LADS -Large Aperture Double Stack Ha telescope

[Peter Drew]

TJ. I suspect, that to avoid other light path issues with a Newtonian, the ERF would have to be placed very near the secondary, hence near the focus = heat.

[Merlin66]

TJ,

Hmmmmm

Certainly feasible, but remember the optimum requirement of a f10 without barlows etc.... not too many f10 reflectors being used today.

If you find one let me know and I'll trial it for you!

Ken

[Tim]

You're on!

[Merlin66]

Update....

The adaptor to mount the double stack PST etalon arrived this morning; PERFECT fit!!

I've now mounted the etalons and just need a couple of minutes of Sun to check, focus and hopefully,,,,,hopefully show the system works as well as, or better than, the theory...

Keep you posted.

( the thin silver section is the new adaptor which holds the entrance lens of the second etalon hard against the outlet lens of the first etalon ie no spacing between the lenses)

Ken

[NickH]

ooooh...yes

[Tim]

Looks good Ken! How do you focus that, is the AE bit on the back a helical focusser?

[brianb]

An 80mm ERF - Where?

Answer

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

Actually you need a slightly larger illuminated cone ... because the sun is not a point object. The solar diameter with 2000 mm focal length is approx. 18 mm, say 20 mm to give a bit of breathing room, or, to put it another way, you want to calculate the distance from the objective based on f/11 rather than f/10. (Work it out by similar triangles if you insist!)

[Merlin66]

TJ

the AE adaptor is just to give the rough distance... I have a Baader micro focuser to get final focus.

[simon84]

This looks and sounds absolutely fantastic but as someone who has acutally only ever viewed the sun in white light am I right in saying that 0.3a means you get to see more surface detail that at 0.5a? Please excuse my ignorance but I've never gone beyond a white light filter but looking at this has really wetted the appetite to get into solar viewing/imaging.

[Merlin66]

Ok

Good new bad news

Good news is that the double stacked PST etalon DEFINATELY works! Give's a resolution around <0.4A (compared with the SM60 double stack) - good crisp focus and with the larger (80mm) aperture clearly shows the surface granulation, umbra and penumbra detail as well as structure in the filaments. All in all, a VERY impressive and satisfying first view.....I'm smiling!!!

Now the downside....

The sweet spot "rings" are much more noticable and trying to tune the double stack to optimise the ring's positions and maximise the useable FOV is a PITA. (tuning for the narrow bandwidth is much easier than with my SM60 double stack!!)

I need more time ( only had about 10 mins - family business!) to set up the DMK and work though some tuning settings and get some images.....

The potential is definately there.....

Ken

[Tim]

Good to hear Ken. Funnily enough, when I tried the double stacks at kelling the etalon tuning was always the bit that makes you go UMMMMM!

[NickH]

OKay this is very interesting as I will have a DS90 very soon to test, so be good to compare

[Tim]

Great day to be testing! Sun looks amazing today

[Merlin66]

The weather today hasn't allow me to do any more testing, but looking at the set-up and thinking about it.....

I now believe the 'sweet" spot issues may be related to the tilt axis of the PST etalons.

This is obviously of no importance in a Modded scope and isn't even noted on the etalon...

With the double stacked SM60 filters I have on the ED80 the T-Max tilt hinges MUST be aligned to each other to prevent crossed interference patterns and give maximum resolution/ bandwidth matching control. This feature needs to be available also in the DS PST set-up. I think the probable mis-alignment of the "tilts" aggrivates the "sweet spot" rings....

OK - so all I now need to determine the direction/ axis of tilt for each etalon and figure a way of getting them aligned and locked.

Makes sense to me....I think....

Ken

[Peter Drew]

I'm following this with interest Ken as I have two good PST Etalons. To date, I've not been a great fan of double stacking for the sole reason that although it enhances disc detail it's always seems to be at the expence of prominence visibility, the feature of Ha viewing that I enjoy most. However my experience of double stacking has been confined to standard PST's, it may be that the larger apertures now available due to modding might redress that balance. Speaking of which it was sunny here this morning and I was able to get first light through a 150mm f8 Helios refractor stopped down to 125mm f10 coupled to a stage2 modded PST. Very good results, better than my 80mm Vixen version but not as good as the 220 mm with sub aperture ERF, this one shows detail in the detail .

[Tim]

Peter, I was privileged to spend quite some time with ds 90mm setup a few weeks ago, with a 2" BF.

The view, including the proms, was fantastic. The DS PST's by contrast always seem a little on the dull side.

[Tim]

That sounds logical Ken. For some cameras I have an extra locking ring which tightens against the camera to hold it at the required rotation, could something like that work on your adaptor, or is the butt joint critical?

Following your guide I did the card thing today with my Vixen 910mm super polaris 80M OTA. The best point of focus was 230mm from the end of the tube. Does that sound about right?

Using a wooden adaptor I taped the black box into place and stopped down the tube with the lens cap and easily got good focus and resolution, I guess that's a good sign? Didn't want to test for more than a few secs in case there was an issue with not using an ERF (aperture in lens cap is approx 40mm)

Inside the tube there are several baffles. The nearest to the rear end is 133mm from the end of the tube, and is approx 45mm internal diameter.

If I was to get a 50mm ERF and put it behind this baffle, would there be enough distance between it and the etalon to avoid heating issues do you think?

Just need to find some kind soul with a lathe now

[NickH]

I'm following this with interest Ken as I have two good PST Etalons. To date, I've not been a great fan of double stacking for the sole reason that although it enhances disc detail it's always seems to be at the expence of prominence visibility, the feature of Ha viewing that I enjoy most. However my experience of double stacking has been confined to standard PST's, it may be that the larger apertures now available due to modding might redress that balance. Speaking of which it was sunny here this morning and I was able to get first light through a 150mm f8 Helios refractor stopped down to 125mm f10 coupled to a stage2 modded PST. Very good results, better than my 80mm Vixen version but not as good as the 220 mm with sub aperture ERF, this one shows detail in the detail .

Nice!!

[Peter Drew]

TJ. With a 50mm ERF you should be able to place it at least 400mm up the tube where the heating should be well within acceptable limits, 130mm seems a bit too close to the focal point. Today, with an 85mm ERF inside a 220 mm refractor, a 10 minute exposure did not result in overheating or image degradation, mainly due to the Sun being lower this time of year. In high Summer I will have to use the air blower cooling system that I have successfully trialled.

[Tim]

Hmmm, not sure it will be possible with the tube I have then, there are three baffles of different apertures down the tube.

Although, if it goes 130mm up the tube, and the focal point is 230mm behind the tube, that makes it 360mm doesn't it?

There was certainly not much, if any, heat on my hand when I had it at the back of the tube this morning, the sun was quite low though. Is there a way to test the heat build up? maybe a thermometer in position?

[brianb]

not much, if any, heat on my hand when I had it at the back of the tube this morning,

Caution is needed here ... depending on what's acting as a heat pipe / sink the actual temperature rise may be small even with a lot of energy being absorbed ... as for instance the heat sink on a solar wedge, which gets just about the full solar input yet remains comfortably warm (15 - 20C above ambient).

The big issue here may well be that even moderate heating may cause severe tube currents, causing havoc with the "seeing". Solar wedges avoid this by having the heat sink outside the optical train.

Heating may also cause tuning shifts in the etalon. How much depends on the construction: Solarscope use quartz which is fairly insensitive to temperature changes, even so they claim stability is around 1A per 200C so 20C is enough to cause a tuning change sufficient to upset a DS rig. I believe the Lunt etalons, made of glass, may be much more sensitive than that to temperature changes. The bandpass drift can of course be "tuned out" but that is an inconvenience.

[Merlin66]

T-J et al,

Your measurements sound pretty spot on. I've just assisted on a Mod where the "back plate" was given a 55mm thread and a threaded tube section ( with a 50mm x 1mm PST threaded end) was used to hold and locate the etalon.

Re heat build-up etc.

In my experience both here in the UK and back in Oz - the PST etalon, by design, can handle the energy from a 50-60mm aperture, so when testing and setting up, an aperture stop of say around 55-60mm can safely be used without an ERF.

I do have some solar energy figures ( I prepared a write-up a couple of years ago for the UKAI forum) which I'll look out for comparison.

Basically if the total energy (excluding UV-IR) on the etalon is reduced to the eqivalent of a 60mm aperture then we're OK - this can be achieved using an ERF or beamsplitter type mirrors.

The issue obviously is not what passes through the sub-diameter ERF (that will be OK) but the heat loading on the front of the ERF and the reflected heat back to the rear of the objective. I think you will be pleasantly surprised with the results....I know from first hand experience that the spectroshelioscope slit at focus doesn't overheat and that Ramsden eyepieces (non cemented types) can and do solar projection with 150mm reflectors where they are located VERY close to focus. The solar image and hence the intensity is proportional to the focal length ie approx. 1/100 the fl. ie 8mm diameter for a focal length of 800mm.

As the D-ERF is a dichromatic design (ie reflection interference filter) there will be probably little heat build-up in the glass itself, but the majority of the heat will be reflected back to the objective...

Let me find my energy calcs.....

On the LADS, I'll measure the etalon tilt axes and use the existing adaptor to rotate them relative to each other - 1/2 a rotation will only add 0.5mm to the spacing which should be OK......

[NickH]

Well, as you may have seen, if this one comes off...this is the kind of imaging result one can expect!

http://www.astroimagem.com/Halpha/AS200/2010-10-17filament_flare-AR1112.jpg

http://www.astroimagem.com/Halpha/AS200/2010-10-17_Active_region_1113.jpg

[Merlin66]

VERY impressive!

Obviously becomes the benchmark. I think the LADS concept has the potential to give the same (or better!) results in a far more compact and user friendly arrangement.

I've found my original write-up on the solar radiation hazards - copy (in two parts) attached.

ken

ICNIRP_review_part1.doc

ICNIRP_review_part2.doc

[NickH]

Literally drooling with the anticipation (must get that fixed...lol)

It's a whole new ballgame at 200mm as those images continually prove, and you can argue about aperture etc...but the proof is in the images, nothing else is coming even close