ERF for PST Modification

[StarryDave]

HI,
I am looking into modifying my PST and have spent a fair bit of time checking the wealth of great info on this site. I don't want to risk damaging my etalon so my plan is to remove the glass from the PST objective and replace it with a 90 or 100mm F10 lens (placed at FL-200mm in front of the etalon). My first question is how easy is it to remove the objective and retaining ring (are they glued in)?
The scope will be used purely for imaging so no eyesight worries. I plan to install a 52mm or 2" ERF at the front of the gold tube to protect the PST innards. The AOK site states that
"A simple way to protect the filter is to use a good normal red filter with 48 mm thread (as for a 2" eyepiece). This works up to max. 4" scopes and f : 10 (mounting on the adapter)."
My second question is whether this is good advice and what filters would be recommended here.
thanks in advance

[marktownley]

Hiya Dave

It's not the etalon that will degrade, it is the blocking filter.

The objective on a PST can easily be removed using a lens spanner.

If you are planning on going the way of a sub aperture ERF you need to make sure the filter you user is a dielectric type (not coloured glass). You need to mount this as high in the light cone as possible.

Mark

[StarryDave]

Thanks Mark,
I don't have a lens spanner but I managed to undo the ring with an awl. Pretty tight but came off easily once started. First hurdle behind me.

My plan to mount the ERF where the original objective was would mean that the sunlight on the filter is concentrated by a factor of 5x with a 90mm F10 objective at 500mm upstream. Do you know of a (reasonably priced) 2" filter that would reduce this energy to a safe level? (bear in mind I don't do visual)

Btw, I owe you a big thanks for an earlier post of yours I found during my research. I had been using a cheap 5x barlow to image small proms but you stated that you shouldn't go above 2.5x with a PST. I switched to my 2.5x Powermate which gave higher fps, brighter image and wider fov. Resizing in post then gave better results that the 5x.

[marktownley]

Sounds a good idea to me. I'd go for a Baader 7nm

https://ensoptical.co.uk/index.php?rout ... r&limit=50

[StarryDave]

Many thanks Mark for taking the time to hunt down a bargain for me - order is in. It's a bit of a twofer because I can reuse it in winter for deepsky work. There is not much sun work at 57N in winter.

I thought I would do a trial run at 70mm to prove the concept and keep the heat load down a bit to start. I managed to bag a Celestron 70/700mm Az Amazon warehouse deal for £84. Package is damaged but contents as new. If I don't cut the tube I can resell it at xmas and swap it for a 90 or 100mm.

Check out the video link below and you will see why I was so grateful for your 2.5x barlow tip. When I started imaging, the tornado looked quite small and I was tempted to switch to a 5x. But I stuck with your advice and run all day with the 2.5x. Lucky I did because the wider FOV caught a lot more action when it lifted off.

https://youtu.be/L7j8oqg7ODI

[AndiesHandyHandies]

Hi

Any ERF infront of the existing, must have one, blocking filter is just needed to keep the heat on it to a reasonable level so as not to slowly wear it out.

For full spectrum replacement for the original coated PST objective you need a UV-IR, Red, and KG3 for visual certainly and Mark notices image degredation without full blocking.

The narrow band deep sky filters have a lower peak H-alpha throughput as they get narrower. My friend says the 35nm degrades the image quality.

You can go for a RED CCD 100nm for maximum throughput. I use a StL Optolong 40nm filter at the moment. Less reflection off the back compared to a Baader 35nm.

As a Quark and CCDs run at 40C+ then their front temperature after time in the sun needs to be no more than 40C I would say.

Cheers. Andrew.

[StarryDave]

Thanks Andrew,

I already have the donor scope and Baader 7nm so my plan is to try those first and see how I get on. Attached sketch shows what I am building. With the 70mm tryout I reckon it will collect 3x more energy that the original 40mm. If that goes well I could go to 90-100mm.

How and where do you measure the temperatures? I have one of those laser temperature guns.

Dave

PST 70mm mod.jpg (330.03 KiB) Viewed 3330 times

[AndiesHandyHandies]

Hi

I use a heat gun as well. After the ERF you can point them up the telescope to get what I call a Black Body Equivalent temperature. They are designed for testing meat temperatures so appropriate for 'meat machine' safety checking.

For the temperature of internal components you need to pull bits out quickly to measure if not immediately available.

I am going through my own PST Mod 2 at the moment.

I did have a 7nm 1.25" by my blocking filter but I have removed it at the moment.

Using a UV-IR and 35nm as ERF behind a moved forward collimating lens. Advantage is light goes back out as it came in.

As you are using the full PST you will have no issues.

Good luck. Andrew.

[StarryDave]

Here is my 70mm PST mod which I just finished and tested. Huge thanks to Mark and Andrew for keeping me on track. I was lucky to bag a set of never used scope rings on eBay for £12 and Mark found me the Baader filter for £110. My Amazon warehouse "box damaged" Celestron Powerseeker 70AZ was in perfect condition for £84 and I couldn't even find the box damage. Odds and sods meant final mod cost was ~£230 - not bad for a 70mm h-alpha scope.

The nice thing about this mod is that it was so easy - all I had to do was remove the 40mm PST objective. This means I can switch back to stock PST very quickly. I aligned the new objective by eye and thought I would have to spend a lot of time aligning it properly with the scope ring screws. However, first light test showed it was pretty well aligned. (see my next post for images)

Great thing is that I now also have a narrowband filter to try deep sky in the winter as well as a winter 70mm GnG scope.

PST70mm.png (1.82 MiB) Viewed 3174 times

[StarryDave]

I managed a few minutes of captures today to test out my 70mm PST mod. All captured on a DMK21 at prime. Seeing was poor with high clouds but good enough to check focus and tuning.

PST70PrimeDMK 20230507104843 P1 X1 104913_avs.png (548.92 KiB) Viewed 3173 times

PST70PrimeDMK 20230507103223 P1 X1 103224_avs.png (1.12 MiB) Viewed 3173 times

PST70PrimeDMK 20230507103030 P1 X1 103030_avs.png (1.11 MiB) Viewed 3173 times

This mod is a great improvement on the stock PST. Much shorter exposures mean lots more frames captured. Can't wait to try out my Powermate 2.5x and catch some prom animations. Already thinking about upgrading to 90mm which the current design will handle by just changing out the donor tube..

[AndiesHandyHandies]

Hi

That looks like a real mod. And you must be pleased with the results.

Cheers. Andrew.

[Rusted]

Keep going Dave. Great mod.

Liked your prom animation!

[marktownley]

Great result Dave!

[StarryDave]

Thanks guys - I am well pleased and looking forward to more captures. My 40mm objective is rusted so I was amazed at how bright the image is now. I have been contemplating a mod for years but was always worried about the complexity and the risk of PST damage. This forum lifted the covers and gave me the confidence to proceed. A few months testing the 70mm and then go for 90mm!

[Mrural]

Hello -
I have a pst w/ a rusted objective lens as well. I'd like to make a similar mod. A few questions, 1) will a SVBONY H-alpha 7nm 2 inch mounted filter work equally well for this? and 2) will this (w/a 70mm objective lens), allow this to be used visually as well for imaging?
best, Matt

[marktownley]

Hello -
I have a pst w/ a rusted objective lens as well. I'd like to make a similar mod. A few questions, 1) will a SVBONY H-alpha 7nm 2 inch mounted filter work equally well for this? and 2) will this (w/a 70mm objective lens), allow this to be used visually as well for imaging?
best, Matt

Hi Matt and welcome to the forum.

Yes, if the svbonyu is mounted as high in the light cone as possible it should work.

[StarryDave]

Matt
I used this stepdown adapter https://www.ebay.co.uk/itm/324302736572 to mount the filter to the front on my PST. It looks to be the perfect place for it because its diameter is very close to the light cone diameter at that point.
I would be wary of visual use. I can tell from the camera exposure times that a lot more light is getting thru than the 3x increase due to the bigger 70mm objective.
My initial design was going to use the Powerseeker 70 OTA instead of the cardboard tube. It would have worked without cutting the OTA but there is an internal baffle that gets in the way. I couldn't remove it but I will try again with a bigger hammer.
Let me know if you need any detail fotos of the mod.

[Mrural]

Thanks for the feedback Mark - I'm still disassembling my PST to check the other filters, but it would be great to get this working again. I'll probably post again as I work through this to troubleshoot the particulars.

[AndiesHandyHandies]

Matt
I used this stepdown adapter https://www.ebay.co.uk/itm/324302736572 to mount the filter to the front on my PST. It looks to be the perfect place for it because its diameter is very close to the light cone diameter at that point.
I would be wary of visual use. I can tell from the camera exposure times that a lot more light is getting thru than the 3x increase due to the bigger 70mm objective.
My initial design was going to use the Powerseeker 70 OTA instead of the cardboard tube. It would have worked without cutting the OTA but there is an internal baffle that gets in the way. I couldn't remove it but I will try again with a bigger hammer.
Let me know if you need any detail fotos of the mod.

You need to have the first internal ERF far enough down the tube so that the sun always falls on the filter whatever way you point the telescope, to be able to view proms on either side of the sun. Dielectric filters do not heat up much as the coatings deposited on optical grade glass. If the sun shines on the filter holder metal or the ota wall you will get heating.

Cheers. Andrew.

[FRAZ]

A Baader erf is worth its weight in gold. Get it right on the front end and you are set up for end of tube or internal as your aperture grows. The investment is really worth it.

Fraz.

https://www.firstlightoptics.com/solar- ... 180mm.html