PST modification - idiots guide

[Montana]

Hello,
I have never been on this part of the forum before and it scares me to death.
Having got nowhere with Solarscope I need to make a Halpha etalon fit on the back of my C11 and I have no other option but to make one myself as I can't afford a Solar Spectrum. I have read Ken's guide to a PST mod but to be honest after the first few pages I glazed over, it is too technical for me to understand.

Is there a straight shopping list anywhere with 'you need to by these items xyz from this place' then put x here, y here and z here and plug it it in?

It needs to be really simple, no technical stuff!

or can I buy all the parts and pay someone to assemble it (does anyone sell them ready done)?

Obviously my first purchase needs to be a PST as I don't want to break mine up as I use it so often as a PST. At least I understand that bit
Apologies for being dumb.

Kind regards
Alexandra

[MalVeauX]

Hi Alexandra,

The hard part is the D-ERF, which you have covered!

You will need these two adapters from AOK swiss (both the front & rear adapter).

http://www.aokswiss.ch/ayo/pst.html

You will need a blocking filter, such as a 10mm or 15mm Coronado blocking filter, you can get that commercially or used. Alternatively, you can use the PST's blocking filter but it requires some work and other adapters, and it's only 5mm so its limiting on larger sensors, but it is an option.

https://optcorp.com/products/coronado-b ... ing-filter

You will need a simple 2" extension of about 80~140mm in total (you can stack these) and these are commercially made as typical 2" extensions in 35mm, 50mm and 80mm lengths commonly. The focus point needs to be 200mm after the rear collimating lens in the PST etalon's housing and the diagonal of your blocking filter will absorb some of that. So it's tyipcal to have 140mm of extension after the rear PST adapter then the blocking filter, and after that is your camera and stuff. You can also get extensions that incorporate non-rotating helical focusers to give yourself fine focus in the imaging train.

Here's examples of 2" extensions commercially, they're common:

https://agenaastro.com/blue-fireball-2- ... nsion.html
https://agenaastro.com/blue-fireball-2- ... nsion.html

Example of 2" extension with 1.25" helical focuser:

https://www.amazon.com/gp/product/B07HR ... UTF8&psc=1

Since you're using an SCT it's a lot simpler and you need less stuff.

The front PST adapter will have M48 threading to allow for standard filters to thread on. This is where you thread on a UV/IR block filter, or a Red CCD imaging filter, or a Baader 35nm HA filter to eliminate the rest of the heat (knowing your DERF passes 393 & 430 so there's still some heat coming through that is not needed/wanted since this is HA with the PST). I would suggest the Baader 35nm HA filter first. After that the Baader Red Imaging CCD-IR block filter.

You insert this into your SCT visual back.

Now, harvesting the PST etalon... that's another story. I have done all of this recently, so I can share via my threads:

Here I harvest a PST:

viewtopic.php?f=9&t=25420

Here I make a PST module that works in SCT, refractor, etc:

viewtopic.php?f=9&t=26560&start=25#p239784

I still ask for help doing this all the time because I'm not the expert, but hopefully this gets you started!

++++++++++++++++++++
++++++++++++++++++++

Here's my current PST module in my C8 Edge with the D-ERF:

C8Edge_PSTMod_BF10mm_09232019.jpg (72.81 KiB) Viewed 4882 times

From left to right:

2" Visual back -> Baader Red CCD IR blocking Imaging Filter (ERF #2) -> AOK Swiss Front PST Adapter -> PST Etalon -> AOK Swiss Rear PST Adapter -> 2" adapter with 1.25" helical focuser -> Coronado 10mm Blocking Filter -> ASI290MM camera

This allows me to focus the primary mirror with the SCT's focuser with the collimating lens in the appropriate spot to be at F10. Then I fine focus with the helical focuser near the camera.

Very best,

[marktownley]

Hi Alexandra,

You just need the 2 AOK adaptors, spacers like Marty says, you can use your blocker from the solarscope and all you have to do is unscrew the etalon from your PST. The black box screws off, just need to unscrew the gold tube. The rest of it is just screwing things together. Once you've done it you won't believe how tricky you thought it was, but in reality is not!

Mark

[Montana]

OK, many thanks guys but I am confused already, where do the spacers come into all of this? and what is a helical focuser?
Which 2" UV/IR blocking filter do I buy or do I buy all 3 mentioned?

Yes I do have a 2" blocking filter already from the Solarscope.
Alexandra

[MalVeauX]

Heya,

The helical focuser is not needed, I just included it because I recently put one inline with a PST mod to add fine focus to the camera inexpensively and simply which also allows for less precision on the collimating lens placement in various scope designs. So for now, maybe ignore this part. It's not needed. Just a nice extra to know about.

So the PST etalon itself has two collimating lenses in the chamber, the front and rear lenses. See my threads above, there are pictures of them and everything. They are F10 each and require -200mm in focus placement on the front, and +200mm out focus on the back. So on your SCT, which is F10, you will simply insert the PST etalon with the two AOK swiss adapters and its stock collimating lenses still in place into your 2" visual back on your SCT, nothing else needed on that scope side of the etalon. Behind it though, you need that +200mm out focus behind it, as that's where the new focus point is. So you need spacers to go from the rear collimating lens in the PST chamber, 200mm out and that's where focus will be. Subtract anything that is in the imaging pathway from that 200mm, so your blocking filter or anything else you might use there. That's where the spacers come in. As it will likely be less than 200mm since you will have a blocking filter in there, so it's common to use shorter spacers in various combinations to achieve your goal based on what you use. Instead of long spacers, for example, I incorporated a helical focuser into this and my blocking filter diagonal to make up that space instead of just spacers. You can just use a spacer and your blocking filter (it just needs to add up in total length to about 200mm to the focus point to your camera).

You don't need all the above UV/IR/filters. Just one to complete the thermal blocking. The ideal one is the Baader 35nm HA 2" filter. It will thread onto your PST front adapter nose and block all unwanted heat other than 656 passing through.

Since you already have a blocking filter, you only need the PST AOK swiss adapters and the PST etalon of course and the spacers needed to get your camera, after the blocking filter, 200mm away from the rear of the PST collimating lens.

Here's a quick illustration to show what I mean about spacing 200mm behind the PST etalon's rear lens to the camera and whatever you use between there needs to add up to around 200mm (so spacers, blocking filter, anything else you put there). I happen to put a helical focuser in line because it adds space and gives me fine focus on top of that.

PST_Mod_200mm.jpg (68.31 KiB) Viewed 4867 times

Very best,

[Montana]

Thanks Marty, I need to go and look what I have

Alexandra

[MalVeauX]

Thanks Marty, I need to go and look what I have

Alexandra

Really all you truly need are the PST AOK swiss adapters (front & rear) and the PST etalon itself. That's the heart & soul of it. You can go wherever you want from there with all kinds of stuff. A basic PST mod is just the AOK swiss adapters and a blocking filter with enough space between the focus point and the PST collimating rear lens to equal around 200mm. Using an SCT at F10 just makes it easier since you don't have to worry about the front train of the setup. Just thread a Baader HA 35nm 2" filter to the front of the PST front adapter and you'll be set, no heat coming out of that system with your D-ERF, very nice.

Very best,

[george9]

Just curious, with an ERF in front of the C11, why add another HA filter in the back? Assuming the ERF is going to cover UV and IR and most of the visible (simple red glass will not). Is the PST etalon particularly sensitive? By the time you get to the blocker, the etalon comb filter gets rid of most of the energy. Seems like one more thing to create reflections.

Does it cover frequencies missed by the the blocker, etalon, and ERF? Is it being added for eye safety or filter safety?

Something like a SolarSpectrum has it's own ERF in front, so no need in that case. [Edit: I mean no need for a SECOND ERF in that case.]

George

[Merlin66]

George,
Your point is well made!
The suggested Baader 35nm is exactly the same spec as the Baader D-ERF.
Belt and braces??
Alexandra,
I find the focuser almost mandatory. Once the etalon is positioned at the majic -200 mm you don’t want to move it. A helical focuser is a simple idea to allow you to focus different eyepieces and or cameras without additional spacers.

Executive Summary
Remove PST etalon assembly.
Add AOK 2” nosepiece to the front
Add AOK rear adaptor
Stack spacers, helical focuser, BF diagonal etc to bring back focus to 200mm behind PST etalon assembly.
Position the nosepiece in the scope to achieve the -200 mm at the PST etalon assembly.

ERF up front or sub diameter ERF should be fitted prior to etalon assembly.
HTH

[langleif2]

Just curious, with an ERF in front of the C11, why add another HA filter in the back? Assuming the ERF is going to cover UV and IR and most of the visible (simple red glass will not). Is the PST etalon particularly sensitive? By the time you get to the blocker, the etalon comb filter gets rid of most of the energy. Seems like one more thing to create reflections.

Does it cover frequencies missed by the the blocker, etalon, and ERF? Is it being added for eye safety or filter safety?

Something like a SolarSpectrum has it's own ERF in front, so no need in that case.

George

You let a lot of radiation through with a SC full aperture triband ERF. I measured the temperature at the 8" SC exit hole to be at least 70 deg C when my PST mod was focused. As an experiment I tried that out swapping the mod with a pair of color glass filter. Both glass broke within few minutes! I'm not letting in that heat to the etalon.

[Merlin66]

Which ERF did you use?

[langleif2]

Aries triband DERF. So I use the Baader 35nm H-alpha filter in front of the etalon. It's safe and will reduce temperature to about 40 deg C.

When using my PST CaK mod I don't need a suitable filter in front because the CaK filter rejects the excess radiation itself.

[Montana]

OK, I am convinced by the Baader 35nm H-alpha filter

Confused again. Is the 20cm behind the etalon a fixed physical distance or a focus distance?
On my C11 the focuser is to the side so when you focus you don't move the bit attached to the scope in and out, so why would a helical focuser help?

Alexandra

[Merlin66]

Alexandra,
The 200mm is a fixed physical distance due to the rear lens in the etalon assembly.
If you use the scope focuser you’re effectively changing the -200mm spacing required for the front lens of the assembly. In effect the position of the scope focus is moved relative to the front lens.
The helical focuser allows you to compensate for the camera etc within the fixed 200 mm back focus

[george9]

Put another way, the helical focuser can help in two ways.

1. Because you are unlikely to find spacers of exactly the right length, the helical focuser lets you set the camera exactly 200mm back behind the etalon. Then use the SCT focuser to bring it into focus. How do you get exactly 200mm? If you can actually measure it, then great. But another way is to remove everything behind the etalon, but a screen (possibly translucent) 200mm behind the etalon, and adjust the SCT focuser to sharp focus. Then put the spacers and helical focuser and camera back on, and then adjust the HELICAL focuser (and spacers if needed) until it is sharp. That should be at 200mm.

2. The SCT focuser is not that great, so once you get everything to the best focus you can get with the SCT focuser, use the helical focuser to tweak it. Yes that will move you slightly off 200mm, but if you only go a mm, it won't hurt.

George

[Montana]

Thanks guys, it is all murky water at the moment but at least it isn't mud it is clearing slowly.

Alexandra

[marktownley]

Have you got a 2" diagonal Alexandra? Also, is the solar spectrum blocker 2" or 1.25"?

[Montana]

I do have a 2" diagonal but I never like using it. My Solarscope blocker is 2"

Alexandra

[marktownley]

With the AOK adaptor, an 80mm and 60mm extension tube, the 2" blocker and a 2" to 1.25" step down, along with the optical length of the camera you will be pretty much in the 200mm ballpark the chip needs to be from the etalon.

[Montana]

Just trying to source and cost things at the moment. Is this a good Helical focuser?

https://www.365astronomy.com/2-Non-rota ... Scale.html

Are these good?
https://www.harrisontelescopes.co.uk/ac ... ube-2.html

1 of them and 2 of them would equal the 14cm Mark suggested (I think). Thinking about it I think I only need one of each.

I need to measure the size of my blocking filter I think.

Alexandra

[LTHB]

I have tried several helical focusers, most of them were more or less wobbly, so this is the point I would pay attention to, but I have no experience with the focuser you linked to.

The best I tried out is this one:

https://www.teleskop-express.de/shop/pr ... ierer.html

Regards,

Frank

[bart1805]

Hi Alexandra,
It all depends on the light path of the blocking filter you are going to use. With a Lunt diagonal, the distance between the back of the rubber ring of the etalon to the front edge of the diagonal is about 11 cm.
If I look at your earlier pictures made with your C11, it seems that focussing is not a big issue. If that is the case than you can use the gold tube of the PST to check if your distance from the etalon to the camera is right. Aim at 20 cm. Screw the gold tube back on the etalon. Focus with the helical focuser. When focussed you have found the right distance. Attach the mod (without the gold tube off course....) to your C11. Focus your C11 an there it is. No need to use the helical focusser for fine focussing.
CS! Bart.

[Merlin66]

My previous message seems to have got lost in space....
You don't need a 2" focuser....
I went for the Borg 1.25" Helical focuser and use it on all the Spectra-L200 spectrographs as well as the SHG.
The focus on the donor scope should be fixed, and any re-focus requirements due to cameras, eyepieces etc, should be done after the etalon.

[marktownley]

Just trying to source and cost things at the moment. Is this a good Helical focuser?

https://www.365astronomy.com/2-Non-rota ... Scale.html

Are these good?
https://www.harrisontelescopes.co.uk/ac ... ube-2.html

1 of them and 2 of them would equal the 14cm Mark suggested (I think). Thinking about it I think I only need one of each.

I need to measure the size of my blocking filter I think.

Alexandra

I have one of those helicals, they're ok. The distance you need from the back lens of the etalon to the chip is 200mm.

Have a look on astroboot, you can often get adaptors and spacers considerably cheaper on there.

[Montana]

So my current plan is:

Order =
1. Baader 35nm Halpha
2. PST front adapter
3. PST etalon
4. PST back adapter (unknown length)
5. Extension tubes (5cm)
6. Blocking filter (5cm)
7. Helical focuser (5.5cm - 9.5cm)
8. 2” to 1.25” reducer
9. Camera
bring back focus to 20cm behind PST etalon assembly to camera chip

Does anyone know how long the PST back adapter is? also how does it slot on to an extension tube? Also what happens when I add a barlow or my 3x telecentric? do I have to take spacers out?

Alexandra