Refractor telescopes and monochrome cameras

[Seby]

Hi,

I have a question about refractor telescopes used together with monochrome cameras. I've read that the benefit of apo and semiapo refractors is that they reduce the chromatic aberration. It's of course very important for visual observations and color cameras but does it make difference to use an achromatic or apochromatic telescope for imaging with a monochrome camera and color filters or narrow band filters? Are there also other advantages in apos beside the reduced chromatic aberration? Thank you for your help!

[rl]

If you're imaging with a monochrome camera all the same theory still applies; light at only one wavelength will be in precise focus. If you focus in the green, the red and blue light will still be slightly off focus. With a B/W image this will look look a slight loss of sharpness. One way around this is to use a colour filter, typically green, or sometimes red to help with seeing. But this costs light, pushing up the exposure time. Not a problem on the sun, moon or brighter planets but a big problem with most else.

The problem is most apparent in short focus achromats..and it is not the only issue. Some of these scopes have a lot of uncorrected spherical aberration for which a coloured filter won't help.  I'm afraid there is little substitute for a triplet or apochromatic doublet if you want good results! Excepting of course the excellent Newtonian in your avatar....(cue heated replies...)

[CraigT82]

Unfortunately it's not that simple, else everyone would be doing it!

Fast achros can even struggle to bring all the wavelengths within just one colour band to focus at the same point. 

However I've seem some very nice narrowband images taken with achros. 

[vlaiv]

In addition to to excellent answer by CraigT82 - not even all fast achros can be used from Narrowband imaging.

Well, you can, but you might not get as good images as you are hoping.

Fast achromats suffer from spherochromatism. That is spherical aberration that depends on wavelength of light. In another words - achromat is corrected for spherical aberration at only one wavelength. Usually that wavelength is green at around 550 - for good achromat - since these are mostly visual instruments.

Unfortunately, only narrow(ish) filter that is in this wavelength is Baader solar continuum and it is useful for imaging Sun in white light and maybe moon.

If you want to image for example Ha - you then actually need your scope to be corrected for 656nm wavelength. More skilled people do this themselves - by changing distance between two lenses of achromat. This changes spherical aberration of the system and you can get enough spherical to counter acts spherocromatism for wanted wavelength and correct for it.

Complex topic. Here is example:

http://interferometrie.blogspot.com/

checkout comparison between three fast achromats there.

[CraigT82]

1 hour ago, vlaiv said:

Complex topic. Here is example:

http://interferometrie.blogspot.com/

checkout comparison between three fast achromats there.

That's a very interesting blog.  Thanks for sharing! 

[andrew s]

Most fast telescopes suffer from field curvature unless they have a built in flattened which is not the norm.

Regards Andrew 

[vlaiv]

49 minutes ago, andrew s said:

Most fast telescopes suffer from field curvature unless they have a built in flattened which is not the norm.

Regards Andrew 

I'm not quite clear on that but I think it is not speed of telescopes but rather design and focal length.

From what I understood when I was researching on the topic (nothing serious, just informative), refractors have curvature somewhere around 1/3 of their focal length - regardless of their aperture.

[CraigT82]

4 minutes ago, vlaiv said:

I'm not quite clear on that but I think it is not speed of telescopes but rather design and focal length.

From what I understood when I was researching on the topic (nothing serious, just informative), refractors have curvature somewhere around 1/3 of their focal length - regardless of their aperture.

Is that the radius of the curved field that's 1/3 of FL? 

[vlaiv]

Just now, CraigT82 said:

Is that the radius of the curved field that's 1/3 of FL? 

Yes, as far as I understand.

[andrew s]

3 hours ago, vlaiv said:

I'm not quite clear on that but I think it is not speed of telescopes but rather design and focal length.

From what I understood when I was researching on the topic (nothing serious, just informative), refractors have curvature somewhere around 1/3 of their focal length - regardless of their aperture.

Yes it's focal length but I don't  know of any fast astronomical telescopes with a "long" focal length. I should have been more precise. 

Regards Andrew 

PS I do but they are professional telescopes 

Edited April 30, 2020 by andrew s

[johninderby]

Nice straight forward explanation. 
https://starizona.com/tutorial/field-curvature/

[Seby]

Thank you for all your replies, now I have understood better!

 

8 hours ago, rl said:

.  I'm afraid there is little substitute for a triplet or apochromatic doublet if you want good results! Excepting of course the excellent Newtonian in your avatar....(cue heated replies...)

Yes, rl, I love my simple f/4 Newton telescope 🙂 but for some large nebulas a shorter focal lenght would be better so I am thinking to buy also a short focal refractor at some point or, maybe, a long focal photo lens.

[vlaiv]

2 minutes ago, Seby said:

Yes, rl, I love my simple f/4 Newton telescope 🙂 but for some large nebulas a shorter focal lenght would be better so I am thinking to buy also a short focal refractor at some point or, maybe, a long focal photo lens.

Until you decide on shorter focal length scope - why don't you try another approach with your F/4 newtonian?

See what sort of FOV you'll be getting on particular nebula and decide on how many panels you need to do. Start simple to get used to doing mosaics. Let's say that you want to do 2x2 mosaic. First make sure you have software support for making panels (SGP has mosaic assistant as an option, EQMod also, not sure about others). Do 1/4 of allocated time on each panel.

Processing will be a bit different - since you used only 1/4 of time (or rather 1/4 of number of exposures), you'll need to make up for lost SNR by binning your data 2x2. That will in the end create same size image that you are used to (in pixel count terms) and it will be as if it was shot by twice shorter focal length.

In fact - it will be like using twice shorter focal length F/4 scope. Not something that can easily be found.

Only issue is processing / stitching of panels. You need to calibrate them properly and remove background gradients for them to blend in seamlessly. There is software out there that can combine panels into mosaic for you (not sure, but I think APP can do it, probably PI as well, and if you don't have licenses for those - ImageJ has plugins that can stitch panels - but it is a bit more manual work to get panels to blend together).

[Seby]

Thank you for your tips, vlaiv, it's an interesting idea. I never tried with mosaics but I think that the tecnique that you describe with shorter times for the single panels is very interesting and the advantage is that I could use the f/4 telescope that I already have. I have to try this with some easy nebula once the lockdown period is finished.