6" PST Stage II progress.

[Rusted]

Hi

I have just taken delivery of a set of AOKSwiss' excellent PST etalon adapters.

Building the optical train with correctly spaced PST components becomes much easier now.

Looking at the ITF/BF5 assembly I noticed it has a parallel 36mm waist when both are screwed together.

This could easily be accommodated in a standard 2" to 1.25" focuser adapter.

It just needs to be bored out a couple of mm from 31.8 to 36mm.

The filter's 'waist' allows 3/4" of 'focusing' in the bored out adapter.

The filter stack then plugs into the top of a 2" star diagonal to maintain its original spacing.

I have also built a dirt cheap D-ERF filter holder from a vintage, stainless steel, P.O. damp sponge holder.

Bored out both faces in the lathe and lined and ringed it with cork to hold the filter.

Then simply screwed it to the middle Celestron 6" refractor baffle with the same aperture as the 90mm D-ERF holder.

It just needs to be shoved in on edge and then turned square to the optical axis to jam it in place.

[Peter Drew]

Looking good, first light looming!  

[Rusted]

Hi Peter

All thanks to your and Merlin's expert advice and experience. 

Still waiting for my new ITF but it's all coming together.

I wasn't sure anybody else was using a 2" star diagonal.

Even though the PST filters have rather small apertures.

It just made so much sense I decided it was worth using the 2"

Plus, the 2" fittings are so much stiffer when joined together in series.

I also get a free push-pull 'focuser' by sliding the filter stack in the adapter.

If I counter-bore the adapter I can increase the focusing range to about an inch without moving the etalon.

[Rusted]

Ooh-er?

My Maier ITF replacement has arrived in the post.

Truly excellent service from yet another company has considerably hastened my H-alpha progress.

But! I've got an O-ring in the package. No O-ring fitted with the original Coronado ITF. No instructions.

I know the shiny green side goes towards the sun/objective. Red towards the EP.

So what about the O-ring? I've searched online but come up completely dry.

Bung the O-ring in first.. then the filter on top?

Or filter first with the O-ring on top?

Eany meany miney....?

[Rusted]

I decided to turn an 18mm,  3/4" plywood ring to adapt the tailpiece of the CR150 to a 2" eyepiece extension.

By turning the plywood in the lathe all surfaces are square, concentric and of exactly the correct size for a perfect press fit.

Now I just need to slide the D-ERF into position in the main tube and reconnect the tail end assembly to enjoy 120mm aperture in H-alpha.

I'll fit it on my Fullerscopes MkIV mounting on its massive steel pier.

Job done. 

My thanks to all those who have shared their expertise to help me avoid costly mistakes. 

 

[Rusted]

Quickly set up on the MkIV mounting in unbelievable sunshine.

I had to replace the longer 2" extender with a shorter one to reach focus.

This needs thought as to whether the etalon is still 20cm inside focus.

There was increasingly intermittent sunshine as the misty sun reached 23° altitude.

Razor sharp on the limb but rather limited field of view. Best with Cemax 12mm for 100x.

Plossls seemed not to work so well due to excessive eye clearance.

First H-alpha light: Spotted several prominences and overall 'orange peel' simultaneously by tuning.

The 11 o' clock sunshine was sheer luck because it soon clouded over to grey.

Need a sun finder and a shade screen. A proper focuser would be nice but not essential. Helical?

[Merlin66]

I've been using the Borg helical focuser on my PST mods for many years.

Works well, but it would be "nice" to be able to fit a motor - something like the SW electronic motor with belt drive. I use this arrangement on the SM60/ED80 and the Spectroheliograph.

 

[Rusted]

Thanks, but the "thin" Borg is far too expensive to contemplate at the moment.

Besides, I may also have messed up my 2" star diagonal, light path calculations.

Making it even more difficult to insert any focuser after the etalon.

I had to fit a much shorter extender to the tailpiece just to allow me to reach focus.

Without noticing, at the time, that I had completely ignored the "20cm inside focus" etalon rule.

The ITF/BF tubular housings are far too large to fit into a 1.25" star diagonal.

So I may be limited to a "straight through" view while I rethink my tail end design.

While it certainly worked, as a first try, I think I need to get back to the correct etalon position first.

Otherwise I  could be throwing away useful aperture.

The long, rear, AOK, 2" to etalon, female adapter could be considerably shortened [in the lathe] if I replant the thumbscrews.

Then a lower, bored out, adapter flange will shorten the light path by a few more millimeters...

[Merlin66]

Hmm,

Something wrong here...

If the etalon front lens is positioned correctly at -200mm inside prime focus then the final imaging focus shoule be 225mm behind the rubber adjusting ring (allowing for the position of the re-imaging lens in the etalon assembly)

This should be enough space for a focuser etc. The  AOK adaptor should NOT be modified IMHO......

 

 

[Rusted]

Thanks. Particularly for the 225mm figure.

To achieve focus I replaced an 80mm [2"Ø] extender [spacer] with a 40mm extender.

The 80mm was correct for the 20cm inside focus etalon position.

So I need to shorten my post etalon light path by 40mm.

Having had another look at the rear fitting AOK I don't think I can reclaim very much depth there.

The 2" spigot of the star diagonal takes up all the "free" length of the AOK before the baffle.

While the ITF 'nose' completely fills the other side of the baffle. 

I can only save a measly 6mm by using a thin flange 2"/1.25 adapter. [Bored out to 36mm.]

Another 2mm from closing the ITF/BF tightly together. [Even more red gunk thread cleaning required first.]

The long light path of the 2" star diagonal is doing all the damage!

But there no obvious way to employ a 1.25" diagonal before or after the ITF/BF.

[Rusted]

Problem solved.

I found an old no-name, 1.25" star diagonal which had an eyepiece holder thread to match the ITF's.

The 80mm extender can go back in the refractor for the correct, 20cm inside focus, etalon position.

The 40mm extender goes after the etalon to bring the light path length to focus. See image:

If I could just fix my terminal man cold I could be out in the sunshine! <sniff> 

[Peter Drew]

I was just about to suggest this solution but you beat me to it, well done!. Hope the "cold" is better soon.  

[Rusted]

Thanks, Peter, on all counts. 

I'm sure I read about the ITF thread fitting a 1.25" star diagonal somewhere.

So I can claim no originality.  My first attempt at matching threads failed.

It was sheer luck I had another star diagonal in the "hardly ever used" box.

Never throw anything away!

[Chriske]

Hi,

Is there info to be found to build such a scope.
A friend of mine is planning and eagre to build one.

Thanks,

Chris

[Rusted]

Hi Chris

Merlin has an excellent, free paper which he will send as a file by email if you ask him.

I listed all the online build sources and videos I could find on my blog:

https://fullerscopes.blogspot.dk/2018/02/links-to-h-alpha-pst-modification.html

[Chriske]

Thanks a lot...

[Merlin66]

Chriske,

If you contact me with an email address, I'll send you a copy of my PST Mod write-up.

 

[Chriske]

Thanks again for the file..!

[Rusted]

Chris

I have no idea as to your plans but don't try to under-size the Baader D-ERF.

I went with the 90mm and it is almost down at the focuser end of my 6" f/8.

In hindsight the 110mm filter would have been much better but was even more expensive.

[Chriske]

Thanks Rusted.

I'll discuss it with  my pal Guy first before we proceed.

[Peter Drew]

2 hours ago, Rusted said:

Chris

I have no idea as to your plans but don't try to under-size the Baader D-ERF.

I went with the 90mm and it is almost down at the focuser end of my 6" f/8.

In hindsight the 110mm filter would have been much better but was even more expensive.

How come? Your 6" F8 objective will be operating at 120mm clear aperture so halfway down the light path will be 60mm. The 90mm D-ERF need be no less than 85mm when mounted. 

[Rusted]

Hi Peter, and thanks for intervening.

I used the focused solar light cone from the objective to judge the desired position of the D-ERF.

Even when only 40cm from the tailpiece the bright circle overlapped the edges of the [original] 85mm baffle on which I had mounted the D-ERF.

I meant to ask you whether it was necessary to physically stop down the 6" f/8 objective to 12cm f/10 with a baffle.

Somehow I had assumed that it occurred automatically as a direct result of vignetting by the small diameter of the PST optics.

At only 138mm ID the CR150's main tube is very close to the light cone or actually stopping down the objective where it joins the fixed cell.

The 1st original baffle could be brought right forwards towards the objective to achieve stopping down.

[Rusted]

An update:  Peter you were right.

The first baffle after the objective is 122mm Ø so will do perfectly to stop down the 150mm to 120mm.

I made a full size 'drawing' with map pins and fine cotton to depict 120mm aperture and 1200mm focal length.

The 85mm baffle/D-ERF can go anywhere beyond 240mm from the objective end of the main tube.

The objective cell adds 100mm so [say] 340mm from the back of the objective.

 

I now have a shiny new, 60mm x 2" WO extender to bring the etalon to the correct distance inside focus. [20cm or 200mm]

Next sunny period, following adjustment, will tell all.

Thanks again.

[Rusted]

Getting closer!

I wasted a couple of hours trying to get focus because I hadn't measured the focal point relative to the OTA accurately.

It turned out to be 27.5cm from the backplate when I focused the sun on black paper through the D-ERF.

My new 60mm x 2" WO extender proved to be the perfect length to match the etalon 20cm inside focus rule.

I still need a 20mm x 2" extender to be able to use a 1.25" star diagonal comfortably.

I could clearly see a large prominence in a Greek Mu form and a pair of closed bracket further down.

Lots of surface detail including a long, dark stripe.

Overoptimism suggested a couple of handheld snaps at the Cemax 12mm EP.

This one is with a straight through configuration <cough> processed in PhotFiltre7:

Stop laughing at the back!

[Chriske]

11 minutes ago, Rusted said:

 

Please don't post this kind of pictures to often, I'm drooling all over my screen now, and you should see my keyboard....