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220mm "PST"


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Intrigued by the set-up used by Rogerio Marcon from Brazil, I thought I would have a play with my 220mm achromat refractor. I placed a 80mm ERF inside the main tube with a holder fitted to one of the baffles some 600mm inside the focus where the solar image was about 65mm diameter. I previously checked the ambient heat with one of my cheeks (facial) and found it to be only warm, in fact the heat at focus wasn't what you would call hot. I inserted my stage one mod PST into the focuser and recieved the best Ha image I have seen so far, the detail and resolution were very similar to the images posted by Pete Lawrence, the only downside that I could see was the pitifully small field of view offered by the BF5 blocking filter. I have yet to refine the build up but so far it would seem to be another avenue worth looking in to as it reduces the cost of a suitable ERF yet retains the resolution of the main objective. So far I haven't experienced any heating effects on the image, yet there is no reason why a cooling fan couldn't be suitably placed. I'm hoping that this mod plus a Celestron 200mm aperture glass white light filter will keep me going for a while although it looks like I might have to fork out for a BF30.

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Spot on James. In general terms, the longer the focal length, the larger the focused image hence lower temperature per unit area. At 600m inside focus where the ERF is placed the solar illumination is around 65mm diameter and barely warm.

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Interesting... I assume this is his website: Astrofotografia e Instrumentação

Some amazing gear. The principal in anyway well illustrated there.

I either dreamed or read somewhere that PERHAPS the objective need not even be an achromat - The scope is, after all, only going to work over a very narrow waveband. Hmmm... I have this big simple lens - But perhaps that is a tad optimistic. :p

Edited by Macavity
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The objective does not have to be other than a single component as Ha is monochromatic, but being a specialist item it might work out just as expensive as a one-off compared to "common" achromats in the popular sizes. As far as I can see, what I have tried differs from Rogerio's scheme is that I have used an ERF and capitalised on the PST configuration to do the rest of the work. From previous posts on this forum it would appear that a stage 2 PST mod might provide even better results, particularly with a larger blocking filter.

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Spot on James.

Did i do something wrong?????

I only ever got called James when i was in trouble.... :p

On a serious note, It still makes me wonder why manufacturers haven't adopted this method of Solar scope build. Is it still as safe as we would like it to be????

Not knocking your idea Peter, though the question did need asking me thinks....

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Sorry Jamie, must have misread your signature. Re the Ha mod, it seems to me that it is still using all the usual elements that are considered safe, the concentration at a given focal ratio, as far as I know remains constant regardless of aperture.

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Further to my initial post, more experiments today under much better conditions resulting in good and bad news. The good is that the image was even better than before, the bad, being as the Sun was higher and in a good clear sky, the ERF heated up eventually to a hotter temperature than I would have liked. I have a 145mm ERF which could be mounted nearer the objective but I was of course hoping that this type of mod might lead to sub aperture ERF's for large long focus refractors. Being as the results so far are so outstanding I hope there will be a way to cool the ERF, a strategically placed fan could help, any further suggestions?

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Right again JAMIE, the nearer to the objective, the less the heat. At the moment I'm trying to rig up a 145mm ERF to try the difference. I've also made up an aluminium blank to substitute for the 80mm ERF in order to see how beneficial a cold air blower may be at this position. I plan to persevere with this as it looks most promising although I wouldn't necessarily advocate buying a large refractor just to do this, at least not at this stage.

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I used a 50/50 mirror as a large diagonal in front of the etalon to reduce the heat while still retaining the resolution

Interesting idea. If you got the right coating, the mirror could reflect most of the Ha whilst transmitting almost all the heat ...

Sub-aperture ERF would need to be made of Zerodur or something similar ... if it heats (even slightly) then its figure will be spoiled ... the nearer to the objective it is, the less the heating (and the larger it needs to be) but the more critical its figure is. I have a sneaky suspicion these factors play against each other. Also don't forget that a warm ERF inside the tube will cause tube currents!

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Thanks for your input Ken and Brian. I hope my initial post didn't give the impression that I had in any way stumbled upon a system that was going to revolutionise Ha viewing, it's just that I'm an avid experimenter and having the components to hand I thought it worth a try. As you have already hinted, there is no Ha "free lunch" and I jumped the gun a bit with the heating issue, but the results are so good I'm hoping to find a way round the problem that doesn't prove out of proportion to the end result. I have some lightly aluminised Mylar which might give enough attenuation to reduce the heat without compromising the image brightness too much for visual use. In any event, the experiment involves very little extra expense or work, if unsuccessful it will have been worth trying and help others avoid wasting their time. If all else fails I still have a good 145mm ERF and a 5" f15 and f10 to try out with a proven approach.

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My worry now (and I mentioned this to people at SSP) is that the new coronado prices make the PST mod seem almost unneccesary, unless you go for these kinds of apertures!

Okay...why?

Well

60MM 0.7A SM60 - £1299 RRP UK/CW BF10

PST - £449

ERF - £390

Vixen A80 - £250

Adapters and chopping - £100 (ERF cell/front and rear adapters for the etalon)

BF10 - £480

this is an 80mm scope with a sweet spot..

If you have the PST, it's more doable, but from scratch now, I would not be doing it, but would be getting the 60 or 90mm Coronados

Edited by NickH
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Totally agree Nick, but having all the necessary components already it's an interesting exercise, specially after seeing the impressive details produced by Rogerio Marcon. At the end of the day I'm trying to maximise my Solar observing as a dodgy chest problem means I have to be frugal with night time exposure. I have a reasonably good budget available but don't want to spend a small fortune unless it is going to give me significantly better performance than my current options.

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I will post up a PST80 image from SSP, where it was average seeing through a 2 minute cloud hole at F30, and whilst I am happy with it, it's nothing compared to the resolution he's getting

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