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Questions about wedge configuration

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#1 PilotAstronomy

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Posted 12 January 2020 - 11:04 AM

I’m trying to finish up my solar setup. 
 

Currently have an ASI174MM, LS80, LE80, and Lunt CaK module. I also have the Lunt solar wedge.

 

I’ve been gaining a lot of experience on Ha on the mostly blank disc and it’s coming along nicely. Got my toes dipped in CaK now and it’s a different challenge but also coming along.

 

My next learning point is about White Light. When I image with the wedge I find I have to drop one of my nighttime 2” color filters (been using the green one) into the wedge to cut the light enough to image at the fastest possible speed on the camera and w it in there I’m barely meeting focal length. I know I’m doing this wrong and I’d rather leave my nighttime filters alone and in my night setup.

 

Two questions:

 

1) what filters should I be adding to the white light wedge for solar imaging? Please be specific as there are a lot of options out there. I’ve seen the debates on continuum/ir/etc. not sure where the thinking lands though.

 

2) what configuration should the imaging train be in w the filters/etc from answering question 1? Currently, I think I’m doing it wrong w/:

sun->LE80->lunt wedge->2” green filter->174mm

I’m pretty sure better to put the filter before the wedge (screwed in it?)

 

Hope these questions are straight forward enough. Thanks in advance!

-Dennis 

 

 



#2 sg6

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Posted 12 January 2020 - 11:42 AM

The Green of a continium is a 540nm filter, unsure of the band width however but being visual I guess fairly wide say 10nm to 20nm maybe??.

 

Not sure what green filter you are contemplating but I guess it is a sort of semi narrow one that is needed. Actually no idea how/why it (green) works.

 

What I would say you lose is the solar surface granularity that Ha and CaK allow.

 

Would be interesting to know if other "narrow" filters do anything as well as the Continiuum. In effect what does a wide OIII or Hb produce as a view. Sonds like worth a laugh.

 

Looks like the Baader Continiiuum is 10nm wide and actually is more like 537-538nm not 540nm.



#3 BYoesle

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Posted 12 January 2020 - 11:46 AM

Hi Dennis,

 

Absolutely do not put any filters ahead of the wedge! The order should be the wedge prism, ND3, and then subsequent filtration. If you're using anc achromatic refractor, the Baader Continuum filter will provide optimum contrast. You can also drop the image brightness with additional ND filtering, or a single polarizing filter - which will allow fine-tuning the brightness. To avoid the extra length of multiple filters you can stack filters in a single filter holder.

 

If you still can't reach focus with your camera, you might try a shorter eyepiece adapter, get a different focuser, or have a competent machinist remove about an inch of your OTA.



#4 PilotAstronomy

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Posted 12 January 2020 - 11:55 AM

Oh I CAN reach focus. It’s just very close and I think stacking more than one filter might make it impossible.

I’m glad my instincts to put filters after the wedge were right.

 

Any other recommendations than just a continuum filter? Thinking about a 1.25” one that could thread into the camera and save some back focus.



#5 vincentv

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Posted 12 January 2020 - 12:27 PM

The color filters are just optimizations to avoid color related aberrations. They are introduced by the atmosphere, telescope optics and simply physics.

 

-Shorter wavelengths (towards blue) allow more resolution from the same aperture. If you let through a large swathe of light the higher resolution image is superimposed with the lower resolution of the longer wavelengths. Be sure that whatever filter you use does not have leaks at other wavelengths. NIR is the usual suspect for leaks but UV can also be present.

 

-Longer wavelengths (towards red) offer less resolution but are also less affected by atmospheric turbulence so under some conditions it can give a sharper image.

 

Since the wedge is providing the safety aspect you can mix and match any filters without risk *after* the wedge.

 

 

Actually no idea how/why it (green) works

Most scope optics are optimized for green. Get rid of the other colors and the result is a sharper image with fewer aberrations. Of course the particular scope design determines how big the improvement is (acrho vs apo, f ratio, triplet vs doublet, etc). The reason green is used has to do with the human eye.


Edited by vincentv, 12 January 2020 - 12:29 PM.


#6 BYoesle

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Posted 12 January 2020 - 12:29 PM

With the filter systems you've discussed, CaK 393 nm, Continuum 540 nm, and H alpha 656 nm, you have your bases covered. Some folks also like the G band - 430 nm, and this gives some improved resolution over 540 nm if your seeing will support it. A suitable G band filter will set you back a couple hundred bucks:

 

430-10 Edmund.jpg

 

 

Most scope optics are optimized for green. Get rid of the other colors and the result is a sharper image with fewer aberrations. Of course the particular scope design determines how big the improvement is (acrho vs apo, f ratio, triplet vs doublet, etc). The reason green is used has to do with the human eye.

 

While both ACHRO and APO telescopes are best corrected for the e line (green) due to the prevalence of nighttime spectral sensitivity of the eye, for solar imaging an achromatic refractor offers better theoretical performance due to reduced spherochromatic aberration:

 

Refractors.jpg

 

This is especially true for fast APO designs at shorter wavelengths like CaK. See here for additional information. An SCT is also generally considered to be a poor performer at shorter wavelengths. A longer f ratio ED or Achro is generally better for CaK, if not going to a reflector design like Christian's 300 mm solar Newtonian reflector.


Edited by BYoesle, 12 January 2020 - 03:14 PM.


#7 MalVeauX

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Posted 12 January 2020 - 04:43 PM

Any other recommendations than just a continuum filter? Thinking about a 1.25” one that could thread into the camera and save some back focus.

With an 80mm aperture, the continuum or just any decent green imaging filter (Baader Green is 500nm) will do good for high contrast and slightly higher angular resolution than red wavelengths, if you want to get as much resolution as you can from the 80mm aperture. You can use any wavelength, but angular resolution increases as wavelength decreases, so 430nm is higher resolution than 540nm, and requires better seeing, and 394nm is higher resolution than 430nm, and requires even better seeing, but you could max out the 80mm aperture if seeing supports it. Ultimately though, for cost, you're likely best staying the inexpensive Continuum 540nm, Baader Green 500nm, or Baader 610nm Long Pass depending on seeing conditions.

 

Very best,




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