

Cheers

Cliff

[JamesF]

For internal 75m ERF holders try www.thorlabs.co.uk/thorProduct.cfm?partNumber=LMR75/M

That looks rather handy!

James

[JamesF]

I'll own up to being one of the potential Tal 100 stage 1 modders. Just waiting to see if a second hand one comes up over the next few months.

James

[Sp@ce_d]

I'll own up to being one of the potential Tal 100 stage 1 modders. Just waiting to see if a second hand one comes up over the next few months.

James

Been following this thread with interest... You can add me to the modding list with the Tal in mind.. although I might go straight to stage 2 as I'm still failing miserably getting the DMK to focus..

[Stu]

I am gradually gathering what I need to do a Stage 1 mod with a Vixen 80M. Will probably try to use a 75 D-ERF internally mounted to save cost. Stage 2 is a future ambition but the blocking filter is too expensive at the moment.

Stu

[JamesF]

I was tempted by the VIxen, but decided I'd prefer the additional aperture of the Tal.

James

[Peter Drew]

Don't discount the standard 5mm blocking filter, it actually gives the best looking result in many cases as the majority of the field is on band. The larger F10 donors have a reduced field which also helps but at the expense of not providing a full disc, just like looking at the Moon with a high power eyepiece. A BF10 will give a full disc view at a 1000mm focal length donor and a BF15 with a 1500mm but will not be fully on band across the whole image at once, you have to scroll round the image to place the on band sweetspot at the area of interest. A Tal 100 tube usually has to be shortened slightly but the current model with 2" Crayford focuser has such a long drawtube you can still use the telescope for normal nightime observation.

[lukebl]

Ah. Just realised that the new TAL 100rs has a 2" focuser. So I'll get one of those instead of the Evostar.

[JamesF]

Do you think we could do a 100RS group buy?

James

[Sp@ce_d]

Hmm wonder what Flo's stock levels r like...