Cheap 100mm Ha scope??

[Merlin66]

Since getting the PST and "hooked" on Ha observations of the Sun, I couldn't help myself..............
Oldfield's ( Hong Kong) blog on El cheapo ERF and pulling the PST apart to use the etalon got me thinking; why not pull the PST apart and use it on the 4" Genesis. Great idea, 6 times the resolution WooooW!!!
Just needed to find a cheap ERF; they don't exist!!!!!!!!! We're talking 100's of dollars, but as you probably know by now I don't give up easy.
I think I've cracked it!!!!!
I needed to machine a couple of adaptors with M50 x 1 threads to mount the etalon ( BTW it doesn't want to be separated from that beautiful gold tube, you need to apply "excessive" pressure, believe me!).
From Oldfield's notes also needed a x2 Barlow, you need to be above f7 (the Genesis is f5), OK found one! A TV x2 "Big Barlow".
For the ERF ( Energy rejection filter) it needs two things:
To remove the UV/IR before it frys the etalon ( or your eye!)
To filter all the visible light other than an few Nm around the Ha
Oldfield just used an old Aerial red lens, but acknowledges the focus get toooo hot and you can't put any human tissue there!
I've found a supplier who does optical filters for stage productions which block the UV/IR ( to stop the make-up from melting....honest!) and bought a 125mm sq filter. This combined with a 4" Wratten #25 filter from Surplus shed will give about 30Nm wide transmission at Ha.
Won't get to put everything together until I get back next week, but I'll keep you posted.
Total cost to date, excluding the Genesis ( you could use an ED80, or the bottom of a Coke bottle; remember you're only transmitting a narrow piece of red light, so you definately don't need an APO!!)
PST $600 ( Can still be used on its own)
Adaptors - slab of VB, plus $11 for the aluminium bar
Red #25 filter $10
UV/IR filter $170
All up say $800 and you've got a terrific Ha scope with 100mm aperture, which would cost well over $4000!!!
I can supply details of specific addresses ( the machining might be a bit more expensive than "mates rates")

[GrahamL]

some pics possible ken ?.. sounds a fun and rewarding project :)

[Karls48]

That’s very interesting Ken. I’m doing something similar. I got Meade ETX 105 that I don’t use and I’m trying to convert it for solar imagining. I have ordered 4” optical flat and 4” red Aerial Camera filter. I’m going to stick silver window tint foil on optical flat and then mount red filter behind it. That (hopefully) will work as ERF. I can not find much information on solar tint foil properties but as it reject mostly heat I hope it will pass light up to 700nm. I that is going to work then I’m going to order 656.3nm filter that I found on eBay for about $80. If sometimes in the future I find cheep (keep dreaming) PST for parts then I would use etalon from it. I been told it will not work but until I try I wont know for sure

[Merlin66]

Can't find any info on the window tint material, but may well be worth trying.
The ETX is not a good choice for a solar telescope... any excessive heat gets reflected to the secondary and can easily cook it and the corrector, Oldfield has tried with a 5"SCT and also gets annoying reflections.
I assume the Ha filter is a 1 1/4" size? Any residual heat etc will be "trapped" inside the tube between the front filter and the Ha. The performance will be limited by the bandwidth, ideally less than 0.8 A is necessary to define the Ha light in the solar image, to show prominances etc.
Check there's no heat at the focus before chancing your camera and or your eye.

[Merlin66]

Trialled the UV-IR (Rosco) filter with the Wratten#25.
ED80 stopped down to 50mm ( didn't want to melt anything first time out!!)
Filters place infront of objective.

Ambient: 7.3deg ( It is winter in the UK!)
All readings after 30sec

Near prime focus, no filters: 36.4deg
Near prime focus, Wratten#25: 24.5deg
Near prime focus, UV-IR filter: 29.6deg
Near prime focus, UV-IR+Wratten #25: 20.4deg

Taking the temp rise with no filter as 100%, then:
Wratten #25: 59.1%
UV-IR : 76.6%
UV-IR+#25 : 45.0%
I had somehow expected the combination filter to significantly reduce the amount of heat getting through, certainly not just to 45%.
I'll do a comparison with the ERF filter on the PST objective, only 40mm, (without the etalon/BF filter), but will still be interesting.

Next thing to do is confirm the transmission curves of the filters with the spectroscope.

[Merlin66]

The 4" #25 filter has just arrived from Surplus Shed. Looks good.
I'll mount the filter in a 4" PVC end cap, with a 100mm clear aperture with the Rosco UV-IR just infront.
The attached photos show the PST etalon and the 2" adaptors which will fit the Genesis.
BTW measured the PST objective, it's 400mm FL, f10 and there's no other optics between it and the etalon, therefore the etalon works in the f10 beam.

If the Sun ever comes back!! I'll do the transmission testing.
Karl, How did you go with the window tint film???

[Karls48]

I’m still waiting for 4” aerial camera red filter from Surplus Shed. I would preferred #25 filter but I could not find it on their website. I’m still thinking that reflective element in the front of lens assembly (as the window tint) is way to go because the glass element that absorbs heat will re-radiate it back to the telescope tube.

[Karls48]

The red filter arrived this morning but it is useless. It got very bad cement breakage between to glass and the filter with only about 60mm clear in the middle. Plus it got 24 point silver star sandwiched between glass and the filter. I wish that they provided better description of some stuff they are selling. Well, it will do for testing but I must look for cheep 100mm red filter. On top of everything it is cloudy all the time.

[Merlin66]

Get back to Andre or Fred at Surplus Shed, you may be able to return it.
The one you want is the RED filter from the M9014 filter set (M2) I bought mine alone for $5. ( My o/n 45866)
It fits nicely into a 4" PVC pipe end cap, just cut/ dremmel a 100mm dia aperture for a neat holder.

[Karls48]

I have send email to Fred, ordering red filter from #M9014 set. It is not worth to return it, shipping would be more then cost of this filter. Now I will have to wait again.

[Merlin66]

I'm sure the guys at Surplus Shed will look after you! The filter is 110mm diameter and 6.6mm thick.
I checked with Patrick at APM ( Agents for Baader C-ERF) and he confirmed that there are NO transmission curves available for the Baader C-ERF, but did say the bandwidth was supposed to be between 600-700Nm only. Sounds very much like the #25 + UV-IR combo.
Maybe the clouds will clear later......maybe
BTW
Anyone considered incorporating an SM40 etalon in parallel to the PST( not infront of the objective, but screwed onto the PST etalon) to give larger aperture opportunities with increased Ha resolution (<0.5A).... An interesting thought!!!!!

[Merlin66]

The attached transmission curves show the UV-IR (Rosco#38000) and the #25 Red combo.
Should achieve 80% transmission at the Ha, and cutoff 600-700Nm
Here's hoping............

[janoskiss]

Call me a coward, but I'd be very scared of putting my eye behind of any home made H-alpha filter. :scared:

[Merlin66]

janoskiss,
The ERF filter is not an Ha filter. It's purpose in life is to reduce the amount of energy which gets through to the PST etalon ( which IS a commercial Ha filter), the light in turn goes through the Ha blocking filter.
I can assure you 100% that after 40 years of building telescopes and optical systems, I would be the last person to advocate un-safe solar filters. You don't get a second chance. Hence the extensive testing of the ERF combo.
It's interesting though that Baader don't even present a transmission graph for their C-ERF,so you have to take their word for the performance.
What I'm advocating is an opportunity to get a large aperture, safe, Ha system at reasonable prices.

[Merlin66]

Though light cloud ( between the big black stuff....it's now raining; again!)
managed to get the filter tested with the ED80.
Set-up: UV-IR, #25 filter, ED80, PST etalon and BF10
Solar image focusses OK; PST etalon "tunes" OK There's a bit of background reflection ( need to tilt ERF?) but all in all a very pleasant sight. No heat build up and a reasonably bright image. No surface detail but a small(?) curtain prom at 5 o'clock ( with diagonal)
Tried to insert the x2 Barlow ( to get >f10) before the etalon but could not focus. Ended up with a 90mm spacer between the Barlow and the ED focusser to get it to image. Didn't see any/ much difference in the performance of the etalon tuning.... actually best views were with the ED (no barlow) and the 21mm TV Plossl.
This at least proves out the capability ( and safety) of the ERF combo.
Weather permitting I'll take some photos a) with the 40mm PST and b) with the ED Combo set-up to allow a comparison.

[Merlin66]

Finally got a bit of clear sky to check out the ERF combo/ PST etalon/BF10 on the ED80.
Firstly; IT WORKS!!!!!!!!!!!!
But,
1. The ERF must be slightly tilted, in my case I used a small piece of cork gasket about 2mm thick placed near the edge of the filter, this "moved" the annoying reflections away from the primary image, didn't seem to have much impact on focus or definition.
2. There must be a "sweet spot" for the etalon/BF focal ratio/ focal length. The best images were with a 10.4mm plossl at the prime focus of the ED80 ( 600mm fl, f7.5), the solar image covered about 80% of the field so it was easy to check around the edge for proms etc. Didn't try a higher magnification. However, when I inserted the TV x2 Barlow infront of the etalon ( It needed an extra spacer, 90mm long infront of the barlow) I found the vignetting from the system (BF filter?) terrible. Through a 21mm eyepiece you could see the edge/outline of the optical train as a "diaphram" about 50% of the field of view, inside which was an image of the sun, which appeared very highly magnified ( an optical illusion, due to the restricted field of view). Not very impressive!
So I have to assume that f15 is a bad number for the etalon/BF, whereas at f7.5 things looked very nice.
I must say, the #25 Wratten filter/UV-IR combo does its job and is a very effective ERF when used with the pST etalon.
I hope Karl ( and others) can verify the above and "home in" on the sweet spot.

[Merlin66]

I set up the Baader spectroscope and checked the transmission of some of my filters. Unfortunately the responce of the Canon 350D chip did not record the UV-IR impact of either the Rosco filter ( or Karl's window tint filter)
You can see the $5 filter from Surplus Shed is almost identical to the photographic Wratten #25.
BTW the Surplus Shed filter is very similar to the one Oldfield uses (no additional UV-IR!!) Hopefully some solar photos to come.
I encourage anyone with a PST to consider adapting it to use a larger aperture; better resolution at low cost!!!!

[Merlin66]

The trick to upgrade a PST to a larger aperture is being able to get enough back focal length to accomodate the 200mm required by the PST etalon.
I've found out that the ED80 and the Genesis will not allow me to position the etalon in the correct position!

I just bought a 90mm f10 objective from Surplus Shed and will construct a square custom wood tube to hold the optics and position the PST etalon in the correct postion ie 700mm from the objective.
I've also machined up a new rear adaptor for the etalon which allows me to fit a SCT focusser ( and use the rest of the SCT attachments I have); this should give the results I'm looking for.
I'll post some photos as the project moves forward.

[Merlin66]

Some photos of the IKEA kit.
Dry fitted and ready for final glue/ screw/ paint!

[h0ughy]

WOW :thumbsup:gee you have put some thought and work into this:2thumbs:. I suppose the big question is "is it still safe?" and the obvious one, what are the views like?

[Merlin66]

Definately 100% safe.
I have a choice of the ERF combo (see separate thread) or a Baader C-ERF.
I'll trial them back to back, see what the differences are, if any.
The other thought is to play around with a double stacked SM40/ PST etalon see if it can be done! THis has the potential of giving <0.5A with a 90mm aperture!!
I've made the "tube" length adjustable to accommodate the necessary barlow lens (to give a collimated beam through the SM40) and a 40mm f10 imaging section ( for the SM40 on it's own!) Let's see.....

[Merlin66]

YES!!! YES!!! OMG!!! YES!!!
First light with the EC90 at 5:48pm ( British time, its still daylight until about 8pm!) today!!!!!
It was ALL ( and more!) that it was designed to be.......
Only visual, need to screw a couple of strips on the sides to fit the HEQ5Pro mounting.....
The view through the BF10 is MUCH better than the BF5 ex-PST.....
With the 90mm f10 ( placing the PST etalon at 700mm) the field in the BF10 with a 21mm plossl is about 60% of the solar disk.
For the first time the "granulation" just jumps out, clear and tight!!!
The filament on todays photos just can't be missed; brightening around the spots... everything looks absolutely great ( wait a moment I'm about to wet myself!!!!!!!!!!!!!!!.... Ahhhh that's better!)
A quick tweek of the etalon and here we have the proms.... MUCH MUCH clearer than the PST and the SM40, God there's SO much detail!!!!!!!!!!!!
OK OK this thing works!!!!
A $50 lens from Surplus Shed, a couple of $$$ of customwood, $12 ( and two days!) on the paint and the $4 Wratten 25 and the $180 Uv-IR ( ERF COmbo) plus obviousy the etalon from the PST and (unfortunetly) the BF10 from the SM40 kit. It would have been MUCH cheaper if the BF5 gave better results..... I'll try it again tomorrow ( after a bit of viewing with the BF10!!!)
I can HONESTLY say if you've got a PST you ain't seen nothin yet until you put the etalon behind a safe ERF and larger aperture!!!!!
I'm happy it worked out soooooooo well.
Tomorrow's another day............

[Roger Davis]

Hi Ken, I have been following your construction with eagerness and am now very tempted to buy the last remaining PST on the shelf just to vandalise it for the Etalon. I have a nice 105mm f/12 lens and an old 120mm refractor tube with no optics sitting at home which now may have a new purpose in life. Here's to ATM!

[Merlin66]

First time in a long time that I've actually got excited about using a telescope; that's got to say something about the EC90!!!!!
It is possible to do this conversion without trashing the PST, so if things don't work out for you, the PST can be re-assembled and re-used.
So far the real key issues are:
1. A safe ERF (A Baader C-ERF or a Wratten#25+UV/IR combo)
2. Able to place the PST etalon 200mm inside prime focus.
3. Getting a good blocking filter to work with your system. The PST double BF at the eyepiece, seems to work OK, BUT I'd say, at the moment, the BF10 or maybe even a BF15 would work even better.
It's not a difficult conversion to do, and the improved resolution and contrast will blow your socks off!!
I think as we move towards the next solarmax you'll see more and more conversions.