Ha Doublestack FS60C or PST?

[pmlogg]

What would be likely to produce the best results for imaging, a PST and SM40 or a Tak FS60C with BF5 or BF10 and two SM40s?

Either solution would allow either single or doublestacking but I presume that 2 x SM40s produce a slightly lower bandpass than the PST plus SM40.  However, the FS60C has a focal length of 355mm compared to the 400mm of the PST (or Solarmax 40) so would give 2.7"/pixel resolution rather than 2.4" with my DMK41.

[Peter Drew]

My experience has been that adding a stand alone etalon and blocking filter to a quality telescope has always produced better results than a bespoke off the shelf solar unit. On that basis I would expect the Tak fs60c to be the best option. I have a good PST with a borrowed SM40 to double stack it and have not been very impressed for the potential cost. As far as I know, double stacking a SM40 on to a PST gives the same reduction in bandwidth as two SM40's would on the Takahashi.    

[ollypenrice]

I'd have thought that the double stacking on the Takahashi was an expensive way of going about it since the FS60 is so expensive itself. I don't know about its performance as a solar scope but it's rather an ordinary performer in full spectrum use, quite honestly.

Olly

[pmlogg]

Thanks for those thoughts. 

I too have tried an SM40 on my PST but haven't found it to offer as much improvement as I expected - but I've not had that many opportunities to work at getting the best out of it, so perhaps I need to persevere.

My 100mm refractor came with an adapter to fit an SM40 to it and I'm about to take delivery of a 2nd hand BF10 so I can try that during the fleeting opportunities before the days lengthen again - but the 800mm focal length means I won't get full disc in frame with the DMK41.  With the etalon(s) only being 40mm the extra aperature won't do me any good so that made me wonder about using my FS60C instead, the added advantage being its compact size.  I don't have a purpose made adapter for it but my plan was to strip out the solar film from a cell I bought from Astroboot and convert it to hold the SM40 (or potentially 2 of them doublestacked) using photographic filter rings.

Searching on the web I've noted that Coronado suggests f/10 - both refractors would be faster than that.   I'm not sure how important that is, given that SM40 adapters are made for <f/10 refractors.

[Merlin66]

I started my journey in solar Ha with a PST...then added the SM40 as a double stack - the results were so impressive I sold the SM40 and modded the PST.....then "upgraded" to a SM60....then a second SM60.....

I now run a TS102 Modded PST and the SM60 double stacked on an ED80 - Majic!!

The double stacking of the PST/SM40 or SM40/SM40 will give the same improvement in the Ha bandwidth, BUT IMHO a SM40 double stack on any suitable refractor will give better definition - dumping the pentaprism and black box is always a good idea.

Shorter focal lengths allow full disk imaging with smaller CCD's......

[pmlogg]

Thanks for that - certainly sounds like a good toolbox for solar work.

The BF10 has arrived so I will try it on the 4" if we get any sun this weekend.  I was toying with the idea of trying a 0.5x focal reducer, to get the focal length back down to 400mm.

In preparation for my attempt to convert my filter cell into an SM40 to FS60C adapter I tried screwing the SM40 into the new female threaded 52x0.75mm filter ring adapter.  I was unpleasantly surprised to find that it would only go on 1 and a quarter threads.  Initially I thought that I'd got a poor quality adapter but then remembered that the FS60C flattener and reducer both have the same thread.  The adapter easily fully screwed onto both.  I then tried to attach the flattener to the front of the PST - and no go, again only a thread and a half. 

So, despite what I have read and seen on several websites I now don't think that the SM40's rear thread is 52x0.75mm but I don't know what it is.  Has anyone else come across this and know what the correct thread specification is?

[Merlin66]

I have a T-Max adjuster from an SM40 etalon in my hand and just screwed it onto a 52mm filter thread....

It goes on perfectly and bottom's out....

The adaptor I have is a KOOD stepping ring 52-55mm

I've used these before to make a front plate for the ED80....an old solar screen housing - punch out the film and replace it with a disk of 0.7mm thick aluminium with a central hole 55mm diameter. Epoxy the stepper ring in the hole and the disk in the empty housing.

Works very well....

HTH

[pmlogg]

Well, I did wonder whether the step-up filter was to blame.  That's why I tried the two Tak components, which I knew did fit another 52x0.75mm filter ring adapter. But, they wouldn't fit onto the PST.

Kood are well known, perhaps they make their adapter rings with a bit more tolerance than the one I bought.

The filter cell I have has a clear aperature just a bit tight to fit an 82mm male thread - so it's an 82mm to 52mm step-down that I bought - aiming to open up the aperature just enough to get a tight fit.  I hadn't thought of epoxy; instead I bought a mating 77 to 82 step up ring, to screw on from inside the filter cell, sandwiching the metal of the front of the cell.   Using it and epoxy would give me more faith in the arrangement.

I see that Kood don't do a single step-down 82 to 52 but do  82 to 77 and 77 to 52, to achieve the same thing.  I have one other thing to try first but a pair of Koods will be the next for me to try.  Thanks for those suggestions.

[pmlogg]

Well, success with the filter cell.  Having already bought a 77mm to 82mm step-up and having a SM40 adapter for a TV85 to hand I bought a 82mm to 95mm step-up ring instead of the 2 Koods. The 77mm to 82mm only has around 2 thread of purchase on the back of the 82 to 95 so I've used glue as well - again thanks for that suggestion.  The excess diameter (94 to 80mm) inside the cell is part filled by a ring of 6mm thick hard rubber topped by a layer of velco.  It feels very firm on the front of the FS60C.  The only play seems to come from the T-max tuner.

I had another llook at the threads on the inside of the SM40 to TV85 adapter and the 52x0.75mm threads on the FS60C flattener and they are not the same.  The SM40 adapter is a coarser thread.  I don't have a thread guage but using a bit of pencil graphite on the threads then rubbed onto paper, they measure at 1mm pitch.  So, I think that the SM40 will probably only fit some 52mm x 0.75m photographic filter adapters.  From your experience the Kood ones fit and I've also seen a picture online of a Soligor 49mm to 52mm step--up ring being used. http://www.astrosurf.com/re/coronado_sm40_takafs60c_20070603-02.jpg.  Unfottunately the 2 no-name brand ones that I have don't fit.

Now I just need some sun.  It only peaked out today but if Metcheck is right there should be a better chance tomorrow.