Question About Solar Spectrum Filters

[marktownley]

I know these filter units are designed to sit in an f30 beam, however my question is will they work in a collimated beam instead?

I can only use an f30 beam on maybe one or two occasions per year due to poor seeing conditions. However would it be possible to mount a negative lens inside the ota focal point at appropriate distance to produce a collimated beam, then mount solar spectrum filter, then a converging lens to bring the collimted beam to focus?

Thanks,
Mark

[brianb11213]

Logic would suggest that a collimated beam should work at least as well if not better than an f/30 converging beam, however I'd double check with their technical support before parting with serious money. And if I couldn't get a satisfactory answer from their technical support I'd look elsewhere.

(Hey, I thought you knew about as much about the practicalities of sub-aperture etalon systems as anyone on the planet!)

Of course the other way would be to produce a f/30 beam for the etalon to sit in & then stick a focal reducer between it and the camera.

[marktownley]

Logic would suggest that a collimated beam should work at least as well if not better than an f/30 converging beam

Indeed, i,m just wondering if they have been 'optimised' for f30...

[colinsk]

No, it is optimized for f/109 (well not optimised but that is the limit of a full disk) but that is not practical. The longer the fl you can get the tighter the bandpass will be. Collimated is OK but telecentric is much better. Etalons want no angles at all.

[markthais]

In a collimated system the bandpass HW stays the same across the field but the wavelength shifts from the center to the edge of the field to the blue wavelengths. This is caused by the field angle of the sun. If the sun where a point source then it wouldn't matter.
You will get a similar shifting in wavelength from a barlow but you also get a boarding of the bandpass from the angle of the beam going through the etalon. Barlow's with long focal length will work OK with filters that are not to narrow. Neg lens with focal length -300mm and longer will work the best.
With a telecentric depending on the etalon spacer and focal ratio, the HW will broaden but the wavelength will stay uniform across the image.
The "sweet spot" that is mentioned in many posts is caused either by the optics or the uniformity of the etalon.
This is something that has been excepted as the normal.
When used correctly the image should be uniform across the field.
Mark

[colinsk]

In a collimated system the bandpass HW stays the same across the field but the wavelength shifts from the center to the edge of the field to the blue wavelengths. This is caused by the field angle of the sun. If the sun where a point source then it wouldn't matter.
You will get a similar shifting in wavelength from a barlow but you also get a boarding of the bandpass from the angle of the beam going through the etalon. Barlow's with long focal length will work OK with filters that are not to narrow. Neg lens with focal length -300mm and longer will work the best.
With a telecentric depending on the etalon spacer and focal ratio, the HW will broaden but the wavelength will stay uniform across the image.
The "sweet spot" that is mentioned in many posts is caused either by the optics or the uniformity of the etalon.
This is something that has been excepted as the normal.
When used correctly the image should be uniform across the field.
Mark

Thank you Mark, That is very concise. To rephrase if I may (please correct me if I am misunderstanding):

Barlow: Wavelength shift across the field of view from entrance angles and bandpass broadening from angles from the rays leaving the barlow.

Collimated: Wavelength shift across the field of view from entrance angles.

Telecentric: Bandpass broadening from angles of the rays leaving the first telecentric pair of lenses.

Thank you,

[marktownley]

Thank you for that Mark - certainly gives food for thought. Thanks for the summary Colin

[markthais]

Hi Colin,
I like the way you exampled it better then mine. It was to the point.
Mark