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PST stage 0.5 mod first light


JamesF

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The Sun is out, the sky is blue, there's not a cloud to spoil the view, so hang work and get out there with the newly modded PST :D At least for ten minutes so I can do a brief "first light" report...

I have no solar finder fitted at the moment, but lining up the shadows of the mounting rings was sufficient to get me on target with my 32mm plossl. I wasn't in the best position for viewing as I'd just put the PST on the EQ3-2 after taking some white light images with the Mak, but even so focus was easy to achieve with the dual-speed controls with the focus point about half-way through the total travel of the focuser and by rotating the tuning ring back and forth I could bring out the proms, filaments and a lovely AR bang in the middle of the Sun. A little tweaking of the tuning ring positioning pin may be required to get it perfect, but I think I need to settle down and get used to the scope first.

Stepping up to a 12mm plossl the view was magnificent with proms easy to pick out, some detail visible in the AR and a lovely filament (? I'm going to have to learn all the right names now :) visble towards one edge with the surface structure also clear. For viewing the entire solar disc this was quite comfortable.

Finally I tried the 5mm Nagler. At this magnification (80x?) the entire disc was awkward to view without having to "look around" the eyepiece but the detail was beautiful. The definition of the proms and filaments and the AR was a joy. It's quite clear that I need more practice with the tuning ring (and perhaps a better understanding of how it works, too). I'm going to have to set something up that's more comfortable for viewing rather than imaging to really get the best out of this.

I also want to use the scope for imaging, but I don't actually intend to use the diagonal. I'm going to make a suitable straight extension to screw the eyepiece holder into so the optical train is straight through. I thought I'd give it a quick go with the camera in the diagonal however. Starting up FireCapture with my ASI120 quickly demonstrated that I can only get about 2/3rds of a disc on the sensor and I didn't quite have enough inwards focuser travel to reach focus (not that I really expected to -- that's what the straight extension will take care of). If I made my adaptor thinner I'm sure it would reach focus, but actually I'd rather have the longer thread for the gold tube to screw into. It did appear that there may be some tilting of the camera because one side of the Sun appeared closer to focus than the other. I think that is almost certainly the diagonal -- another reason why I don't want it in the optical train for imaging.

Overall I'm really very pleased. The eyepieces I tested reach focus in the 10mm to 20mm range of the focuser (which runs from 0 to 30mm) so there's a good balance there. The focus is very smooth to adjust, especially compared with the focuser on the stock PST which I found quite "grabby". I'm really looking forward to a longer session with it, perhaps at the weekend. Now I really need to go and do some learning about the Sun :D

James

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Honestly hard to say. I couldn't pick out any immediately obvious vignetting visually. The proms were faint compared to the rest of the surface but then I expected them to be. I'm going to have another crack at it after lunch and take some time to set up somewhere comfortable to sit so I can concentrate better. Using my mount as set up for imaging wasn't really the ideal, but this has been a fair while coming and I was desperate to get out and at least check that it all worked :)

I also think I'll need some sort of shade because concentrating on the eyepiece is difficult with the Sun at 90 degrees to my face, especially when viewing from the side of the scope rather than behind it. I need to sort out a finder first though, so I can make sure it won't be compromised by the positioning of the shade.

James

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Well, despite people hassling me about work I've had another play this lunchtime.

This time I started off with a 19mm Pan to get the image centred. I quickly moved away from that and a 13mm Nagler however as I found they were reflecting a lot of light around which was very distracting. My 9mm BGO on the other hand was just gorgeous. The Sun was framed nicely across about two thirds of the field of view, all in focus. Definition seemed to suffer towards the edges of the field of view, but with the Sun centred that really wasn't a problem. Hopping up magnifications with each of the BGOs the views just got more detailed -- beautifully sharp once I got the hang of focusing. The BGOs appear to want a focus point well up the eyepiece holder meaning there was only a few millimetres of focus travel left, but if there's enough, there's enough :) Unlike it's larger cousins the 5mm Nagler didn't suffer from reflections, so I'll be quite tempted to use that for "close up" viewing. I'll be swapping between it and the 5mm BGO to see which I prefer the "feel" of for a while I think.

At 80x I picked one AR and could pick out a dark area at the centre with a very bright region to one side of it, surrounded by the characteristic "petals" that are also visible in white light. Shifting the edge of the disc to the centre of the field of view I sat watching an enormous prom that looked a bit like a wind-swept tree for five minutes or so. There needs to be an "Earth to scale" in-eyepiece PST mod :)

Movement of the focuser is so much nicer than the standard PST with the possibility of smoothly shifting the focuser by the tiniest of amounts. For me personally this makes the mod worth doing regardless of any other benefits. Obviously there's now the imaging side to work on, and I want to get to grips with the positioning of the etalon tuner, but thus far I am very happy with the scope. It's just so more pleasant to use than it was.

James

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It's hard to say specifically that the image quality is better I think Stu. My initial goals when doing this mod were threefold:

Firstly to resolve the problems with prism alignment because mine was well out of true. I've tried twisting it back into what I believe to be the correct position but didn't seem to want to really stay there. I thought (though problem 2 may be related to this) that it was contributing to the entire image not reaching focus at the same point. Obviously as the prism is no longer in play that is now resolved.

Secondly I found the stock PST focuser mechanism very "grabby". I think that's probably down to the way the prism is sandwiched between the threaded carrier on the end of the focuser screw and the material on the other side of the black box. It never really felt smooth to me. Obviously this is now also sorted.

Finally, I wanted something suitable for both visual and imaging use without having to hack bits off cameras. My ideal solution was to find another PST eyepiece holder unit and make a spacer of suitable length to fit the focuser with a thread on the back to accept the eyepiece holder giving a straight optical path and avoiding any issues caused either by the prism (in the original PST) or diagonal (in my "visual" setup). As I've not been able to find one yet, I'll just unscrew the holder from the diagonal and screw it into the spacer.

Obviously as I've not made the spacer yet (I wanted to get everything tested and have a good idea of how long it needs to be before doing that) I can't comment on the last issue, but I'm much happier with the usability of the scope now and it does appear that the entire image is in focus at the same time. Getting a camera onto it will really be the proof of the pudding there I think.

There may have been a bit of a gamble in attempting this mod not knowing it would turn out well, but the practice at metal-turning is welcome and I always intended to get an external focuser for the C9.25 anyhow so in the event that it didn't work out I had little to lose.

James

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Thanks James. It certainly seems like you've solved all your issues, I think it looks great too :-).

The focussing on my PST is quite smooth so it's never been an issue, the etalon adjustment on the other hand is quite grabby so is something I'd like to sort when I hopefully attempt a stage 1 mod at some point.

Look forward to hearing about further results, have fun.

Stu

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