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PST modification - idiots guide

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langleif2
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Re: PST modification - idiots guide

Post by langleif2 » Wed Oct 09, 2019 2:57 pm

As mentioned, you don't need the helical focuser. Especially if you want to remote control the focuser. You just need a back adapter or extension tube at the length of 14cm as seen on this picture:

Assembled%20Tube%201.jpg
Assembled%20Tube%201.jpg (155.34 KiB) Viewed 94 times

Eventually you can take a look at my thread: "Is a C8 and a PST mod really useable". There you can also see a picture of my mod. I just took a SCT to 2" adapter and a 2" Skywatcher T2 visual back and added a T2 spacer and finally a T2 to 1.25" adapter. I took what was left of my spare parts. You can ask: Will a SCT to 2" adapter fit the etalon back? Yes, it's not exactly the same tpi but it fits nicely.

You don't need your 3x telecentric. If you want to you must remove the lenses in your etalon. But this will bring out your focal length where the magnification will be too much for the seeing and it will further open up the etalon bandpass. Not the way to go with an airspaced etalon. You can do it none the less but you will most likely need a suitable reducer.

If with your C11 the magnification is not enough with the PST mod natively, you can use a Barlow between the BF and the camera.
Last edited by langleif2 on Fri Oct 11, 2019 9:21 am, edited 1 time in total.

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Re: PST modification - idiots guide

Post by marktownley » Wed Oct 09, 2019 8:05 pm

Montana wrote:
Wed Oct 09, 2019 9:30 am
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
That should work. The PST back adaptor is just a 2" female holder, so the extension tube just slots in, the same as an eyepiece would. Once you've got that magic 200mm set it just stays set as it is. A barlow would go between it and the camera, your 2x or probably 2.5x pm would be a good combination.
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Rusted
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Re: PST modification - idiots guide

Post by Rusted » Fri Oct 11, 2019 4:46 am

I have some recent direct experience with helical focusers. Sorry, Frank, but my short TS was both hopeless and dangerous.
Mine soon developed a lot of slack. Moreover it is only held together with a long riveted pin which is heavily leveraged by the design.
Once the riveting breaks loose, which it will eventually given the poor design, the focuser literally falls apart.
You may have an expensive blocking diagonal, a binoviewer and some costly eyepieces dangling from that floppy little focuser!

I recently replaced my push-pull focusing using a series of extenders, with an Omegon 2" helical focuser.
This has worked very well and had enough travel to allow a Barlow to be used, or removed from the camera nose, without dismantling.

https://1.bp.blogspot.com/-U_hfUJVZfOI/ ... %2Brsz.JPG

On the subject of D-ERF safety: The focused beam from my 90mm, internal, Baader D-ERF in my 6" f/8 is hot! You would not want to place your hand in that red beam! The PST etalon seems to tame the heat but it makes you wonder how much heat it can tolerate. I often track the sun for hours. Capturing videos and processing them continuously throughout the day.

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Montana
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Re: PST modification - idiots guide

Post by Montana » Fri Oct 11, 2019 9:48 am

Thanks guys for the OK, I will start ordering my list now. Hopefully pick everything up at the IAS show in November. Is anyone going?

Alexandra

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