Performance Istar lens in narrowband imaging and UV-IR

[arnaud]

I have a question about using an achromatic lens for narrowband imaging. Perhaps this has already been addressed, but I could’t find a thread.


I’m considering using the 6 or 8” f/6 achromatic lens offered and build it into a deepsky instrument. Using a flattener I would like to image deepsky objects or perhaps the sun. Because I observe from a bortle 9 sky, I’m restricted to narrowband imaging.


As I understand it, the achromatic lenses are corrected for differences in focus for different wavelengts, although not as good as an APO refractor could be. Also, the achromatic lens corrects for spherical aberration, designed usually for a specific wavelength (550 nm?).

So if I buy this lens should build a telescope in which:


• not all colours do not come perfectly into focus.

• spherical aberration is only well controlled at a single wavelength and deviates when other wavelengths are used for imaging.


I hope I understand this correctly. If I use narrowband filters, the first issue should not be a problem. I can refocus between narrowband filters and in such a small bandwidth (10 nm) I should be able to obtain sharp images. So my questions are:


What about the spherical aberration at other wavelengths than the design wavelength? At which point would the strehl value increase to unacceptable values? Suppose I were to use the telescope for solar imaging in the 393 nm Ca K-band, would the strehl value still be acceptable (>0.8) Or would it be much lower?


Our camera sensors nowadays have a range of 350-1100 nm, but most spot diagrams are still restricted to visual wavelengths only. Would it be possible to calculate the performance, for example of the 6 or 8” achromatic lens offered on your website, and see how it performs at 350, 800 and 900 nm as well?


Finally, the lens is coated with an AR coating. Will those coatings block or decrease the signal from IR or UV?

The IR and UV are interesting for me because they are usually unhindered by light pollution, so I have some opportunities there.


Al lot of questions, but I hope you can help me with some of them. In any case, thank you for your time.


Arnaud

[arnaud]

I see I already got the advice to contact some of the other owners of achromatic telescopes on an earlier post. I did not yet read them when I posted this question. I will ask them.
But if anyone still has advice or comments, I would welcome them.

[Ales - iStar Optical]

May 22, 2021 3:39:01 GMT -5 arnaud said:

I have a question about using an achromatic lens for narrowband imaging. Perhaps this has already been addressed, but I could’t find a thread.


I’m considering using the 6 or 8” f/6 achromatic lens offered and build it into a deepsky instrument. Using a flattener I would like to image deepsky objects or perhaps the sun. Because I observe from a bortle 9 sky, I’m restricted to narrowband imaging.


As I understand it, the achromatic lenses are corrected for differences in focus for different wavelengts, although not as good as an APO refractor could be. Also, the achromatic lens corrects for spherical aberration, designed usually for a specific wavelength (550 nm?).

So if I buy this lens should build a telescope in which:


• not all colours do not come perfectly into focus.

• spherical aberration is only well controlled at a single wavelength and deviates when other wavelengths are used for imaging.


I hope I understand this correctly. If I use narrowband filters, the first issue should not be a problem. I can refocus between narrowband filters and in such a small bandwidth (10 nm) I should be able to obtain sharp images. So my questions are:


What about the spherical aberration at other wavelengths than the design wavelength? At which point would the strehl value increase to unacceptable values? Suppose I were to use the telescope for solar imaging in the 393 nm Ca K-band, would the strehl value still be acceptable (>0.8) Or would it be much lower?


Our camera sensors nowadays have a range of 350-1100 nm, but most spot diagrams are still restricted to visual wavelengths only. Would it be possible to calculate the performance, for example of the 6 or 8” achromatic lens offered on your website, and see how it performs at 350, 800 and 900 nm as well?


Finally, the lens is coated with an AR coating. Will those coatings block or decrease the signal from IR or UV?

The IR and UV are interesting for me because they are usually unhindered by light pollution, so I have some opportunities there.


Al lot of questions, but I hope you can help me with some of them. In any case, thank you for your time.


Arnaud



Hello, here is a response from my friend, optical designer here in Tucson. I hope his response helps.

Yes, that understanding is correct. You can refocus for different wavelengths but there will be uncorrected spherical aberration at wavelengths other than the nominal design wavelength, usually 587nm. In fact, in a fast, all-spherical achromat, even the center wavelength has some residual spherical. Looking at it in Zemax, it could give decent results in H-alpha (656nm) but not great results at OIII (500nm). It gets pretty bad any farther blue or red from that. I would not consider UV or IR with probably even an apo refractor, and definitely not an achromat.

For H-alpha you can get a Strehl of about 0.8, which is fine for deep sky imaging, and maybe okay for solar when you account for the fact that most solar setups like a Daystar would increase the focal ratio and have other optics involved. I use a 6" f/8 achromat for H-alpha solar and it works great. Things get bad in the blue very quickly for an achromat. For an 8" f/6 at 393nm in CaK the wavefront error is over 3 waves! In theory, you can correct for this by increasing the spacing between the objective lens elements, but clearly, this is something you would only want to do once for a dedicated CaK scope and not something that could be changed easily.

So I guess I would say it would work for H-alpha, both deep-sky and solar, but not for much imaging beyond that, unfortunately.