A 6" f/8 PST mod? Are we kidding ourselves?

[Rusted]

Hi,

I have ordered a Baader 1:1.25 GPC to complete the PST mod on my 6" f/8 to correct it to f/10.
Then the sun came out briefly. So I measured the solar light cone at the etalon 200mm inside focus.
After all, this is where the GPC wants to sit close in front of the etalon.
Guess what? The solar light cone measures somewhere around 30mm Ø at that point.
I'm afraid the remaining cloud made it hard to get a properly sharp disk.

It seems much of the light from a 6" f/8 never passes through the 20mm Ø PST etalon.
All it does is heat the etalon surroundings. i.e. AOC etalon adapter.
Am I mistaken about this or is this something we have all overlooked?
Should we all be looking for a much better matching, smaller aperture, shorter focus, f/10, donor refractor?

[MalVeauX]

Heya,

I use a 150mm F8 Refractor ans mask it to 120mm F10 and it works excellent. I extend my collimating lens forward with extensions rather than cut the OTA (and I can extend it any amount with simple 2" extensions) so that none of the heat is hitting the etalon, instead, it's just hitting the front lens and its holding cell and collimates to F10 so only a collimated F10 beam passes through the PST etalon and out the back. My unscientific "palm test" after the etalon shows very little heat after that point, so most of it is being handled with the internal Baader 2" Red CCD-IR block filterthat I use in front of the first collimating lens inside the OTA.

I have not used it as a full 150mm because I do not have the Baader GPC 1.25x, but I have wanted to get one. I just need to figure out how to mount it in the system I use for my PST module and internal D-ERF some how (behind the D-ERF). The front of my module can handle typical M48 threaded 2" stuff, so if I can mount the Baader GPC somehow in there, that would be possible to test it out. Plus, without a costly front D-ERF.

I'm very interested to see how you or anyone else mounts the Baader GPC?

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Very best,

[Rusted]

Thanks. I don't argue with any of that. Your images are the envy of many here.

Let me rephrase the question:

How vital is it that the entire solar light cone passes through the 20mm Ø PST etalon at 200mm inside focus?

It is easy to show that adhering to the 200mm inside focus rule means that only a 50mm f/10 light cone falls completely inside the etalon.

Everything bigger than 50mm aperture at f/10 spreads a light annulus around the etalon. i.e. Heavy vignetting.

Does this vignetting affect resolution? Are we still getting full "value for money" for every millimetre above 50mm at f/10?

How much does the extra aperture of a 6" @ f/10 really add to the image if it has a 32mm Ø spread at the etalon? What about a 12" f/10? Or a 20" f/10?

The edges will fall off in light level but is that all?

[Rusted]

Hi Marty,

I should have added that my front AOK etalon adapter has a baffle. A GPC could be fitted to that.

Without having yet seen the Baader 1:1.25 GPC in the flesh, I am assuming it has a T2 male thread.
This can be threaded into a female T2>2" push fit adapter. Though some lathe work might be needed.
If only to reduce the dimensions of the adapter to fit in the tight space inside the AOK.

More on this when I have the GPC to hand. I still have no idea of its clear aperture.
Which is really what prompted this thread. I suddenly doubted whether I was wasting my money on my 6" f/8.
Logic suggests that the PST etalon works fine with larger instruments.
I just need to be reassured.

[MalVeauX]

Thanks, I am very interested to see more info on this. I too am quite interested in adding the Baader 1.25x GPC and how it can be easily added to the imaging train to operate a 150mm F10 option from time to time. Especially from a thermal handling standpoint, as my 120mm F10 version handles the heat great. I'm curious how much the full tube will be effected and if the wee internal D-ERF will be sufficient or not. My AOK Swiss front adapter also has an aperture but it is not threaded and very thin and is the diameter of the etalon itself it seems, so shouldn't effect anything in that sense. I would imagine the GPC has to be in front of the collimating lens of course. I just don't know how to mount it. I need it to be D-ERF (Baader Red CCD-IR Block Filter) -> Extension -> Baader GPC 1.25x -> Extension -> Collimating lens -> Extensions -> PST Etalon -> rear imaging train.

Very best,

[Rusted]

Thanks Marty.

I use a 90mm Baader D-ERF mounted on an original Celestron baffle at about half way down the main tube.
Carefully placed to ensure the solar beam passes through without vignetting or heating the baffle.

The D-ERF sits in a stainless steel container which I found in a thrift/charity shop.
This was chain drilled to take the bottom out and and then I filed the hole smooth and round.
The filter is nestled in a cork lining and retained with a leather strap simply and crudely screwed through the edges of the pot.
The flat side of the OTA baffle faces the Sun so reduces the risk of fire.

My D-ERF passes enough residual heat to be hot at the focus without the PST components in place!
This was a surprise and required double checking the little arrow on the edge of the filter for facial orientation.
The objective side is much more mirror like.
Only the considerable expense prevents me from buying a 160mm to go in front of the objective.
It just feels like a much safer option.

I have ordered the Telescope Express recommended Baader T2 > 2" adapter along with the 1-1.25 GPC.
Though I may have to turn it down to fit the AOK.
My collection of T2 and other adapters will be carefully studied first to see what might work right off the shelf.
Of course I shall share some images of what works for me.

[marktownley]

Nice adaptors!

[Merlin66]

Rusted,
No one has really answered your original question Re etalon size v’s mod donor size.
This issue is raised on page 4 of my PST mod write-up.
The solar image is close to 1/100 the focal length of the donor. For your 150mm operating at f10 the effective focal length would be 1500mm, solar disk size 15mm.
At the etalon the beam size would then be (200/10) = 20mm plus the 15mm = 35mm.
Yes, this does result in some vignetting, but as evidenced by many successful large mods it is not a major issue.

[Rusted]

Thank you, both, for your responses.

Despite extensive searching online I could not find any mention of the relationship between mechanical vignetting and resolution.
Though I was able to discover that it [vignetting] is extremely commonplace.

If we put a smaller baffle in front of the objective lens we know it will affect image quality. The lens becomes smaller.
If we put a smaller baffle after the lens then it vignettes. But what else?
I suppose the full lens aperture is still feeling the centre of the image but the light level falls off away from the axis.

The relationship between focal length and image size guarantees vignetting in increasing apertures with a fixed 20mm PST etalon size.
I was trying to quantify the results if only in a general sort of way.
With a rather expensive GPC on its way I wanted to be sure it would be beneficial to our PST modding cause.
I know others here have had success with this relatively weak GPC.

It ought to be possible to roughly determine these effects for myself. Simply by fitting smaller and smaller stops in front of the PST etalon.
But my sun has been stolen and hidden behind thick cloud for days now. So there is no chance to play!

[bart1805]

With a 6 inch f/8 and a 1.25x Glasspath you place fhe glasspath at minus 160mm. The front and backlens work at their best in a f/10 beam. Hence the importance of the glasspath or masking the front lens to 120mm.

[Rusted]

With a 6 inch f/8 and a 1.25x Glasspath you place the glasspath at minus 160mm.
The front and back lens work at their best in a f/10 beam.
Hence the importance of the glasspath or masking the front lens to 120mm.

Thank you for your continued guidance Bart. That is useful information.

Going back to my worries about larger apertures at f/10 and a fixed 20mm PST etalon diameter:

What I was probably missing was that image scale will increase in step with aperture and focal length.
Meaning that the camera, or eyepiece, sees a smaller field of view but at increased scale.
Within that reduced field diameter the image will contain more information [higher resolution] thanks to the increased objective aperture.

[AndiesHandyHandies]

Hi

There was a test of front mounted DERFs in CN and they pass quiet a lot of light into the telescope.
Roughly convert a 6" to a 4" in terms of thermal load.

You are better masking in front of the objective to get the required F10 for the PST.
Silver reflective front.
This stops unwanted light heating the inside of the telescope.
Any internal vignetting will reduce resolution anyway.

A Baader 2" 35nm is better than a red coloured filter. It reflects the rejected light so reduces internal heating.
And safer as the filter is cooler, the red filter absorbs light and so heats up so could crack.
Astrograph uses a 2" Baader 35nm in refractors up to 6" as an internal ERF.
Calculations show the Baader 1.25" 35nm will be safe up to 4" as an internal ERF, so good with on a 6" with a front DERF.
Limits light into the etalon to 35nm passband.

Any Barlow infront of the PST Collimating lens should be placed so that the bright beam is within its aperture.

Roger Marcon uses a large piece of aluminium with a hole in it as required for the etalon beam on a 8" F15 Zeiss refractor. No ERF.

I have calculated for my Vixen 110mm VMC F9.4 baffled to F10 that the filters safely reduce the observed beam to just the H-Alpha line. I have extra baffles on the secondary and front of the main mirror baffle to limit the beam on the PST Collimator lens to be within its holder. And baffle out stray skylight as comercial scopes are for nightime use and want to claim a larger field, though not fully illuminated.

I use a 1.25" Baader 35nm after the moved forward PST Collimator lens. It does pass long IR over 1100nm. The Collimator is moved forward to enable focus to be reached. By placing the Baader after the collimator lens means the beam on it is parallel and when the rejeced light goes back out through the Collimator lens it is spread out as it came in so not hot spot on mirrors or lenses. Or infront of the telescope.

I use a Omega 25mm 40nm ERF filter in place of the PST Collimator lens which has long IR blocking. Mark T says blocking long IR is better for imaging.

And a Beloptik UV/IR KG3 in the eyepiece for visual use which blocks long IR.

Any one of those filters with the PST Etalon is safe for visual for a bit. In case one fails.

On my 127mm Triplet I use a 2" Baader 35nm moved up the telescope with 2" extensions so it just contains the beam. I use a 48-43mm step down ring on it to baffle the inside of the focusser tube. The Omega 25mm 40nm acts a second ERF.

Andrew

[Rusted]

Thank you Andrew. A lot to take in. I'm afraid some of it makes no sense to me.

Is there any record of an internal Baader D-ERF breaking due to heat stress?

Under which circumstances would CN be discussing D-ERFs?
There is a total ban on any mention of amateur solar equipment making or of modifying commercial solar products.

[AndiesHandyHandies]

Hi

Are you using a 2" red filter as an internal ERF?
My friend used one and forgot to check after adding a Barlow and cracked a red filter. Still useable though as not at focus and seemed to have clear cover glass holding it together.

There are various Solar discussions on CN. A review of front DERFs would not be a dangerous mod suggestion.

Andrew

[Rusted]

Hi

Are you using a 2" red filter as an internal ERF?
My friend used one and forgot to check after adding a Barlow and cracked a red filter. Still useable though as not at focus and seemed to have clear cover glass holding it together.

There are various Solar discussions on CN. A review of front DERFs would not be a dangerous mod suggestion.

Andrew

I use a 90mm Baader D-ERF half way down the main tube of my 6" f/8.

My Baader 1:1.25 Baader GPC has arrived along with a Baader 2" binoviewer nosepiece. Both from Telescope-Express.de
The Baader GPC has a clear aperture close to 25mm. Probably adequate for placing 40mm in front of the PST etalon.
After the plastic ring [supplied] is pressed into the 2" nosepiece the GPC is pushed into the plastic ring.

The large knurled ring is now removed from the nosepiece. Allowing the nosepiece to slid freely inside the WO extender.
The AOK 2" etalon nosepiece slides into the WO extender pushing the GPC into a suitable position at 40mm where it cannot escape.
Those not using a WO extender must ensure the nosepiece & GPC cannot escape if the telescope is pointed nose down.
The WO extender has a slightly smaller diameter where I screw it to my home-made backplate.

See images attached.

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P1390919 rsz 800 gpc + pst.jpg (72.36 KiB) Viewed 783 times

[Merlin66]

Can someone post the link to the Baader D-ERF discussions on CN.
The normal search function doesn't find it.

[AndiesHandyHandies]

Hi Merlin,

I had a good look but cant find it.

Transmissions from memory were in the range 60-40%.

Andrew

[Rusted]

Hi Merlin,

I had a good look but cant find it.

Transmissions from memory were in the range 60-40%.

Andrew

Is it possible you are mistaken in thinking this was mentioned on CN?
I am struggling to imagine any scenario where CN would tolerate such discussion.

I was barred from the CN forum for daring to mention non-commercial solar telescopes in response to a direct request for information from another member.

My argument that full and accurate knowledge would increase user safety was summarily dismissed by mods and a vitriolic forum owner.

As was my expressed belief that they were protecting commercial sales to avoid direct competition which might dent precious profit margins.

The sky high prices, of even the smallest of solar instruments, is a direct block to countless amateurs with solar observing and imaging ambitions.
Not to mention the now, well known failures of expensive commercial equipment to perform as advertised due to variability and [seemingly] poor quality control.

[Rusted]

Meanwhile, back at the ranch: I tried the Baader 1:1.25 GPC in my 6" f/8 today.
The focus was shifted 18cm away from its original position. I made no effort to move the etalon to match.
But built instead a ridiculous and flexible length of multiple 2" Ø adapters to allow my ZWO 120MC to come to focus.
The image was greatly enlarged and far softer than I was used to.
Sadly it was a day of fast moving cloud with only brief glimpses of the "boiling" sun.
I shall experiment further when the sun obliges with its presence unfettered by cloud.

Meanwhile I have fallen back on a cardboard objective stop to bring the 150/8 to an officially approved 120/10.
Too early to say whether this provided any improvement over 150mm due to the poor seeing conditions.

[bart1805]

Rusted, I don't understand what you are building. It is far more simple.
Place the Glasspath really close to the front lens of the etalon. In a f/10 telescope you would place the front lens of the etalon 200mm before the focus point. In a f/8 with a 1.25x Glasspath you place it 160 mm before the original focus point. After the etalon everything is the same: place the sensor of the camera 200mm after the back lens.

[AndiesHandyHandies]

Hi

Whats the design back focal position of the Baader 1.25x Glasspath to give that magnification?

Rod tested 2x Barlows at -200mm and they actually gave 4x.

Have you measured the image size without and with the Glasspath?

Andrew.

[Rusted]

Rusted, I don't understand what you are building. It is far more simple.
Place the Glasspath really close to the front lens of the etalon. In a f/10 telescope you would place the front lens of the etalon 200mm before the focus point. In a f/8 with a 1.25x Glasspath you place it 160 mm before the original focus point. After the etalon everything is the same: place the sensor of the camera 200mm after the back lens.

Thanks again Bart.
I had no chance to measure the exact position of the GPC.
I simply pointed the telescopes at the zenith and assumed the GPC in the loose, 2" nosepiece would slide back down against the etalon group.
Now I think about it, the GPC was probably stopped from doing so by the AOK 2" nosepiece.
So now I need to fit the GPC inside the AOK nosepiece. I may have to turn my own adapter on the lathe.
Fortunately the GPC is just a push fit into the supplied, plastic ring. I just need a slightly smaller diameter adapter to duplicate this.

[Rusted]

Hi

Whats the design back focal position of the Baader 1.25x Glasspath to give that magnification?

Rod tested 2x Barlows at -200mm and they actually gave 4x.

Have you measured the image size without and with the Glasspath?

Andrew.

Not yet. Heavy cloud for most of the afternoon made any real measurements difficult.
I took some digital snaps of the monitor screen, with and without the GPC for comparison.
Hoping to compare the radii of the solar limb. They were too soft to be worth posting here.
Perhaps I should try taming them in imppg? ;-)

[Rusted]

I hope the new GPC position satisfies Bart.

First the AOK 2" nosepiece was bored to 28mm Ø in the lathe. This allowed the 'nose' of the GPC to pass through.
The etalon now holds the GPC gently up against the inside of the AOK baffle using rubber washers for packing.
Moving the washers under the GPC flange would allow an even closer spacing to the etalon if desired
Weather too awful to test the new GPC position. Should be better tomorrow.

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[bart1805]

If it it satisfies me is not so important, if it works is the question. Yes, this is perfect.
now find the minus 160mm spot and place the GP there. Good luck!