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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!
WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!
With the imminent arrival of a used PST, in the post, it is time to think seriously about conversion components.
A larger aperture, than the PST's own 40mm objective, would provide much greater resolution. This raises serious issues of overheating the PST inner components! Not to mention the potential dangers to the users eyes. The answer is to install a Baader D-ERF energy rejection filter near, or more often in front, of the main telescope objective. Do NOT confuse the specialist Baader D-ERF energy rejection filters with cheaper camera filters. The camera filters offer NO protection from the intense heat focused by the telescope!
The Baader energy rejection filter is a large, round piece of 8mm [5/16"] thick glass and very costly in larger sizes. It needs secure and properly designed support. One can buy a commercial filter adapter but they seem foolishly expensive to one, like me, who habitually uses DIY [or ATM] to avoid such high expenditure.
Moreover, the commercial adapters are made to securely fit telescope dewshields. Not to be placed inside the main tube. Where a smaller diameter filter can be moved nearer the objective's focus.
This saves the high cost of a full aperture filter. The highly reflective, multi-coated filter surface sends any focused heat back out through the telescope objective. Which protects the PST's lenses, filters and etalon [and the user's eyes] from excessive heat. The filter is marked with an arrow on its edge to indicate the surface which should face towards the sun.
This saves the high cost of a full aperture filter. The highly reflective, multi-coated filter surface sends any focused heat back out through the telescope objective. Which protects the PST's lenses, filters and etalon [and the user's eyes] from excessive heat. The filter is marked with an arrow on its edge to indicate the surface which should face towards the sun.
The plan is to convert my rarely used, Celestron 150mm f/8 refractor as the donor for a much larger, solar telescope. It's not a completely free lunch though. Because of all the other components needed for the conversion. The PST's rather small, 20mm etalon stops down the 150mm aperture to about 120mm. Still a huge improvement on a 40mm aperture telescope. Which would cost many thousands in a commercial Solar Telescope in larger objective sizes.
The obvious answer, to save the high cost of an adapter, is to use some of my 10mm scrap aluminium sheet. The filter itself is 8mm thick x 90mm diameter. The filter rim could be safely seated in a strip of cork inside the turned, slightly oversized hole. This should safely avoid any chance of shock damage from the instrument being dropped or badly bumped. Small metal tabs, front and back, can retain the filter from escaping its holding ring. [See image alongside]In reality, the 150mm Celestron's tube is only 138mm in internal diameter. Leaving one to question whether this doesn't stop the telescope down, being so close to the back of the objective! It must surely make grazing incidence more more likely and far more difficult to hide with later baffles. I have some true 6" Ø tubing which might make more sense. Though this would need an adapter for the screw-on objective cell. It was only later models of the CR150HD/CR6 which had collimateable cells.
The outside diameter of the adapter ring can be turned [in the lathe] to match any telescope tube's inner diameter. By using slightly thicker sheet, from which to turn the ring, cork can be inserted inside the filter securing tabs. Alternatively the 10mm adapter ring could be thinned on the lathe to match the filter and the tabs provided with cork facings over their entire inner surfaces. Long screws with Nyloc nuts will safely hold the tabs in place 120° apart around the filter rim. The tabs could be washers or alloy disks slightly overlapping the glass filter. With cork lining facing the glass.
An alternative design is to leave a narrow ring to back up the filter hole rather than having a straight, through hole. The filter would still need a couple of glued cork tabs on the projecting ring to protect it from shock and distortion. The filter is a highly accurate, optically polished components and must not be squashed, bent, twisted, compressed or forced into a tight space. In fact it should be treated with the same respect and care as the telescope objective itself. In larger sizes ithe D-ERF filter will cost more than a similar sized achromatic objective! Or even a complete telescope!
Extra thicknesses of cork placed in 180° opposition are usually suggested to tilt the filter slightly. Parallel, plano optics can cause artifacts by repeated, back and forth reflection with other flat surfaces like the etalon or a flat-backed Littrow objective.
An alternative design is to leave a narrow ring to back up the filter hole rather than having a straight, through hole. The filter would still need a couple of glued cork tabs on the projecting ring to protect it from shock and distortion. The filter is a highly accurate, optically polished components and must not be squashed, bent, twisted, compressed or forced into a tight space. In fact it should be treated with the same respect and care as the telescope objective itself. In larger sizes ithe D-ERF filter will cost more than a similar sized achromatic objective! Or even a complete telescope!
Extra thicknesses of cork placed in 180° opposition are usually suggested to tilt the filter slightly. Parallel, plano optics can cause artifacts by repeated, back and forth reflection with other flat surfaces like the etalon or a flat-backed Littrow objective.
The edges of the adapter ring can be radially drilled and tapped to take the screws which will hold it securely inside the main, telescope tube. Large ventilation holes drilled in a ring around the adapter ring might be a good idea. Otherwise any build up of warmth would be trapped between the filter and the back of the objective. Though the main tube in this area itself could be perforated to allow any heat to escape if it was thought desirable. Spacers between an undersized adapter ring and the inner tube wall could provide the same ventilation purpose. They might also allow much easier handling of the adapter with the filter already fitted.
Those interested in obtaining more details from real experts in the field of Solar observation and PST conversions should enjoy the Solar section of Stargazer's Lounge.You'll need to join to ask questions.
https://stargazerslounge.com/forum/207-observing-solar/
A similar specialist forum exists on Cloudy Nights:
Solar Observing and Imaging - Cloudy Nights
Please read the serious safety warning at the top of the page! No responsibility is taken for your cack-handed, half-arsed-backwards constructions based on anything you read here! I cannot personally supervise your work. Nor your ab-use of my own ideas and illustrations. I have decades of experience in safely observing the Sun in white light but still seek advice from those who have constructed numerous examples of these PST conversions. Several of whom are regular advisers on Stargazer's Lounge and CN.
I scribble my thoughts here for my own amusement and endless analysis of likely problems. Treat the text as a guide, by all means, but not as explicit instructions. Your own safety is your own responsibility. "Ambulance chasers" are no replacement for self-discipline, common sense and careful study of the implications of your own actions. No amount of "lottery prize compensation" will cure permanent blindness caused by your own foolishness. Consider yourself warned!
Click on any image for an enlargement.
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