PST Evo Star 90 done!

[Sqyff]

Finally it is done.

I managed to get a bit of late sunlight Thursday evening (10 mins or so) to measure the focal point of the scope and to my utter amazement and delight the tube is shorter than the 200mm required for the mod. In fact by sheer blind luck the length of the tube combined with the tube plug I had made are perfect for the mod, in other words........no tube cutting!!!

I spent last night putting it all together and managed to get first light this morning in a strong breeze and on a flimsy EQ2 (as you can see from the pictures).

The contrast and detail was amazing but there seems to be a slight vignetting, nothing too unusable but a bit annoying (any ideas how to get rid of this would be welcome).

The D-ERF holder is a temp construction until I get a better one made.

All in all I'm quite pleased with the way this went, although a new heavier mount is now on the horizon. Might pop up to Telescope House this afternoon to see if there is a second hand mount available.

[JamesF]

It does look just a smidge under-mounted

James

[Merlin66]

Looks good!

How does the vignetting compare with the original PST set-up.

You're using a zoom eyepiece? Did this focus easily on the original PST?

[Sqyff]

The zoom worked quite well on the standard PST and I don't think it vignetted that I can remember.

I have a feeling it might be the plug. Basically this is a 50mm deep aluminium slug with a 2" hole through it. There is no cut back on the inside and having just measured the PST front adapter this is about 15mm shorter so I think the edge of the light cone is being cut by the edge of the center hole. I hope that makes sense.

[Peter Drew]

In what way are you describing the vignetting?. The 5mm blocking filter of the standard PST can pass a full solar disc for an objective of 400mm focal length, the longer focal length of your mod will present a larger solar disc for which the 5mm BF is too small. The remedy, (expensive!) is to buy a larger BF. You are only vignetting the solar disc in the same way as you would by looking at the Moon with an eyepiece whose magnification does not permit seeing the whole of the lunar disc, you are not vignetting the main aperture and so you are not losing resolution.

[Sqyff]

Hi Peter.

Basically I am getting a very un-sharp fuzzy edge which seems to be narrowing the view, a bit like tunnel vision.

I've had another look at the scope and plug and now don't think it's the plug cutting the light cone as the 2" center hole is no smaller than the baffle hole a few inches ahead of it.

Could it be that I have not quite measured to the correct place and this is causing the vignetting even though I can get focus? I have been a bit confused about which part of the PST etalon assembly I should be measuring to be inside the 200mm .

Hmm, I've just re-measured everything and I think I might be out a few millimetres. I can give you the numbers so you might be able to see what is wrong.

From the focal point to the front edge of the aluminium plug is about 205mm. When I put the PST adapter on this has a plate in front of the threaded part which is 3mm deep. So the front edge of the etalon focuser is 202mm from the focal point..........I think.

[Steep]

Move it in a bit if you can but also check to see that the pst is square to the front element and centered.

[Merlin66]

It's usual to measure the majic 200mm from the front edge of the rubber knurled ring on the body of the PST etalon.

As I mentioned previously +/- 5mm will give you good results.

[Peter Drew]

What is the condition of the blue ITF in front of the blocking filter, these are known to deteriorate and is evident by a red ring that starts from the outer edge and gradually encroaches into the centre, this could eventually cause a fuzzy edge to the field.

[Sqyff]

Thanks Merlin and Steep. I tend to be a bit thick sometimes.

So I need to take about 15mm off the scope tube, would that be about right to bring the etalon into the correct focal area?

[Sqyff]

Hi Peter.

Not sure about the ITF filter but as the unit was only bought new in October I would hope this is OK. As I said, the standard gold tube configuration has no vignetting it's only when I connect the PST to the donor scope I have this trouble.

[Merlin66]

You have to remember the relationship between the size of the solar disk and the size of the final blocking filter...

At 400mm fl (original PST) the sun's image is around 4mm diameter against a 5mm blocking filter - looks OK.

When you mod, the fl has increased to 1000mm, solar diameter - 10mm, hence the usual feeling of edge fading/ vignetting.

The answer, as you've probably guessed, is to go for a Stage 2 mod with a larger blocking filter......

[Sqyff]

Hmm, didn't think the effect would be that pronounced. Oh well, thanks Merlin. I still think I may need to knock a bit off the tube when I get the chance as i'm sure i'm a little bit short on the correct distance.

I will probably go for a stage 2 at some point but £450+ for a BF15 and over £1000 for a BF30 plus shipping, import tax etc is just silly money. How they can justify this sort of price is beyond me. Are there no alternatives to the Coronado that are a more reasonable price?

[Merlin66]

Yes,

I agree. The prices for the blocking filter assembly are extraordinary!!!

Limited market, and degree of difficulty manufacturing I think are driving the pricing.

We are currently in discussions with possible suppliers for "replacement" mini-erf/ITF filters and also for the final blocking filter. Unfortunately the required specification still seems to give an ITF price around $250 (combination narrowband Ha with additional UV-extendedIR blocking) and we're still waiting on the 8A Ha blocking filter price.

I think the final pricing will still be higher than we'd hoped for.......

[Sqyff]

Anything below $500 for a BF15 alternative is still a saving.

In fact I was just reading your discussion about this in a certain solar observing forum. Seems like there are a lot of extremely knowledgeable involved so I have faith that a solution will be forthcoming.

[Sqyff]

Oops! Forgot to put 'people' after 'knowledgeable'.

[Peter Drew]

I'm not overly concerned by the price of ITF's and blocking filters when comparing the cost saved by modding a PST to that of a large bespoke Ha telescope, what I would like to see is these expensive items having a longer user life.

[Sqyff]

Good point Peter. I would have thought these parts would be more robust.

[Merlin]

In the instruction sheets that came with my LUNT LS50F, it states that, with care, the kit will "last a lifetime". Evidently not, as the ERF inside the B600 has a "bloom" and needs replaciing. I'm currently using the LS50F with great success as a double-stack on the unmodded PST.

By the way chaps, is the internal thread that takes the PST objective cell compatible with the rear male thread on the etalon that screws into the black box? If so, I can use the length of gold tube I sawed off surplus to a stage 1 mod for a stage 2 mod ( including a helical focuser ).

[Merlin66]

Merlin,

Good thought!

Unfortunately the objective thread is much smaller than the required M50 thread....bumma!!