LADS -Large Aperture Double Stack Ha telescope

[Merlin66]

I'll use this thread to document the trials and tribulations of preparing a large aperture telescope with a sub-diameter Energy rejection filer (ERF) fitted with two etalons from the PST and a BF10 blocking filter, to allow effective double stacking performance of less than 0.5A in Ha.

Besides being an interesting project which I don't believe has been tried anywhere else, it may show the capabilies of such a system and be of interest to members who either have modded their PST already, or would like to consider the mod and demonstrate a cost effective upgrade to high resolution Ha Imaging.

I have a write-up (6Mb) available which describes how to do the Stage 1 and Stage 2 mods on a PST. This LADS scope will start by building on an existing Stage 2 mod.

The set-up I have is a 90mm Baader D-ERF (effective 88mm clear aperture) on a Meade 90mm f10 donor OTA, fitted with a PST etalon and a Coronado BF10 blocking filter. It gives excellent results and compared with my SM60 double stack is giving a good 0.7A performance.

The second PST arrived today, and believe it or not could be easy dismantled by hand! No BOA wrenches required - the gold tube just unscrewed...and then the etalon...one of the easiest pull downs I've had this year. The etalon looks good, but I haven't had any sun to properly test it. That can be done later.

The attached diagram shows the proposed layout for the LADS. I'll trial the double stacked etalons before moving onto the larger aperture.

The front and rear threads on the PST etalon are 50mm x 1mm pitch, so to position the etalons I'll need an adaptor with a female/ male thread. Hopefully I can get this machined next week.

In the meantime I have a "collection" of SCT adaptors etc which have a similar (but not identical) thread.

This will be the starting point.....

[Merlin66]

Well....

At 12:35pm on the 10/10/10 I managed to demonstrate that the double stacked PST etalon certainly has the potential of working!!!

I set up the 88mm modded PST and added the second etalon (held in place with spacers and gaffa tape (!) and managed to focus the Sun's image through the BF10 and a 10mm eyepiece.

Unfortunately the joint wasn't sturdy enough to allow the etalons to be tuned, but just getting a good focused image is enough encouragement at this stage!!!

The back focus distance was shorter than anticipated, only 185mm but I think that may have been due to the "gap" between the etalons ( about 10mm).

The next stage is to get a M50 threaded adaptor made up to hold the etalons snuggly together.....

It can be done....LADS with a 0.3A resolution!!

Onwards and upwards.....

[NickH]

ooooh come to papa!

That's a second etalon on the cards now then!

[Merlin66]

Nick,

IMHO now that I'm 100% sure we can focus there's no technical reason why the double stack shouldn't perform as well as the theory. It opens up a whole new ball game for modded PST's and certainly the potential of sub 0.4A performance has a certain appeal.

Let's see how quickly I can get these adaptor(s) made up.......

[NickH]

Sub 0.4A 80-100mm scope... it's a no brainer...especially as it can easily be switched back to 0.7A for the proms

[Peter Drew]

Interesting stuff Ken!. You may recall that I am currently experimenting with a 220mm aperture refractor and sub diameter ERF, I had'nt considered double stacking but I do have two PST etalons so it will add to the interest.I have acquired a high volume output air blower with flexible hose to try and get round the ERF heating issues but a trip to Tenerife has held things up. I've also recently modded my stage 1's to stage 2 as advised by my Ha Peers, I was initially reluctant to do this as the stage 1 results were so good, however stage 2 is even better. I've retained the original black box, less prism, as a homage to the designer, I did this by opening up the back end to accept a 1.25 diagonal and the suitably spaced B/F.

[Merlin66]

220mm aperture:icon_eek:.....what size ERF are you using?

How the devil do you get the f10 beam for the PST etalon????

telecentric barlow?

I've ordered the adaptor for the PST etalons, if it works as planned I can supply the necessary sketchs to get one made.

Really does feel VERY promising.

Ken

[Peter Drew]

Ken. 145mm aperture ERF or 85mm ERF depending on position in tube. OG is F12 . Peter.

[Tim]

Sounds very interesting Ken.

What stops the heat building between objective an ERF in your sketch though?

Thanks

Tim

[GazOC]

Looks interesting Ken, whats the maths for determing the distance from the objective of the ERF for given sizes of objective and ERF?

[Merlin66]

Nothing....

That's why I suggest an ERF/objective ratio >50%. I honestly don't have rigorous data to determine the exact heat loading but I can give you an example of a 100mm f8 objective focusing the Sun's image onto a stainless steel slit (no ERF, no filters) and the slit only gets warm to the touch. (The 8mm diameter (approx) solar image is used in a spectrohelioscope.)

If the thermal effects appear to affect the resolution, I'd probably "break" the tube and "bend" the optics by 90 degrees using a 50/50 flat beam splitter at 45 degrees. This would allow 50% of the heat/ energy to exit the tube. ie a 200mm objective would be reduced to an "energy equivalent" of a 140mm, then have the ERF....

I used this concept on the EC90 (El Cheapo 90mm Ha scope) a couple of years ago - infact I used two 50/50 beamsplitters instead of an ERF! The energy reduction of 0.5 x 0.5 =0.25 applied to a 90mm brought the "energy equivalent" down to a 45mm - which is well within the acceptance of a PST etalon. So the resolution of a 90mm aperture with the energy loading of a 45mm with a PST etalon..worked well ( It ended up a double stack configuration)

Ken

[Peter Drew]

TJ. My current experiments with a sub aperture ERF single stack etalon arrangement leave me to believe that there would be no serious (or dangerous) heating effect with a 50% of the objective diameter ERF placed around halfway up the focal length.

GazOC. Without using any maths, you could draw a circle on white paper equal to the diameter of the ERF and offer it up to the objective whilst pointed at the Sun (or the Moon if you would prefer), provided that the resulting illuminated disc is less than the ERF diameter it will not vignet the objective. The smaller the ERF, the nearer to the focal point it would have to be, and the hotter it will get. Worth a go though as the results are awesome.

[Merlin66]

GazOC

Use similar triangles ie 200 f10 = 200/2000

An 80mm ERF - Where?

Answer

(80/200) *2000 = 800mm from the focus ie (2000-800)= 1200mm behind the objective.

[NickH]

In essence then, a slotted OTA where you could place the ERF "in the slot" and then seal it back over (like a large filter drawer), but which held the OTA rigid enough, would be ideal

Taking a 150mm scope (common and not too costly for say an AR6/C6), with the appropriate focal length, et voila

[Merlin66]

Geeez Nick, I just hate that when you look over my shoulder!!

You must have seen the sketches I have drawn up....either that or fools seldom differ:cool:

[GazOC]

Thanks for the answers Peter and Ken. It makes sense that you'd have to be more and more concerned about the heat the further you had to place the ERF from the objective.

[NickH]

Regretting selling the AR6 now... :-(

[Merlin66]

I have a lot of respect for the Sun and the possible damage it can do. Using spectroscopes, spectrohelioscopes, Ha scopes makes you aware of the possible dangers and precautions MUST be taken at all times...

One of the versions of the EC90 and what can happen if you leave your telescope unattended during the day!!

[Merlin66]

OK I just had confirmation that the double etalon adaptor is in the post, should arrive tomorrow.

Weather permitting I hope to demonstrate the double PST etalon performance over the weekend.

The next step will be the hacksaw......

Ken

[Tim]

Ah, yes I see now, keeping the ERF closer to the objective keeps the heat down

So an OTA which all screwed together, with or without the ERF in place, like a Borg could make for a nice dual purpose instrument.

So, and excuse my mathematical ignorance, If I was to get a 70mm DERF rather than a 90mm, saving £££, how far down the Vixen A80 tube I have would it need to go? Mind you, a problem might be the internal baffles I suppose, not sure how many there are, unless the first one back from the objective could be used to hold the ERF against in its cell.

Question then, does the ERF still need a tilt to the optical path to avoid reflections?

Thanks.

[Tim]

Shame you couldn't put an ERF in front of the secondary in a Newt then, ????????

[Merlin66]

Same formula as per message #13 and yes the ERF still should be tilted to reduce the opportunity for unwanted reflections.

[Astro_Baby]

I really must get over and see you at some point and pick your brains on solar

[Peter Drew]

TJ, in order to benefit from the main (only?) advantage of sub diameter ERF's- cost, I think you would be OK with something like a 50mm ERF half way down an 80mm Vixen tube.

[Tim]

Thanks Peter I have number blindness, formulas might as well be in Chinese

So, is the ERF in front of a secondary mirror of a newt a non-starter then?