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The PST etalon group expects to see an f/10 converging light beam. When fed with a typical f/8 beam, from one of the popular 6" refractors, the etalon narrows down the beam to f/10. The problem is the loss of aperture as a result. The telescope effectively becomes a [slightly emaciated] 120mm refractor. You lose the aperture you paid for.
Today I discovered a thread on the Solar Chat Forum. Where the Baader 1:1.25 glasspath corrector is used to achieve an f/10 beam. So that the PST etalon "sees" its intended, light cone angle. Whether the f/8 beam is vignetted will depend on the position of the GPC and its clear aperture. One user suggested that it made no difference to their modified PST instrument whether the GPC was placed against the etalon group or spaced more towards the telescope objective lens. This matters because of limitations in mechanical support available off the shelf. Most PST etalons sit between the two, AOK adapters. It would need a unique adapter to place the Baader GPC in the front AOK.
The solar image diameter of any telescope is focal length/109. So a 1200mm focal length will have an image diameter of 11mm. Sadly, this is not a figure we can use because the etalon is usually placed 200mm inside the focus of the donor telescope. This is because the original PST used this layout. Even though the PST's internal prism folded the beam to achieve a more compact instrument.
So we need the diameter of the f/8 light cone at 1000mm. Not 1200. If only the sun would shine it would be a piece of cake to place something at the desired point and measure the almost focused, sun's bright spot. Preferably before it bursts into flames! Though not with a D-ERF already in place.
As I typed those words the sun came out for the only time all day. I dashed over to the observatory, opened the shutters, turned the dome and swung the telescopes onto the sun. Thick cloud instantly intervened with large areas of clear blue sky teasing me overhead. Grr!
The PST etalon/lens group has a 20mm clear aperture. So the GPC needs to be a minimum 20mm clear aperture not to vignette the light cone any more than the etalon itself. The further forward of the etalon the GPC is placed the greater the risk of vignetting.
I have stared at all of the sales images online and the Baader GPC looks to be over 20mm. Perhaps I am an optimist and the images were taken in the hands of a petite female? But that would make me a cynic or [merely] paranoid. Or both. Guessing wildly, I'd say the GPC aperture is somewhere between 22 and 25mm. Which would make good sense for an expensive binoviewer with a larger clear aperture. 25mm?
Exact positioning within the light cone also affects the power of the negative lens [GPC] because it is so far from the eyepiece. Instead of the useful correction of f/8 to our [hoped for] f/10 it could become even more. F/11 or f/12 or worse. It is no use placing the GPC after the etalon group because it won't have the desired effect on the etalon but work only as a Barlow. A GPC may act like a Barlow but is is designed not to produce false colour and other optical aberrations when placed a long way inside focus.
One must hope that the numbers Baader publish to describe their GPCs are accurate in the context of their intended role in binoviewers. Do the large Baader binoviewers have a similar glass path length to a PST filter stack? Probably not. Most of a [modified] PST filter stack is just fresh air. Most of a binoviewer is solid glass.
The 1:1.25 Baader GPC is supposed to provide 30mm more inward focus when used in conjunction with their binoviewer. This is supposed to save the owners of valuable instruments from having to saw lengths off the main tube. Just to be able to reach focus. The 1:1.25 has a remarkably gentle effect compared to the 2x, and more, GPCs offered for "lesser" binoviewers. Low magnifications will help to maintain a wider field of view.
Another question is how the modified focal length affects the position of the etalon relative to the focal plane. Is 200mm inside focus relevant to our [highly theoretical] 1500mm, new focal length? It seems obvious that I must accurately measure the true focal length of the system after the GPC is inserted in front of the etalon.
Or, should I leave out the etalon group and measure the bare focal length? Then place the etalon in the correct place relative to the bare focal length? I know from experience that my supposedly neutral, etalon group affects the focal length of my 6". From, completely untrustworthy memory, I think it made it slightly longer. I believe the etalon group should be present because that is how they designed to perform in the original [unmodified] 40mm f/10 Coronado PST.
First get your 1:1.25 Baader GPC! Yet more expense as we approach the predicted, longest solar minimum since they last held winter fairs on the ice of the Thames. It has to be said that the last numbered, solar "blemish" [AR] wouldn't have embarrassed a prom queen on the biggest night of her life! More on this when my 1:1.25" GPC turns up in the post.
I have already realised that the recommended [and expensive] Baader T2-2" adapter will be utterly worthless. Since it won't fit inside the AOK, front, etalon adapter. Another Grr! It might be possible to cut a slice from the adapter to make it fit inside the AOK. Then the GPC can screw into an internal T2 thread in a short ring.
I will just have to console myself that I am saving myself the expense of buying a new f/10 6" objective. Or even a better 6" ED OTA to replace my ageing Celestron CR150HD. There's a nice AP 6" Apo for sale which is f/9. Closer, but still no cigar. The cost of a divorce easily outweighs any such temptation.
What about this for a mad idea: Offer 6" [152mm] D-ERFs with weak, negative power to correct 6" f/8s to f/10 for all the PST H-alpha modifiers? Nah. The number of buyers would be absolutely minuscule.
The cost of the 160mm D-ERF is now close to 1000 Euros. That is more expensive than an iStar objective of the same size! But, it would also keep most of the sun's heat out of the telescope. Better instrumental seeing than an internal D-ERF? The images from external D-ERFs are certainly enticing. Yet an instrument builder, with very real experience, claims that it makes no difference where the D-ERF sits. He says there is no sign of any thermal effects inside the instrument due to an internal D-ERF.
If I had a tube worthy of drilling I could fit a computer fan and blow air across the tube to an exit port. However, the CR150HD has a thread for the objective cell. So it is not easy to fit the objective cell accurately to any other tube. An oversized tube would make sense because the light cone grazes the inside wall just behind the objective of my CR150. Presumably a desperate attempt to keep the tube diameter to a minimum to save costs.
As I typed those words the sun came out for the only time all day. I dashed over to the observatory, opened the shutters, turned the dome and swung the telescopes onto the sun. Thick cloud instantly intervened with large areas of clear blue sky teasing me overhead. Grr!
The PST etalon/lens group has a 20mm clear aperture. So the GPC needs to be a minimum 20mm clear aperture not to vignette the light cone any more than the etalon itself. The further forward of the etalon the GPC is placed the greater the risk of vignetting.
I have stared at all of the sales images online and the Baader GPC looks to be over 20mm. Perhaps I am an optimist and the images were taken in the hands of a petite female? But that would make me a cynic or [merely] paranoid. Or both. Guessing wildly, I'd say the GPC aperture is somewhere between 22 and 25mm. Which would make good sense for an expensive binoviewer with a larger clear aperture. 25mm?
Exact positioning within the light cone also affects the power of the negative lens [GPC] because it is so far from the eyepiece. Instead of the useful correction of f/8 to our [hoped for] f/10 it could become even more. F/11 or f/12 or worse. It is no use placing the GPC after the etalon group because it won't have the desired effect on the etalon but work only as a Barlow. A GPC may act like a Barlow but is is designed not to produce false colour and other optical aberrations when placed a long way inside focus.
One must hope that the numbers Baader publish to describe their GPCs are accurate in the context of their intended role in binoviewers. Do the large Baader binoviewers have a similar glass path length to a PST filter stack? Probably not. Most of a [modified] PST filter stack is just fresh air. Most of a binoviewer is solid glass.
The 1:1.25 Baader GPC is supposed to provide 30mm more inward focus when used in conjunction with their binoviewer. This is supposed to save the owners of valuable instruments from having to saw lengths off the main tube. Just to be able to reach focus. The 1:1.25 has a remarkably gentle effect compared to the 2x, and more, GPCs offered for "lesser" binoviewers. Low magnifications will help to maintain a wider field of view.
Another question is how the modified focal length affects the position of the etalon relative to the focal plane. Is 200mm inside focus relevant to our [highly theoretical] 1500mm, new focal length? It seems obvious that I must accurately measure the true focal length of the system after the GPC is inserted in front of the etalon.
Or, should I leave out the etalon group and measure the bare focal length? Then place the etalon in the correct place relative to the bare focal length? I know from experience that my supposedly neutral, etalon group affects the focal length of my 6". From, completely untrustworthy memory, I think it made it slightly longer. I believe the etalon group should be present because that is how they designed to perform in the original [unmodified] 40mm f/10 Coronado PST.
First get your 1:1.25 Baader GPC! Yet more expense as we approach the predicted, longest solar minimum since they last held winter fairs on the ice of the Thames. It has to be said that the last numbered, solar "blemish" [AR] wouldn't have embarrassed a prom queen on the biggest night of her life! More on this when my 1:1.25" GPC turns up in the post.
I have already realised that the recommended [and expensive] Baader T2-2" adapter will be utterly worthless. Since it won't fit inside the AOK, front, etalon adapter. Another Grr! It might be possible to cut a slice from the adapter to make it fit inside the AOK. Then the GPC can screw into an internal T2 thread in a short ring.
I will just have to console myself that I am saving myself the expense of buying a new f/10 6" objective. Or even a better 6" ED OTA to replace my ageing Celestron CR150HD. There's a nice AP 6" Apo for sale which is f/9. Closer, but still no cigar. The cost of a divorce easily outweighs any such temptation.
What about this for a mad idea: Offer 6" [152mm] D-ERFs with weak, negative power to correct 6" f/8s to f/10 for all the PST H-alpha modifiers? Nah. The number of buyers would be absolutely minuscule.
The cost of the 160mm D-ERF is now close to 1000 Euros. That is more expensive than an iStar objective of the same size! But, it would also keep most of the sun's heat out of the telescope. Better instrumental seeing than an internal D-ERF? The images from external D-ERFs are certainly enticing. Yet an instrument builder, with very real experience, claims that it makes no difference where the D-ERF sits. He says there is no sign of any thermal effects inside the instrument due to an internal D-ERF.
If I had a tube worthy of drilling I could fit a computer fan and blow air across the tube to an exit port. However, the CR150HD has a thread for the objective cell. So it is not easy to fit the objective cell accurately to any other tube. An oversized tube would make sense because the light cone grazes the inside wall just behind the objective of my CR150. Presumably a desperate attempt to keep the tube diameter to a minimum to save costs.
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