6" PST Stage II progress.

[JamesF]

30 minutes ago, Merlin66 said:

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)

 

Yeah, I just can't put it all together in my head at the moment   It's the difference between "knowing" and "understanding"

James

[Rusted]

2 hours ago, Merlin66 said:

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)

I had tried to convince myself that this wouldn't work with non-f/10 systems.

However a simple drawing [with the positive lens removed] shows the obvious truth:

The etalon Barlow will force parallelism on the converging light cone from almost any objective.

Converting the objective into an effective f/10 by vignetting at the etalon group's own aperture.

The wider cone of light from the 'faster' lens is simply ignored being outside the etalon.

As if physically stopped down by a solid baffle.

 

[Rusted]

I discovered something else interesting today.

My etalon group has [effectively] slightly negative power at 20cm inside focus.

I measured ~3cm longer focus on my 150/8/1200 with the etalon group in place.

It could be the optical, glass path length of the group affecting matters.

Because it has matching, negative and positive lenses the group behaves much like a thick block of glass.

[Rusted]

On 3/16/2018 at 17:36, Peter Drew said:

@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.    

Peter,

I'd value a clarification of your "SW Barlow screwed to the nose piece of the binoviewer."

I'm trying to use my Orion Shorty Plus Barlow and my shiny new S-T binoviewer.

My binoviewers have a male threaded 1.25" Ø, plain, chromed spigot.

Minus the binoviewer spigot and the Orion Shorty eyepiece socket, both have female threads.

You'd need a short, matching, male threaded, doobriwotsit to join the two.

Wouldn't you? The shorter the system, the better, to hold the power down.

Any chance of an image to show exactly what you've done?

S-T offer a 1.6x glass path corrector [NB.not 2x] which would also help to keep the magnification down a bit.

BTW: I should have matching pairs of 26 & 32mm Meade S4000s by tomorrow.

You're costing me an arm and a leg and I'm already in debt to the local butcher for several kidneys I no longer have!

Any chance of a bit of sponsorship? Seeing as it's all your fault!

[Peter Drew]

@Rusted.  I use the SW Barlow as when the black cell containing the lens is unscrewed it has a male thread that screws into the female threaded chromed nosepiece of most of the binoviewers.

As far as sponsorship is concerned, my normally very expensive consultancy fees are being offered free! 

[Rusted]

Thank you, Peter,

Just got back from the shed with tears running down my cheeks.. 

The Orion Shorty-Plus has a much larger diameter thread.  Sniff!

Now I have to order an SW Barlow with at least a tenner for postage alone and several days for delivery.

Should I really be looking at the 1.6x T-S Glass Path Corrector @ €39 rather than the SW Barlow? 

Any advantages?   [Hint: I already have a Barlow and don't need another except for binoviewing.]

[Peter Drew]

@Rusted. The reason why I use the SW Barlow and recommend them is because in the first instance it was the only one that I had that fit my binoviewer and being as it worked so well, and on other mods that I've made, there has been no incentive to experiment with an alternative as yet. 

[Rusted]

Thank you, yet again, Peter.

The De-Luxe Skywatcher 2x Barlow was about the same price as the T-S 1.6x Glass Path Corrector.

So I went with one of those since it is intended for the T-S binoviewer anyway.

No doubt you'll hear all about it when it arrives. 

The T- S binoviewer is quite a nice bit of kit.

A little more then the Revelation but with compression bands instead of thumbscrews.

I'm always losing screws so I needed something more childproof.

[Moonshane]

Just for clarification. The barlow element screws into the chrome nose when it is on the BV  not into the BV when the nose is removed.

[Rusted]

49 minutes ago, Moonshane said:

Just for clarification. The barlow element screws into the chrome nose when it is on the BV  not into the BV when the nose is removed.

Really? Are your referring to the T-S binov and its GPC?

I thought they all enjoyed the same standard filter thread?

[Merlin66]

Rusted,

How did you measure the final focal length?

If it was distance from end of OTA without the mod v's distance from end of OTA with the mod, then I'd comment that this would vary due to the mechanical length of the etalon assembly.

In your above diagram -the red section shows the parallel beam from the front lens - but simplistically the re-imaging rear lens could be anywhere along this section - at 10mm or 300mm.....and you'd still have the same effective optical focal length but different mechanical "focal" length.

Hope this helps.

 

[Moonshane]

The Skywatcher barlow element threa

1 hour ago, Rusted said:

Really? Are your referring to the T-S binov and its GPC?

I thought they all enjoyed the same standard filter thread?

The Skywatcher barlow element has the same thread as a 1.25" filter.

[Rusted]

9 hours ago, Moonshane said:

The Skywatcher barlow element threa

The Skywatcher barlow element has the same thread as a 1.25" filter.

Okay. If that is the case then why cannot it be screwed directly into the binoviewers body without the chrome spigot?

Normal use would probably include fitting it into a star diagonal. The extra protrusion when using both items might be damaging to the mirror or prism.

My understanding from Peter's comments is that the Barlow's optical 'nose' is fitted directly to the binov body after discarding the upper part.

Perhaps I am misunderstanding something here because i have never owned either a binoviewer nor anything which would screw into it.

EDIT: Sorry, gentlemen. It's so hard to get the staff, these days.

You are both right! I falsely assumed that the binoviewer would have a standard filter thread in its body because it was so [very] close to filter thread size.

The binoviewer body thread is actually smaller than a filter thread!

So it was back to the shed [twice] to compare threads. Close, but no big, fluffy rabbit.

I now have three non-compatible threads to deal with and still nothing that fits my binoviewer except its own, bare spigot.

A Barlow, a Barlow, my kingdom for a Barlow!

[Rusted]

9 hours ago, Merlin66 said:

Rusted,

How did you measure the final focal length?

If it was distance from end of OTA without the mod v's distance from end of OTA with the mod, then I'd comment that this would vary due to the mechanical length of the etalon assembly.

In your above diagram -the red section shows the parallel beam from the front lens - but simplistically the re-imaging rear lens could be anywhere along this section - at 10mm or 300mm.....and you'd still have the same effective optical focal length but different mechanical "focal" length.

Hope this helps.

 

I agree. I think. 

First I measured the focal distance from the temporary plywood backplate to the sharpest image of the Sun.

Then I added just the etalon [at the correct distance inside focus] and remeasured to find the focal distance was now 3cm longer.

I ignored this new figure as irrelevant to my calculations for subtracting 20cm to find the correct etalon distance.

The internal D-ERF was present in both cases. Hope this helps?

[Rusted]

Rant/

The Teleskop-Service GPC/Barlow duly arrived in the post today. Now what?

The problem is the sheer, drooling idiocy of those who designed the [XXXXXXXXXX] thing. [Insert a suitable expletive here!!]

Firstly, it's diameter is undersized for the standard 1.25" focuser/eyepiece.

Secondly, it has unwanted length but won't screw directly into the severely impoverished Teleskop-Service body thread!

So, once screwed into the S-T binoviewer nose piece it offers ZERO support for all those silly moments when its great length pushes the binoviewer right out of a fitting!

Normal fittings like bog standard 1.25" star diagonals!!!! Normal fittings like a PST eyepiece holder!!!

It makes the S-T binoviewer completely and utterly useless unless I am willing to look "straight through" without a diagonal.

It makes using a PST completely impossible with the S-T binoviewer!

The only way to overcome this crippling, [brain dead] designer malfunction is to force a spare eyepiece sleeve over some electrical tape wrapped tightly around the brand new T-S GPC!!

Then it will provide some meager support as it is inserted halfway into a bog standard, 1.25" fitting with little chance of getting a secure grip from any known locking thumbscrew.

Grrr!

Angry of Rural Wotsit! 

\Rant

[Rusted]

Some images to help clarify the effect of adding the T-S GPC to their own binoviewer:

It instantly becomes impossible to use a 1.25" star diagonal!

[Peter Drew]

!

[Rusted]

Hi Peter,

I presume your Skywatcher Barlow is standard 31.8mm OD?

 

[Moonshane]

I suppose it's a moot point but this is one of the main reasons I went for the Baader Maxbrights and t2 connectivity.

[Rusted]

17 minutes ago, Moonshane said:

I suppose it's a moot point but this is one of the main reasons I went for the Baader Maxbrights and t2 connectivity.

I quite like the T-S binoviewer, at its lower price point, but is has some very silly design flaws which seriously detract from its true potential.

[Rusted]

Good Friday: Had hours at the telescope in an almost cloud free, sunny sky until after lunch.

Then later after 5pm [CET] in poorer seeing.

Very even surface texture with lots of detail including the clouds in the prominences using the 12mm Cemax.

No focuser, just slide and lock the EP with the PST screw. Not very smart!

Still thinking about the best way forward for giving the D-ERF fine tilting adjustment via threaded rods. [Studs or all-threads.]

I'm still getting an asymmetric red blush at times. Hope its not the PST etalon.

No binoviewing in H-alpha yet. The binoviewer GPC badly needs compression ring support. [Coming soon.]

The present 90mm [white light] Vixen will be swapped for the 7" iStar for WL viewing when the dome is finished.

I have the Baader solar film filter already made for the 7".

It was used on the Mercury transit with far more detail and image scale than the Vixen could ever manage.

I presently take the telescopes back down after each use for winter weather protection.

Not easy when the ladder has to be managed one handed, going both up and down, with icy treads.

Both the 6" and 7" telescopes will eventually be permanently placed on the big mounting.

It has occurred to me that optics for solar observation should be as clean as possible to increase contrast.

Mine aren't!

 

[Rusted]

Easter Monday and another quite sunny morning between clumps of cloud.

Two large prominences, while the bright feature continues to plod across the Sun's surface in H-alpha.

First success with the T-S binoviewer + 1.6x GPC + 40mm [2"] extension in H-alpha.

Tried 26, 20 & 10mm pairs of Meade 4000 Plossl EPs.

The 26mm were best and easiest to use because I was literally bending over backwards to look upwards to stay on axis.

After removing the binoviewer I took some handheld snaps through a no-name 20mm Plossl using a short zoom Canon.

The trick seemed to be to give the camera a chance to adjust automatically before fully depressing the shutter.

Too early and the contrast was far too much. Wait, on the half-cocked button and the surface texture showed up more evenly.

I ought to thank all those who have helped me get this far more quickly than I could ever have managed alone.

We all stand on the shoulders of those who have gone before. [Sometimes,  it's the only safe way to reach the eyepiece!]

 

[Rusted]

I'm wondering whether the 200mm rule [inside the refractor's focus] still holds with GPCs?

I can just reach focus with my T-S binoviewer +1.6x T-S GPC + T-S star diagonal by direct insertion.

The Etalon adapter tube is inserted directly into the 150/8's tailpiece via an adapter plate I turned myself.

No spacer tube is needed nor even possible to achieve focus.

Does the GPC actually break the 200mm rule? Or does the GPC's optical compensation safely ignore the rule?

I'm rather tempted to get the 2.6x T-S GPC to allow more back focus with the binoviewer.

The image shows my latest set-up. I still need to fit the PST eyepiece holder with a compression band.

Or, add two more thumbscrews to get a safe decent grip on the GPCs.

The blocking filter lives in the bottom of the PST eyepiece holder which makes 1.25" star diagonal modification a bit "iffy."

Can it be safely spun in the lathe to turn a groove for a compression band? There should be enough meat in the eyepiece holder wall.

[Peter Drew]

As all the focusing components are subsequent to the position of the etalon I think it is important that the position of the etalon is set as near to the -200mm dimension as is reasonably possible. The final train should be adjusted to achieve focus depending on the configuration in use. The PST etalon is designed to operate at the field angle and illuminated full 20mm aperture, major deviation from this can reduce the performance. 

[Rusted]

Thank you, Peter. 

That probably explains the rather large but featureless 'tomato' I saw through the binoviewer. [Technical term.]

<sniff> Now I have to start again completely from scratch. 

Good fun, this H-a thingy, isn't it? 

While I have your attention: Should I have prominences AND very even surface detail in perfectly tuned "focus" all at the same time?

I keep turning the tuning ring but they will insist on being razor sharp, both at the same time.

Why do I feel so cheated?