Converting a WO Meg72 into Solarscope

[brantuk]

Hi Folks

Any advice on this would be gratefully appreciated cos I've got absolutely no idea.

Basically I want to know if it's possible to turn my Meg72 into a Solarscope. If so then what bits would I need? and how would I do it?

If you can point me to any useful websites or links for the equipment I'd need that would be great. I'd like to see flares, proms, spots, ... everything the sun has to offer really.

Cheers

Brant

[Merlin66]

http://stargazerslounge.com/diy-astronomer/105516-they-dont-arf-glue-them-psts.html

Brant,

Have a look at the write-up I'd done on PST conversion.

Not sure the WO is the best choice for a donor OTA.....

Ken

[brantuk]

Thanks Ken

I just flashed through it - looks like exactly the sort of thing I was looking for. I did catch the bit about aperture and focal length minimums. Looks like it might not be a great idea with the Meg as you say.

I'll give it a good read at least to learn how it's done.

Cheers

Brant

[brianb]

Lunt or Solarscope 50mm or 60mm Ha etalon at the front, blocking filter at the back end ... easy conversion but far from cheap.

[brantuk]

Thanks brian - lets see if I dare look at the prices lol Someone else told me Ha isn't cheap - and that "etalon" word just smacks of big bucks - oh well here goes

[brantuk]

Ugh!! Looks like that upgrade is a long way off lol

[Capricorn]

If looking at straight white light observing then an aperture filter is the obvious. I know Astro-Engineering make a screw-in one's for the Meade ETX's, never heard of one for the WO 72.

Equally I was down at the Norman Lockyear Obs a couple of years ago and someone had some there but made by another manufacturer.

Check the net to see if they are having an open day again soon and maybe try a trip down. Their open days tend to be dedicated to solar observation owing to Norman Lockyear. So solar filters pop up a lot.

[brantuk]

Thanks for that Capricorn - I'll look into it - I just missed a s/h PST in the for sale section - I woulda had that - gutted lol

[kev 102 42]

Id like to know what scopes can be modded and sizes besids the Vixen,.........kev

[Merlin66]

kev,

Due to the limited size and design of the PST etalon, IMHO the "donor" OTA needs to be a refractor with a native focal ratio around f10 ( ie without barlows etc)

The aperture really depends on the $$$$$ you want to spend on the ERF. I'd say the overall costs of modifications wouldn't be justified below a 70mm objective.

There's a section in my write-up which compares various f ratios, and focal lengths to the 20mm etalon.....

Ken

[kev 102 42]

Ken could i chop my 102 evostar

• Telescope Focal Length 1000mm f9.8
  

[brianb]

IMHO the "donor" OTA needs to be a refractor with a native focal ratio around f10

At least f/10. If you have a f/9 objective you could stop it down 10% to avoid issues with light passing through the etalon at too steep an angle, causing bandwidth spreading. A f/15 objective would work just great ...

[Merlin66]

Kev,

The nearest ERF size would be the 90mm diameter, assuming you are left with a clear aperture of 85mm after it is mounted, then the 102mm would be "stopped down" to an 85mm f11.8 system, which would be a good choice.

Not sure if it needs to be chopped! You might be able to remove the focuser and just add a spacer....

Ken

[Peter Drew]

I'm still puzzled by the question of focal ratios suitable for stage 1 PST mods. I've been experimenting along these lines for a while now, I have a 145mm ERF which gives me the opportunity to try out a range of apertures. So far, I've tried a 6" f8 Skywatcher stopped to 125mm, a 102 Vixen Flourite at 100mm, a Sky 90 at 80mm, a 100ED at 80mm and recently the "popular" Vixen A80M at 80mm. All have given much better performance than the standard PST and some of the best views have been with the Sky 90 which is at odds with the logical f10 optimal focal ratio. What I have noticed, is that as the focal length goes up, the small 5mm blocking filter vignettes the field so that only a portion of the Solar disc can be seen. This can be mitigated by raising the eyepiece with an adaptor or using binoviewers although I'm not sure if this results in the blocking filter vignetting the objective aperture. Based on my experiences, I would advise a would-be modder to have a go with what they've got to hand within reason, it at least should provide an improvement over the original.

[Merlin66]

Peter,

The f10 is based on the optical design of the original PST and is not mandatory, but when combined with the limited diameter of the etalon (20mm) and the built-in optics on the front and rear of the etalon (-200mm barlow at the front, +200mm lens at the rear), the only configuration which can give the optimal parallel collimated beam through the etalon is when it is placed 200mm inside prime focus of the "donor" objective and the final focus is 200mm behind the etalon. You mention raising the eyepiece or adding binoviewer - this indicates to me that the rear focus distance has significantly changed, so the beam through the etalon can no longer be parallel.

My original spreadsheet, and the table in my current write-up, show the changing diameter of the solar image at the etalon:

Aperture fratio Focal length Sun’s diameter at etalon

80mm f7 560mm 34.1mm

80mm f8 640mm 31.4mm

80mm f10 800mm 28.0mm

80mm f12 960mm 26.3mm

80mm f15 1200mm 25.3mm

Table 1 Solar image vs. focal ratio

(Interestingly, if you calculate the numbers, a 35mm f13 system would give an image of 20mm which would have been better suited to the PST etalon.)

Ken

[Peter Drew]

Thanks Ken for the further explanation and your write-up which I've not yet had chance to read, there's obviously a lot more mileage on this topic!. I purchased a Sky 90 some time ago with a view to adding a 90mm Coronado front etalon but eventually bottled out on the high price. Having seen with my own eyes ( and witnesses), the Sky 90 with unmodded PST rear end, on the day match two other Sky 90's with 90mm Coronado etalons I'm more than pleased with the saving. I'm looking forward to a stage 2 attempt in the near future, one of my PST's was assembled with "cherry picked" components and has one of the best etalons I've seen.

[brantuk]

I'm glad I started this thread - hadn't previously considered solar viewing till recently - but it's very interesting hearing what people have to say - and I'm learning a lot too.

Thanks to all contributors