6" PST Stage II progress.

[Rusted]

Anybody know of a decent 20mm optical length 2"-1.25" EP adapter with a compression band or self-centering mech?

I've been searching all the dealer's websites but most don't mention the optical length.

I need 20mm more optical length to reach focus with a good grip.

It can't be done with tubular extensions below 35mm because the inside and outside diameters would overlap.

Leaving no material to hold it together.

Any bright ideas to add 20mm optical length after the etalon by any other means?

Thanks

[Rusted]

Before I crank up the lathe tomorrow, to make a special 20mm deep 2"-1.25"adapter:

Does anyone remember the thread size of the ITF tube into the PST's black housing?

This is the same thread as some 1.25" star diagonals have for the female EP socket.

I might be able to find a commercial 20mm extension to push the filter stack 20mm away from the etalon.

I measure about a 32mm hole in the PST housing.

So M32 x something? M32.5?

Thanks

 

[Merlin66]

I'm sure I have that info somewhere.....just need to find it!

Are you sure about the 20mm spacing? I have 130mm of spacers between the rear PST thread and the front of the diagonal....your arrangement looks shorter.

I also use T thread spacers to provide the flexibility of spacing. You could consider an Orion 2" to T thread adaptor behind the etalon then use T thread spacers and a Baader T thread to 1.25" or Borg helical.

 

 

[Rusted]

Thanks again, Ken.

I arranged the leading edge of the rubber band on the etalon precisely at 20cm inside focus with the D-ERF in place.

My "straight through" collection of extenders was indeed quite long.

That was the setup I used for my handheld 'snap' at the eyepiece.

I was amazed I captured anything through the Cemax 12mm.

Then I inserted the 1.25" star diagonal with the ITF/BF stack screw directly into its top thread.

By holding the star diagonal, exactly one MkI index finger width away from the AOK-etalon tailpiece , I then had perfect focus.

I carefully measured the MkI index finger width with a precision Mitutoyo Vernier caliper.

Hence the desire for a 20mm spacer and several hours of poring over countless online sales websites.

Now it's back to more nose dripping on the lathe as I make a 20mm thick 2"-1.25" adapter.

After the considerable expense of the PST + MkII mods my life would be seriously shortened if I spent any money on a Borg helical focuser.

[Rusted]

Hi Ken

You got me thinking: The T2 standard makes a lot of sense. It depends how I value my time. 

Hours on the lathe in sub freezing conditions to make a naked metal adapter. Or buy a couple of pretty bits from TS.

https://www.teleskop-express.de/shop/product_info.php/info/p200_TS-Optics-Adaptor-2-inch-to-T2---Focal-Adaptor.html

https://www.teleskop-express.de/shop/product_info.php/info/p209_Visual-Back-T2-to-1-25----with-additional-T2-male-thread---short.html

This is the shortest optical path length conversion from the AOK-etalon 2" female tailpiece to T2 male thread.

Then female T2 to 1.25" compression ring to firmly hold the star diagonal directly mounted with ITF/BF/eyepiece holder stack. 3+15 = 18mm, hopefully.

As you say, if I do need more optical path length, the possibilities are endless with T2.

[Rusted]

Bright but misty sunshine with light winds and 43F today. Several prominences clearly visible with some surface texture in a ring.

So I checked the optical path length of straight through and using the 1.25" star diagonal.

The straight through distance was 216mm from the leading edge of the etalon band to the underside of the gold band on the Cemax 12mm.

Removing the 80mm extension and fitting the star diagonal required the EP be withdrawn by 22mm to reach focus.

Just waiting for the T2 to 1.25" adapters to arrive now.

 

 

[Merlin66]

The optimum back focus, when the etalon is at the majic -200mm is 225mm behind the rear of the rubber knurled ring. I don’t think the etalon spacing is 100%

 

[Rusted]

Thanks, Ken.

I noticed the 60mm extender had moved out of the temporary plywood ring by a couple of millimeters.

This shortened the after-etalon focus. Pushing the extender back in again helped but obviously not quite enough.

It looks as if I shall have to move the 60mm extender in a bit further. A few millimeters counter-boring should do it.

I'll turn a solid aluminium backplate when I have it all spot on. I'm thinking of adding extended control rods for adjusting the D-ERF.

Thumbscrews projecting from the backplate to fine adjust the tilt the baffle carrying the D-ERF.

Unfortunately the forecast is no sun for the next week. The moment any appears I shall be back out there.

The ease with which I am seeing prominences is quite amazing after decades of waiting to see them 'live.'

There was a large 'curtain' yesterday and a 'pointy' mountain which suddenly snapped into a detailed Xmas tree at best focus/tuning.

Bit short on the correct terminology at this stage. Thanks for your continuing advice and inspiration.

[Rusted]

Brief sunny periods this morning between heavy cloud combined with 45mph gusts.

I had re-bored the plywood backplate to allow the etalon to be pushed even further in.

Focus was now half an inch outwards from the Cemax 12mm being fully home in the PST EP holder.

The 40mm x 2" extender was removed.

The first baffle was moved to the objective end of the tube for 120mm aperture.

Two jet type prominences and an arch were nicely sharp at my 8 'clock in the eyepiece.

Far sharper than the online H-alpha images for today!

Surface texture depended on the degree of clarity between clouds.

The background had a red blush on one side of the solar image.

This was absent on the other side. Does the D-ERF need a nudge?

Or is it more likely to be the PST etalon?

 

 

[Merlin66]

Sounds more like a ERF issue than the etalon - vary the ERF tilt back/ forth to see what happens.

 

[Rusted]

Thanks Ken. I'll try that next time we see any real Sun.

Here's my latest back focus geometry:

This is the best h-alpha image yet! With superb, feathery detail in the prominences at only 100x.

And, needing no re-tuning to see both prominences and surface structure simultaneously.

I bored the plywood backplate deeper to bring the etalon even nearer the objective.

I was struggling with only brief glimpses between heavy cloud in a 45mph gale yesterday.

I kept being blown off my old beer crate, hop-up. The telescope was rock solid though.

[Merlin66]

Another 20mm and majic will really happen!!!!

 

[Rusted]

Well, you're the [Solar] magician. So you must be right. 

 

[Moonshane]

hi Ken

I don't doubt you are right (naturally on such matters you are!) but if you have the etalon further back like above, presumably the whole light cone will pass through the etalon? Just for my info (read so I can get it into my thick skull) , could you please give a simplified summary of why the position of etalon is so critical assuming it's beyond -200mm as in the above scenario?

I can get why it should not be forward of -200mm but cannot understand the other side.

[Rusted]

1 hour ago, Moonshane said:

hi Ken

I don't doubt you are right (naturally on such matters you are!) but if you have the etalon further back like above, presumably the whole light cone will pass through the etalon? Just for my info (read so I can get it into my thick skull) , could you please give a simplified summary of why the position of etalon is so critical assuming it's beyond -200mm as in the above scenario?

I can get why it should not be forward of -200mm but cannot understand the other side.

In case Ken has adopted the inverted position for some shuteye..

The etalon needs parallel light to function optimally. It also needs an f/10 light cone.

Regardless of aperture a certain focal ratio always produces the same included angle of light cone.

Parallel light can only be achieved when the lenses which sandwich the etalon can manage that task.

The first [negative] lens acts like a Barlow to make the f/10 light cone parallel.

After passing through the etalon the light is bent [condensed] back to the original cone angle of the objective by the following [positive] lens.

If the negative lens was inside its optimum position it would throw the solar image further out.

Equivalent to a 'slower' f-ratio than PST optimum for the following filter elements.

The incorrect cone angle would upset the [PST optimized] f/10 optical geometry of the etalon & ITF/BF stack.

Similarly if the "Barlow" lens was outside its optimum [20cm] position the result would be equivalent to a "shorter" focal ratio.

Again, this would not provide the etalon & ITF/BF with the correct light cone for optimum [PST based] performance.

I have squinted through the etalon sandwich at distant objects.

The sandwich has a strange optical effect but neither magnifies nor reduces what is seen through it.

At least, not by enough to notice any difference.

The PST is really a rather sophisticated collection of optimized components based on f/10 geometry.

No doubt Ken will be along soon to correct my assumptions from reading most of the online PST modification resources.

[Moonshane]

Cheers Rusted

I suppose I am coming from the experience that I cannot see that much difference in my scope (Tal based stage 2 mod) when the etalon is in positions differing by 10mm or so. I generally always get the same levels of good detail although perhaps if I got it spot on, the results would be even better? I do use BVs and get full disk at around 50x with the original 5mm BF which confuses me even more! 

[Rusted]

My actual observation is still limited to short trials between clouds.

I only reached the 20cm optimum etalon position today and it never cleared after that.

How do you keep the power so low using BVs?

You usually need a Barlow which doubles the power.

So you'd need 40mm EPs which surely wouldn't match the tiny BF5mm?

This rather puts me off investing in a binoviewer plus all the extra EPs on top.

Which particular BV and EPs are you using?

All my EPs are Meade 4000s so I'd need to search the s/h market to double up for binoviewing.

[Peter Drew]

@Rusted. I think your "assumptions" re the reasons for the etalon position are most likely correct, Ken, being also my mentor will no doubt give chapter and verse in due course. I have also found @Moonshane that a surprising shift either side of the optimum position for the etalon still gives good results, but why not get it right!. I use entry level Plossl eyepieces and an entry level binoviewer and have not achieved better results with better components. I use a standard 2X SW Barlow lens screwed to the nosepiece of the binoviewer, my estimation comparing magnifications with single eyepieces is that this provides around 4x amplification and a magnification of 150x on my 150mm F10 with 40mm FL eyepieces. Like Moonshane, I can get a full disc solar image with a Barlowed binoviewer on a 4" F10 solar mod at a higher power than it ought to, the jury is still out on how this is possible.    

[Rusted]

Thank you, Peter.

As I am already struggling with a tiny field of view your amplification factor fills me with trepidation!

I could rarely go over 120x on the Moon or planets with the 150/8 due to the poor seeing.

The 40mm is the one EP I don't own. So I'd need a pair.

Is there any better option to the Meade 4000 while I'm virtual window shopping?

The M4000 40mm is still available at just under £50 a bottle.

I'd have to order two. Any discount for bulk? 

EDIT: The old problem of limited field of view with a 40mm in a 1.25" fitting comes up again.

A 32mm Plossl usually has a similar field of view to the 40mm but increases magnification even further.

[Peter Drew]

@Rusted. With your shorter focal length you can get away with a pair of 32mm Plossls.  At 150x which is my lowest magnification with binoviewers I can get about 1/3 of the solar disc in the field, plenty enough for my requirements as I like close up high res views, I have smaller solar scopes if I really want a full disc. You really must try binoviewers for solar, the improvement with the Barlow attached makes it obligatory IMO.   

[Merlin66]

Being in Oz I do the night shift for the forum......

 

The above detail is generally correct.

1. The PST etalon was designed for use in a 40mm f10 system

2. The front "barlow" has a focal length of -200mm (the Majic 200) (It is positioned very close to the front edge of the knurled ring)

3. When the barlow is placed at the -200mm the lens acts as an aperture stop and "converts" the donor to a f10 light cone.

4. When the barlow is placed at the -200mm, the emerging beam is a parallel beam which maintains the design requirements of the etalon.

5. If the barlow is positioned <> 200mm (+/-5mm) the emerging beam is either a "converging" or "divergent" beam when it hits the etalon. This causes a path difference across the etalon gap and can impact on the central wavelength transmission and sweet spot.

6. When the barlow is correctly positioned there is no need for a focuser in front of the etalon assembly. The original Gold tube in the PST never changed it's length!!!

7. The final focus is produced by a +200mm lens (which sits 25mm rear of the knurled ring) - the optimum final focus is therefore 225mm behind the rear edge of the knurled ring.

8. Some adventurous modders have changed the barlow/ rear lens to "better match" their available donor telescopes. IMHO the classical f10 system seems to give best results.

 

Any questions/ concerns/ comments always welcome.

 

 

 

[Rusted]

Thanks, Ken.

So I ended up not so red faced with my less precise description of our "needy" little PSTs and their mods.

Downside with solar is becoming very red faced, from wind burn.

This was from standing outside for hours and hours in 40mph+ winds at -1C under a complete overcast waiting for a sucker hole!

As soon as it was dark the sky cleared. [Of course it did!]

This morning it is -5C and will soon be blowing just as hard again!

Upside is an almost cloudless sky with bright sunshine.

Though I'll need to upgrade my safety gear on my observing beer crate to save me from being toppled in gusts.

Do you think crampons and an ice axe are a bit over the top for solar observation? A could belay myself to the pier, I suppose.

Meanwhile Peter is poking me in both eyes until I cough up another wad of cash to go binoviewing.

Am I having fun yet?

[JamesF]

10 hours ago, Merlin66 said:

3. When the barlow is placed at the -200mm the lens acts as an aperture stop and "converts" the donor to a f10 light cone.

This is the bit I'm struggling to get my head around when used with an objective that is not f/10.  I think I might need to draw some pictures of the light cones.  Or do some maths.

In particular I'm wondering if, when using an objective with a focal ratio faster than f/10, one couldn't place the etalon unit "further back" so it doesn't act as an aperture stop.

James

[Merlin66]

James,

the focal length of the barlow is -200mm, that's where it needs to be relative to the prime focus - to give the required parallel beam though the etalon. The aperture of the barlow (20mm) at this -200mm then acts as as an aperture stop and forces the outcome to be an f10 "system" (200/20=10)

 

[Moonshane]

9I think mine is about right but will check next time out ?

Re the bvs I use Baader Maxbrights and a 1.25x GPC and 25mm TV plossls to give 50x and full disk with lots of space around. I also use 20mm (62x) and 15mm (83×) and I seem to recall getting full disk with all. I certainly did with the 1.7x GPC at 25mm (68x) and 20mm (85x). All that said I still tend to get best results in the central sweet spot so centre proms etc for best results.

I used the shortest possible components after the etalon  to allow min magnification.

Here's my stage two mod binoviewer ready

 

And in the scope