Short Focal Length Telescope with External Etalon

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
Post Reply
kornfeld1

Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

Are there any limits to the focal length of the telescope you use with an external etalon? I have a 60mm Solarmax etalon, and am considering trying it out on a 350mm focal length refractor, with the goal of being able to fit a full solar disc onto a DMK31 or 41 sized chip. This thread:

http://solarchat.solarastronomy.org/ind ... -evolution

has a scope with a focal length in that range, but I wanted to see if there are any concerns with going to an even shorter FL with a larger etalon. :thanx:


colinsk

Re: Short Focal Length Telescope with External Etalon

Post by colinsk »

If you divide the fl/107mm you will get the solar image size. This should match your chip size. A external etalon does not care about the f/l or the focal ratio.


kornfeld1

Re: Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

A external etalon does not care about the f/l or the focal ratio.

Perfect, that's the exact info I needed!

I found this excellent site for simulating how things will appear through eyepieces or in a camera:

http://www.12dstring.me.uk/fov.php

The list of potential telescopes and cameras is incredibly encyclopedic.


astrodanco

Re: Short Focal Length Telescope with External Etalon

Post by astrodanco »

Yup, that's the site I like to use.

The Astronomics.com website also has a handy calculator that appears on the right hand side of the screen as you're browsing CCD cameras on their site.


User avatar
robert
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 3069
Joined: Fri Oct 14, 2011 1:49 pm
Location: N.W.Scotland
Has thanked: 4 times
Been thanked: 1274 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by robert »


A external etalon does not care about the f/l or the focal ratio.

unless it has a narrow band effect and this is smaller than the full disc, like my LS75 which is perfect at higher FL but is hard to use for full disc shots
Robert


images and animations http://tinyurl.com/h5bgoso
2024 images https://www.flickr.com/photos/69734017@ ... 0313830045
2023 images https://www.flickr.com/photos/69734017@ ... 0304905278
ED80. ED100. Celestron-150mm-PST mod. C8 edge. ES127
LS60PT-LS60F-B1200. B600-Cak. PGR-Ch3-IMX265
colinsk

Re: Short Focal Length Telescope with External Etalon

Post by colinsk »

That is a good point. The etalon will still see rays coming from .5deg if you want a full disk shot. That is enough to degrade the performance. But a longer f/l telescope and a bigger chip would still have the same problem.


User avatar
robert
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 3069
Joined: Fri Oct 14, 2011 1:49 pm
Location: N.W.Scotland
Has thanked: 4 times
Been thanked: 1274 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by robert »

I wonder if the etalon in the LS75 could be made to pressure tune, would that produce a flatter filter effect with no banding?
I can imagine fitting some o-rings and a syringe to give this a test.
Any thoughts?
:idea:
cheers
Robert


images and animations http://tinyurl.com/h5bgoso
2024 images https://www.flickr.com/photos/69734017@ ... 0313830045
2023 images https://www.flickr.com/photos/69734017@ ... 0304905278
ED80. ED100. Celestron-150mm-PST mod. C8 edge. ES127
LS60PT-LS60F-B1200. B600-Cak. PGR-Ch3-IMX265
colinsk

Re: Short Focal Length Telescope with External Etalon

Post by colinsk »

How far do you tilt the etalon to get on band? If you flatten the etalon so that the reflection of the sun is in the field of view (at your normal viewing temperature and altitude) then start adjusting you should seen the reflection go outside the field of view. If the scope is on band just as the reflection leaves then it likely would not be helped by changing the tuning method. You will have to tilt it this much anyway. If you keep adjusting for a few turns then your etalon might benefit from a different tuning method.

So for air spaced etalons:

* increasing n (air pressure) is red shifted

* increasing l (spacer thickness, ie heating) is red shifted
(This assumes the expansion coefficient of the spacers is greater than the change of pressure caused by heating air... )

* increasing theta (tilt) is blue shifted

See if you have a broader sweet spot on a very cold day.


kornfeld1

Re: Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

Would there be any concern with putting the etalon in the fridge or freezer before an observing session? It's pretty heavy, so imagine you could get at least a little observing in before it warmed up.


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by Merlin66 »

Phil,
Don't even think about it!!
Seriously, there are no major issues using an external tilt etalon on 99% of most of the scopes we use....
I use the SM60 - single and double stack on a SW ED80/ BF15 with no issues.
(if and when I get back to Oz, I'm going to re-machine my SM60 adaptor plate to fit the new TS 102/1100 achromat..that way I can have a "universal" 100mm PST mod, a SM60 (single or double stack) and a Baader solar wedge on the same scope..with quick interchangeability)


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
kornfeld1

Re: Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

Phil,
Don't even think about it!!
)

Don't throw my stuff in the freezer, or don't use a short telescope?

Or are you saying, "don't worry about it!" Meaning it's fine to do that stuff?

(Sorry if I'm being dense, I was wine tasting all day yesterday, and my brain is pretty gummed up today.)


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by Merlin66 »

Too much Shiraz can be a major problem! ;-)
No freezers, please!
Just use the etalon you have on the short focus scope you have - and enjoy the view!!


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
kornfeld1

Re: Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

Too much Shiraz can be a major problem! ;-)

Life is rough. :)

I was thinking it might be more helpful if I asked the original question sort-of-in-reverse...what would make a telescope not compatible for use with an external etalon? Let's assume that you can mount the etalon, and you can mount the diagonal. What other criteria might rule a telescope out?

Also: how will a short focal length scope affect the resolution of the final image? I seem to remember there being big drops in resolution with small drops in aperture; are there going to be big drops in resolution with decreasing focal length as well?


brianb11213

Re: Short Focal Length Telescope with External Etalon

Post by brianb11213 »

Would there be any concern with putting the etalon in the fridge or freezer before an observing session? It's pretty heavy, so imagine you could get at least a little observing in before it warmed up.
Ummm .... it wouldn't be usable until it warmed up to the dew point & then still some time would be needed for the condensation to evaporate.

Even if the surfaces weren't misted over, thermal currents would undoubtedly ruin the image. The major optical components need to be at a steady temperature (where the amount they gain balances the amount they lose) to have any chance of getting reasonably steady images.


brianb11213

Re: Short Focal Length Telescope with External Etalon

Post by brianb11213 »

what would make a telescope not compatible for use with an external etalon? Let's assume that you can mount the etalon, and you can mount the diagonal. What other criteria might rule a telescope out?
Can't think of anything offhand - if the ERF is mounted with the etalon in front of the scope, the size, focal ratio & optical design of the scope doesn't matter a foetid dingo's kidney.

how will a short focal length scope affect the resolution of the final image?
The resolution is (in theory) determined by the free aperture of the objective (allowing for any masking by the etalon, ERF or aperture mask) and the working wavelength only.

In practice very short focus objectives are hard to figure accurately and, in the case of reflectors, tend to require excessively large secondary mirrors, which has an effect. But Newtonians can be made to work very well indeed at f/4 or even faster, provided you don't want a large field ... off axis coma degrades the image much more seriously in the Newtonian design with short focal ratios.

However, for imaging, the sensor needs to resolve the detail: how big the focal ratio needs to be depends on the pixel pitch of the sensor. We can get into all sorts of theoretical arguments involving Rayleigh's limit, Airy disks and Nyquist's Sampling Theorem, but what it boils down to is that to extract all the information in a diffraction limited hydrogen alpha image, the focal ratio needs to be about 4 times the pixel pitch measured in microns. This works out at f/22.4 for the DMK21 camera (5.6 micron pixel pitch); empirically the optimum focal ratio in good seeing is in the f/20 - f/24 range; significantly less than that and the sensor isn't resolving the detail in the image, significantly more than than gives a fainter image i.e. longer exposure time or more gain needed = more image noise but without resolution of any more detail due to increasing diffraction softening as the focal ratio is increased further.

Of course the image made by a short focus objective can be multiplied in size by negative (barlow) or positive (transfer lens) projection to achieve the desired effective focal ratio at the sensor. The actual focal ratio of the objective is (theoretically) irrelevant.


User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20440 times
Been thanked: 10246 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by marktownley »

However, for imaging, the sensor needs to resolve the detail: how big the focal ratio needs to be depends on the pixel pitch of the sensor. We can get into all sorts of theoretical arguments involving Rayleigh's limit, Airy disks and Nyquist's Sampling Theorem, but what it boils down to is that to extract all the information in a diffraction limited hydrogen alpha image, the focal ratio needs to be about 4 times the pixel pitch measured in microns. This works out at f/22.4 for the DMK21 camera (5.6 micron pixel pitch); empirically the optimum focal ratio in good seeing is in the f/20 - f/24 range; significantly less than that and the sensor isn't resolving the detail in the image, significantly more than than gives a fainter image i.e. longer exposure time or more gain needed = more image noise but without resolution of any more detail due to increasing diffraction softening as the focal ratio is increased further.

Completely agree with this Brian ;)


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
colinsk

Re: Short Focal Length Telescope with External Etalon

Post by colinsk »

Brian and Mark, Thank you for pointing this out. I have done very little imaging being a visual observer. I will log this in the back of my noggin. Hopefully it will resurface one day when I need it. :oops:


kornfeld1

Re: Short Focal Length Telescope with External Etalon

Post by kornfeld1 »

Are there any issues with using external etalons on Schmidt-Cassegrain telescopes? I feel like it must be fine to do this because I found adapters like this one:

http://www.astronomics.com/main/product ... t_id=APM10

It seems like this would be a way to maintain the long focal length, but get a telescope in a more managable size...but I wanted to see if there were any issues I should be aware of.


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by Merlin66 »

There should be no issues, if the front etalon is firmly mounted square to the axis.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
brianb11213

Re: Short Focal Length Telescope with External Etalon

Post by brianb11213 »

There should be no issues, if the front etalon is firmly mounted square to the axis.
Well I think you'd want the off axis plate manufactured so that there was no shadowing of the etalon by either secondary mirror or the edge of the primary. Since SCTs tend to have about 33% central obstruction that means you'd need a 30 cm or greater aperture SCT to service a 100mm etalon ... working at f/30+ which would limit the control you had over the image scale. And the thing would be at least as heavy / clumsy as a 100mm short focus refractor.

SCT with full aperture ERF & a back end etalon (Daystar/Solar Spectrum)? That might be interesting .... no chance of full disk images without mosaicing dozens of frames but should be very good for high resolution closeups ....


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Short Focal Length Telescope with External Etalon

Post by Merlin66 »

Fair comment, Brian.
You could do the same with a 100mm ERF and a PST "mod" - it appears it IS possible to reach the majic -200mm with an SCT.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Post Reply