Small (50-60mm) PST Mod

[Moonshane]

I have been hankering for ages after a BF10 to give me full  disk visual images of the sun with my 100mm Tal based PST stage 2 mod. That said, I'd still have to centre proms etc into the sweet spot. The detail is superb with this scope but I always have the itch to see full disk. I cannot really afford a new BF10 without selling something I want to keep, and they come up for sale so infrequently that I either miss them or they are sold at daft amounts. I can currently see what feels like 2/3rds of the sun with my current scope with one limb just disappearing out of view as the other appears on the other side of field. I am using the standard pst 5mm blocking filter.

It struck me that another option might be to use a cheap refractor of 50-60-70mm but preferably f10 to take advantage of the whole aperture and pst etalon. Given the current portion seen of the whole disk I am hopeful I might get away with a 600mm focal length but 1) can I get such a scope? and 2) has anyone tried a scope like this for a pst mod?

 

I am assuming 1) I can get and fit (perhaps with a bespoke adapter and a section cut off the OTA) a focuser to adequately hold my 'bits' 2) that I won't need a DERF but maybe could get a ha/IR rejection filter (will be looking into this in more detail)

 

Any ideas/comments would be appreciated.

[michael8554]

I have a Stage 1 PST mod using a Meade 70mm F10 Achromat, they are also branded Bresser, Celestron, Skywatcher etc.

It's a while since I did the mod but I'm pretty sure you MUST have an ERF if you're going to fit a PST to a larger OTA, a HA/IR filter won't protect your eyes.

Hopefully Merlin will comment on this.

Look at the mods section here:

http://solarchat.natca.net/

Michael

[Merlin66]

The original BF5 was designed to give a full disk coverage with the PST 40mm f10 optics.

Any solution involving a longer focal length will be compromised..

Based on our practical experience it is possible to use a <60mm aperture without an additional ERF but an internal IR or  Baader Ha filter is recommended to minimise the heat loading on the BF assembly.

 

[Stu]

There's a 70mm f10 Meade refractor on Astroboot for £13 at the moment Shane, no focuser. Might be worth experimenting with?

I don't recall seeing a 60mm f10 but there must be some around.

[Peter Drew]

Shane. Out of interest I've just rigged up a generic 60mm F11 objective with a PST body and BF5. I can confirm that this will give a full disc image. BUT, it's fairly tight and you would still have to move the view off centre to see decent size prominences properly so IMO not particularly satisfactory. Having done lots of experimenting with various Ha components I've come to the conclusion that there is no complete "free lunch" with PST mods, the 20mm diameter of the etalon is the limiting factor as this determines the size of the sweet spot. I can easily get a full disc image with my 6" F10 PST mod with a BF15 but it still has the sweet spot size of the original PST and I have to scroll round the image to view detail on band. However, the big inrcease in resolution and the higher possible magnifications more than offset this issue. Not to mention the cost saving over a bespoke 6" Ha scope!.  

[Moonshane]

cheers all.

lots to think about. for now I might just continue to enjoy the slightly closer in views afforded by the 100mm aperture and bf5. I have little to complain about really it's an awesome visual Ha scope.

[Moonshane]

Still thinking about this and now a slightly different tack. I have thought before about simply adding a 2" Baader 35nm filter onto the nose piece of my PST stage 2 mod (i.e. the bit you normally insert into the focuser of the shortened Tal). I appreciate the need for a f10 system and that if inserted into my f6 80mm triplet, that the etalon (which would be behind the filter) would clip the aperture at 48mm. However, this would allow at least some observing in Ha when away with the caravan and prevent the need to take another scope. Assuming this would be safe (would it?), I'd get a view which approximates something between the PST and the 50mm Lunt at full disk. The other option is to create a front end cap to allow the 35nm filter to be used as a objective ERF before the light goes in (again reducing aperture to 50mm (48mm at the etalon).

This might all seem to be a bit mad/OTT but at around £100 or so for the filter plus the costs of a filter holder at the objective is a reasonably cost effective and compact method of getting dual solar plus night time all in one small scope. If I do add it at the objective end there's obviously less stress on the whole system.

Does this make any sense?

[Merlin66]

What you suggest will work well.

The Baader 35nm Ha filter is the same design and construction as their D-ERF.

 

[Moonshane]

 Cheers Ken. Next challenge is whether I have enough spare in focus to get the best etalon position :0)

[Moonshane]

Alas.  I fear the in focus will not be adequate.  

[moriniboy]

I tried Ha with my Equinox 80 and PST stage 1 and also found that I did not have enough in focus. Luckily at the time Astroboot were selling replacement short tubes made for binoviewing and it worked brilliantly.

Just swapped the original tube back for astro viewing.

[Moonshane]

How jammy!

I am using a Meade 80mm f6 triplet which has odd threads unfortunately so my chances are remote. 

 

[Peter Drew]

Don't overlook the F10 "rule" unless the lenses either side of the PST etalon are changed. If a 80mm F5 telescope is used for a mod, the 20mm standard PST etalon will vignet it to 40mm, the same aperture as the original PST so not much of an improvement other than a better objective! 

[Moonshane]

Cheers Peter

Yeah, I am starting to revert to my original plan I reckon, a 60mm f10/11 which I can get cheap and then cut down as necessary. The issue might be getting a decent focuser but I could always get a reducer for the pst mod and insert into a 1.25" fitting.I was really hoping to avoid another scope though is possible. The f10 rule would make my 80mm f6 a 48mm f10.

[Moonshane]

Update. I bought a cheapie 60mm f10 (it is definitely 600mm focal length apparently) off Astroboot for £20. It has a 1" focuser but I'll probably be cutting down the tube anyway and adding (probably) a helical somewhere in the chain. I'll be on the lookout for a 2" UV/IR rejection or a 35nm Ha filter allowing me to put the filter in the pst-2" adapter that goes into the focuser, and add some (probably Munsen) rings/a dovetail. I'll need to get a bit of cash together and although it will mean taking two scopes away, this one will be pretty light and easily packed into a corner somewhere. I'll send pics when any progress is made and also report first light. As this has a slightly wider field than Peter's f11 I hope it will have a bit more space around the sun, with more of the sweet sport covering the disk. We'll see

[nightfisher]

good luck with this shane

[sungazer]

Hi, I have been through this problem,and have made many different mods, but I now have built into a 90mm tube a 85mm F7.5 lens ,which have been available on Astrobuysell . This gives a full disc using the standard PST minus lens. 

If you contact me via email with your phone no I will be happy to help you.

[sungazer]

Shane,I should have said you MUST have an ERF In the light path full or sub apature. It is also possible to increase the PST eyepiece :hole: to 5.5mm. With great care ,this gives a slightly larger field of view.  All the best with the project,john.a

 

[Moonshane]

Cheers John

I have a 100mm pst mod with sub aperture DERF but based on what I read inc comments above I hope you agree that for 60mm aperture a good uv/ir cut filter would be perfectly adequate?  This Is after all an ERF of sorts. My understanding is that the Erf's job is heat reduction not brightness reduction.

[sungazer]

Shane,I should have said you MUST have an ERF In the light path full or sub apature. It is also possible to increase the PST eyepiece :hole: to 5.5mm. With great care ,this gives a slightly larger field of view.  All the best with the project,john.a

 

[Moonshane]

Cheers John  will take into account.

 

 

[Moonshane]

I have decided that unless someone with a lot more knowledge than me suggests otherwise, I will be buying a Baader ha 35nm filter.

[sungazer]

Shane,Thanks for your reply,if you already have an ERF ,then it should be coated to give UV/ IR protection.the coating if applied, looks purplish , and is on the front facing surface.

i do have 2 lenses of minus 125&98mm ,which I purchased from Surplus Shed in USA which enables one to avoid shortening your telescope tube .these fit into the front of the PST Etalon in place of the normal 200mm one. Should you wish to borrow them please let me know.

A phone call may help, if you Email me your number to:  john@ the-longs.co.uk  I can call you this evening if convenient .John.

 

[Moonshane]

cheers John

the DERF I have is the 90mm one of these http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3245_Baader-D-ERF-75mm-Energieschutzfilter-fuer-Sonnenbeobachtung.html

This is internally fitted in my 100mm Tal based PST stage 2 mod. I bought the parts from experienced solar modders so am confident they work safely and effectively.

The issue I have currently is that I want a smaller unit to take away and a cheap 60mm f10 seems ideal. I will basically be putting whatever filter I buy on the nosepiece of my stage2 mod and inserting the whole thing in the focuser. Based on what has been said here I think the Baader Ha 35nm filter would be suitable http://www.baader-planetarium.de/sektion/s43d/bilder_filtersticker/gross_BP43dTN2458381-01E__Baader_Filtersticker_large_H-alpha-Filter_35_1.25inch.jpgas it has the same spec as the DERF allowing through more or less only what you use for Ha observing.

 

[Moonshane]

I suppose the next question is 1.25" or 2"? I suppose either would be OK on the basis that the etalon is 20mm. Thanks for the offer of further help. I may well take you up on that but for now, as I'll be using the stage two mod on two scopes, I'd prefer to retain the unit intact for now I think.