PST mod revival (again)

[GlassWalker]

I think I first thought about doing a mod late 2012, and... suffice to say I've got distracted and done nothing about it last year. So this year, I will finally get round to the mod!

To recap, I have two PSTs, one I'm keeping stock, one I'm modifying. I had all sorts of wacky ideas, but have now decided to go the tradition route and have ordered a TAL 100RS. The only other major component I need then is the D-ERF. I remember it was a common trick to get an under-sized one and insert it into the tube part way down. Here's where I'm getting confused. My previous notes suggest it was a 75mm one, but now I can only see a 70mm one on sale. Next one up is 90mm and also comes with a hefty price lift. Was it actually 75mm or is my memory failing me? I don't think I have much choice other than go for the 70mm.

[allcart]

Still got this one listed here.

http://www.teleskop-express.de/shop/product_info.php/info/p3245_Baader-D-ERF-75mm-Energieschutzfilter-fuer-Sonnenbeobachtung.html

[GlassWalker]

I'd prefer to order from UK.

I have compared the part number between the UK 70mm ones and the 75mm on Baader's web page, and it is the same. Just different measurement method?

Edit: then again, the one from TS works out cheaper...

[allcart]

Don't forget you may have customs duty and postal handling charges.

[GlassWalker]

No customs fees due from Germany as we're one happy euro family, for now. Postage is reasonable, which may mean "slow". Then again it has taken me over a year to get this far...

[keithatrochdale]

Hi get the Baader one, it is very, very good.

As said there are no charges buying from another EEC country.

My PST conversion and the man (Peter Drew) who did it.

And here is the holder and cover plate.

The slot is cut centeredat 15.5" from the front of the lens holder.

The tube is cut to 26.5" also from the front of the lens holder.

I removed all the baffles from the tube and flocked it.

[Peter Drew]

The 75mm is the size of the filter unmounted, this will be nearer 70mm clear aperture when it is. 70mm is more than large enough for a 100mm objective and represents a considerable saving financially over a full aperture front mounted filter. 

[GlassWalker]

Thanks all, I'll put the order in once the coffee has kicked in, perhaps lunch time I was only looking at the Baader, but my confusion was that UK sites seem to list it as 70mm, whereas older notes and Baader themselves call it 75mm.

Any other useful accessories I should consider that might come in handy? Like something to hold the filter?

[GlassWalker]

Filter ordered. Postage was a bit more than I thought earlier, but I guess as I wasn't signed in that was probably german postage, not to UK. Sill going to be cheaper anyway.

I'm having search fail looking for a filter holder of some kind though...

[Peter Drew]

Thorlabs do a holder for a 75mm filter LMR75(/M).  £38 + delivery. 

[GlassWalker]

Thanks, will order that in the morning. The D-ERF has shipped and due Friday! It might overtake the scope in that case as I'm not expecting that until Monday.

[GlassWalker]

Scope arrived today, although it is still in its box. D-ERF is still on target to arrive tomorrow. I wont be working on the scope at weekend anyway since tools are at work!

Anyone used the Thorlab holder before? Debating if the spanner wrench is required since it is double the price of the holder, or could I bodge it with other random bits as a one off job? I assume there's a screw in ring that holds the filter in place? I'm also wondering if the spanner wrench may be handy for other uses in future, but chances are it'll be so rare it isn't worth it.

[Uplooker]

I used one for my PST mod with internal ERF. With regards to the spanner - don't bother. It is easily possible to tighten the ring by hand. You certainly do want it to be too tight incase you set up stresses on the glass if it warms up at all. When I say warm, I mean only slightly as anything would if sat out in the sun for any length of time, which hopefully it will be if you are viewing / imaging the sun -

Ian

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[lukebl]

Good luck with the mod. I may be in a minority here, but I was disappointed by my Stage 1 PST / TAL mod, primarily because of the apparent cropping of the field of view you get using the tiny 5mm BF filter in the PST eyepiece holder.

In fact, I prefer to now just use my reassembled unmodded PST, leaving my expensive front-end 110mm D-ERF and TAL unused. However, a smaller than expected tax bill this year means I'm considering ordering a 10mm BF to get a wider view and make the whole expense a bit more worthwhile!

[GlassWalker]

I know the BF will be a limit factor also, but since I have two PST I got a stock one for the wider image, and can use the mod to pick off interesting details. A bigger BF would make sense, apart from the high cost I seem to recall they demand.

[clubjoker]

For others reaeding this, it's possible to use a combination of two photographic filters as a cheaper alternative to the Baader ERF.

Really interested to hear how this turns out. In a fit of too-many-telecopes-buy-more I am the proud owner of a Stellarvue 80 achro, which has been cut short for solar PST work. So I've got the OTA and the "ERF" substitute, but no PST yet :/

Please keep us up to date, and photos!!

[Steve Ward]

That sounds wrong and dangerous to me ... !!!! 

More details please , I would say that the filters you refer to are totally inadequate for the purpose you suggest ... 

[Steve Ward]

Come on CJ , wakey , wakey .... still waiting for theses filter specifications ...... 

[clubjoker]

That sounds wrong and dangerous to me ... !!!! 

More details please , I would say that the filters you refer to are totally inadequate for the purpose you suggest ... 

Come on CJ , wakey , wakey .... still waiting for theses filter specifications ...... 

Hi Steve

Yes, to anyone considering solar astronomy, I would urge the utmost caution. Be absolutely sure about what you're doing. Especially if looking with your eyes rather than a camera, as infra-red light will permanently damage your eyes without you feeling pain.

I must be clear: I know very little about solar astronomy. I am merely referring to information I had seen elsewhere on alternatives for Energy Rejection Filters. Here is some info on using 2 photo filters instead of a Baader ERF to filter out IR:

http://solarchat.natca.net/index.php/en/this-is-solar-chat/12-solar-scope-modifications/96974-pst-mod-complete

And posts #31 onwards here:

http://stargazerslounge.com/topic/108905-my-70mm-stage-2-pst-mod/page-2

Noe that post #32 suggests photographic filters' optical quality not being good enough. However, Dennis Put in the Netherlands (an accomplished solar photographer) tells me he used them successfully (and he's taken some amazing photos):

"For the ERF I used the photographic filters as can be seen in the pictures that I sent. If you look at the filter transmission curve Baader promises for its Baader D-ERF filters, then one can conclude that the combination of the two photographic filters that I used will very much approach the desired passband in red light, blocking IR and UV and only passing a red band of light. My experience and that of the previous owner are that the etalon and other internal parts do not suffer from thermal heating. A friend of my has measured the transmission of a Baader D-ERF 110mm by himself with professional equipment, and the outcome was that there were distinct transmission peaks in both UV and IR, the areas of light most harmful to the human eye (especially far IR). It is not said that his method of measuring is the best and optimized for such equipment as these filters, but the transmission peaks do not arise from nothing. Now, with a modified PST you will have a ITF filter behind the blockingfilter (an IR blocking filter), but to further optimize I used a 2" 12nm H-alpha filter as mentioned earlier. Nevertheless the Baader D-ERF are good filters for imaging purposes and I have made good results with them."

Clear skies

John

[Steve Ward]

Ta ...

[Peter Drew]

If, as I believe, the Stellarvue 80 achromat is F5 then it won't be a suitable donor telescope for a PST mod which requires a minimum of F10 to retain the full aperture of the objective. A 80mm F5 objective would be vignetted to 40mm aperture by the 20mm PST etalon.    

[clubjoker]

If, as I believe, the Stellarvue 80 achromat is F5 then it won't be a suitable donor telescope for a PST mod which requires a minimum of F10 to retain the full aperture of the objective. A 80mm F5 objective would be vignetted to 40mm aperture by the 20mm PST etalon.

Hi Peter

This one is an old Stellarvue 80/9D, f/9.4, so it will work ok. The guy I got it from, Dennis Put is a real enthusiast. Just in December, he won the first Patrick Moore Prize from the BAA, but was too modest to mention to me why he'd come over to London in December (to receive the award, I guess!).

What do you think of the use of photographic filters for the ERF?

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[Merlin66]

IMHO photographic filters are no good as an ERF. Optical quality and stability being the main concerns. They are designed for photography not precision astronomical work.

I think you'll find that the resonance peaks for the D-ERF are so low that the attenuation is still well within safe limits.

A combination which includes a BelOpik UV-IR/Kg3 gives absolute maximum protection.

[GlassWalker]

Back to the original topic (my feeble attempts at PST modding), to recap I have the TAL 100RS although yet to be unpacked. Also received the 75mm D-ERF which is a depressingly small piece of glass for the cost. I know it's high precision and all that, but still... then again, per area, I'm sure other astronomical filters I have cost even more. The outstanding item is the filter holder, which is ordered and hopefully arriving some time next week.

Sky looks surprisingly clear at moment. If I can find the solar film I made up previously I might test out the TAL in white light and get some practice in I haven't done any solar in a long time.

The photographic filters, interesting but personally the only reason I might look at them is if they bring much bigger sizes down to a reasonable cost. In a quick look at the general filter curves, you get increased losses from having more glass, plus I hate to imagine working out any stray reflections!

Oh, any tips on BF upgrading? In practice, how much of an upgrade would the Coronado BF10 be? I'm more imaging than visual, so I hope that would help significantly in field of view and light falloff. Now, does anywhere in Europe sell it at a reasonable cost? By reasonable, put it this way, it is $500 in the US. Importing it is one option, even with customs fees. I'd pay a bit more to get it more locally as long as it is sane.

[Peter Drew]

Good to hear that the objective was F9.4 and as such should be a good donor. I share the views expressed by Merlin66 on the filter issues. For Ha observation the priorities are safety, performance and cost in that order. The larger blocking filters are nice for visual and almost obligatory for imaging due to the small field of the basic 5mm blocking filter.