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Using a doublet for mono imaging?

Refractors Acquisition Monochrome Solar System
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Photon_Collector avatar
Photon_Collector avatar
I have the Zenithstar 61 small refractor from William Optics. I will send my bigger Askar 120 to a remote observatory with a OSC camera, and would like to buy a mono camera to use from home (light polluted) while my better rig is imaging from Bortle 1. 

I've heard doublets are actually good for mono imaging, anyone have experience(s) they can share?
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Tony Gondola avatar
Tony Gondola avatar
Yes, you certainly can. I've done a lot with a cheap ST-80 doublet using dual band filters and a relatively small sensor camera (585). Shooting mono would work even better as you don't have to deal with the different best focus points you get with dual-band OSC.
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Oscar avatar
Oscar avatar
if you don't take luminance and you had an achromatic scope (single element, worse than doublet), and if you refocus per RGB filter, you can get away with no CA (because CA comes from R, G, B focus points that vary from each other a little)
Charles Hagen avatar
Charles Hagen avatar
if you don't take luminance and you had an achromatic scope (single element, worse than doublet), and if you refocus per RGB filter, you can get away with no CA (because CA comes from R, G, B focus points that vary from each other a little)

That is a wildly inaccurate oversimplification. Red, Green and Blue all are ranges of wavelengths that can all experience CA within their respective band passes. Even if you are shooting narrowband which is effectively a single wavelength that will not experience CA, your singlet will likely not be optimized for that wavelength and you will get other extreme aberrations. If you are looking to take nice images, a high quality doublet can be fine for narrowband, but I would not recommend anything cheap. There are many other considerations to be made beyond the number of elements in the scope.
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Oscar avatar
Oscar avatar
Charles Hagen:
if you don't take luminance and you had an achromatic scope (single element, worse than doublet), and if you refocus per RGB filter, you can get away with no CA (because CA comes from R, G, B focus points that vary from each other a little)

That is a wildly inaccurate oversimplification. Red, Green and Blue all are ranges of wavelengths that can all experience CA within their respective band passes. Even if you are shooting narrowband which is effectively a single wavelength that will not experience CA, your singlet will likely not be optimized for that wavelength and you will get other extreme aberrations. If you are looking to take nice images, a high quality doublet can be fine for narrowband, but I would not recommend anything cheap. There are many other considerations to be made beyond the number of elements in the scope.

thank you for correcting me
Taman avatar
Taman avatar
I have the Zenithstar 61 small refractor from William Optics. I will send my bigger Askar 120 to a remote observatory with a OSC camera, and would like to buy a mono camera to use from home (light polluted) while my better rig is imaging from Bortle 1. 

I've heard doublets are actually good for mono imaging, anyone have experience(s) they can share?

I have the Zenithstar 73 and have successfully used it for narrowband. Provided your stars look reasonable with a colour camera, you should have no problems with mono. Here's one of my better examples:

https://www.astrobin.com/0fue8u/C/
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