CaK disc comparison between SS and DS

[krakatoa1883]

Having definitely missed the CaK PST when it was available and therefore the possibility to assemble a CaK unit by myself, I have nothing left to increase image contrast but to stack two Lunt B1200 units. The difference between SS and DS is already apparent during the capture (through 90 mm refractor stopped to 50 mm f/10) without any processing applied

comparison.jpg (63.89 KiB) Viewed 601 times

and is more clear after (the same) processing

comp processed.jpg (302.91 KiB) Viewed 601 times

although is less than I expected but worth the purchase of a second module. For those that may be interested stacking is made by removing the blocking filter of one module (I suggest to check which one is better between the two as quality may vary) converting the final male T thread to a female one by means of a converting ring that can be attached to the second module after the eyepiece holder has been removed

stack.jpg (106.63 KiB) Viewed 601 times

a very simple and fully reversible mod. I tried to remove from one module only the filter assembly, without the full barrel, in order to shorten the system but I have been unsuccessful and eventually gave up.

[EGRAY_OBSERVATORY]

Thank you Raf - for an interesting and informative comparison.

Cheers
Terry

[Stardust5858]

Thanks Raf, that's a very interesting comparison. Thanks for sharing your findings.

[Montana]

Very nice indeed I don't think I could afford to buy two though

Alexandra

[torsinadoc]

Thanks for the information. I am considering double stacking my lunt 1800.

[Bob Yoesle]

Very good results with the Lunt DS! Some folks are skeptical about DS improvement in CaK, but for me it was obvious.

Triple stacking CaK will provide a more subtle but perceptible improvement, but just as with H alpha pushes exposure time up. However, for my DIY system TS'd CaK exposures were no more than double stacking H alpha:

viewtopic.php?t=24617&start=75#p225539

[krakatoa1883]

Thanks for comments

The idea of stacking two modules came in fact from Bob (thanks, Bob!) in the thread he linked and at least in another one. Stacking requires more gain but not so much to make the image noisy. Better to have a very transparent sky however as one can see from the above, imaging of the full disc with only a 50 mm aperture is already possibile and with the Sun at less than 30° above my horizon when I took those images. I have still to try in hires mode by using of my 90 mm scope which has a dedicated focal extender that corrects for SA in the near UV. I am planning to replace the original Lunt BF with the blue glass from the blocking filter of Quark Calcium + a KG3 glass, I already noted that this assembly works fine with the Lunt filter and passes more light than the series BF.

[marktownley]

Excellent Raf! Very clearly demonstrates the benefits of DS CaK.

[JochenM]

Excellent results, Raf.

[Carbon60]

Hi Raf,

Just to be clear on this, you’ve double stacked the yellow filters in the Lunt modules, correct? The purple/blue filters (two per module) are in the long barrel (on the left of your image) and as you have only one barrel in your setup, the second pair of transmission filters from the second module have been set aside (not used). Is this also correct?

Thanks.

Stu.

[christian viladrich]

Excellent. Thanks for sharing !

In addition to the image of yours and Bob's images, here is another comparison.

First one with a 0.37 A FWHM Ca K filter :

http://astrosurf.com/viladrich/astro/so ... ASI290.jpg

Then a 0.24 nm FWHM :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
The layer of the chromosphere is a bit higher, so the small pore is less visible.The frontier of the bright plages become a bit fuzzier (because magnetic tubes are expanding with height).

Double stack 0.37 A + 0.24 A filters :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
Here we get higher in the chromosphere. The pore is virtually hiden by the canopy. Vertical fibrils become visible in the bright plages.

[krakatoa1883]

Hi Stu

Both modules have the same long 2" barrel containing (I presume) the blocking assembly as most of the light is blocked here, I only removed one of them. The rear barrels with the Lunt logo including the "yellow" filter remain untouched.

[pedro]

Excellent comparison Raf, thanks for sharing

[krakatoa1883]

Thanks to everyone for comments.

Interesting comparison, Christian, the difference between 0.37 and 0.24 is already striking but the DS still add something more.

[marktownley]

Very good thread everyone! I'll leave a while longer here for further discussion and then move to the solar reference library so it doesn't get lost.

[MAURITS]

Excellent comparation Raf.

[marktownley]

The other thing to throw in the mix you may want to consider in your trials Raf. I found when I did this with CaK PST filters that there was a preferred order which gave a more contrasty view, so, try swapping the 2 filters round and seeing if it makes any difference.

[krakatoa1883]

Thanks for your suggestion, Mark. I didn't consider swapping the filters so this will be my next try. The good thing of this mod is that Lunt modules (I mean the filter assembly) are T-threaded at both ends so it's easy to experiment with different combinations, one needs just a couple of inverting rings.

As seen on this image, the small filter becomes the "aperture" and the image space and object space become parallel through it..

I am not sure to have understood the mod you suggest, it seems you mean the following: sun--> scope --> ERF --> barlow --> filter --> DCX --> camera, however a Barlow is a diverging lens while in the pictures I see the aperture placed between two converging lenses. Do you mean is the second filter that shall be placed between the DCX's ?

[Bob Yoesle]

The narrowband filter itself, should be placed between these 2 optics. It is a quick an easy double telecentric and This made the spicule layer and prominences explode into view... I cannot explain how it works, but it does.

Hi Apollo,

These optical systems are no different than exists with H alpha (or any other) filter or etalon.

In the above diagrams for the telecentric lens arrangements, if the first lens (positive or negative) is placed with its focus coincident with that of the objective you will have the exiting rays parallel, and placing the filter after this lens results in that you are using a collimator lens/filter system, not a telecentric system. The second convex lens merely becomes a refocusing lens - which may or may not be telecentric. Having the filter in a collimated system likely reduces the FWHM of the filter compared to the usual non-collimated objective-filter system.

machinevisionsystems4.png (70.14 KiB) Viewed 396 times

If you place the filter after the second convex lens, and the axial rays exiting it are also parallel, you then have a telecentric lens system.

Having the filter receive parallel axial rays in the collimator configuration will, just as with H alpha filters in a collimated system, result in field angle magnification, and therefore might produce a Jacquinot spot depending on the FWHM of the filter. In this case it might be unnoticeable due to the wide FWHM and the resulting larger Jaquinot spot size.

If the optics produce the parallel axial output from the second positive (refocusing) lens, and it is sufficient long convergence, then just as with an H alpha mica filter system, results in a reduced FWHM across the filter. This might be the preferable configuration. But just as with H alpha filters, may result in excessive EFL.

If the axial rays exiting the second positive lens are not parallel, then it is not a telecentric lens/filter system, and it is effectively a Barlow lens system, and will result in a widened FWHM and possibly a Jacquinot spot as well.

appearance01.jpg (227.57 KiB) Viewed 396 times

The "aperture" in these diagrams simply indicates the position where no vignetting occurs.

[krakatoa1883]

Very useful clarification, thanks. I have a Edmund -100 mm PCV that I can place inside the focal point to generate a collimated beam. If I have understood well both filters shall be placed in this bundle as the etalon in a H-alpha system. Which focal length should I use for the refocusing lens ?

[Bob Yoesle]

That depends on what the objective focal length is. Here's an example for what you are currently using:

If your objective has a 500 mm FL (i.e. 50 mm f10), and your collimator has a (+/-) 100 mm FL, your field angle magnification is 500 mm / 100 mm = 5 x.

So now a 100 mm FL refocusing lens will result in having 5 x its normal FL. Knowing this, you can get an identical original EFL by using a refocusing lens with a 100 mm FL. If you want and EFL of 750 mm, you would solve X x 5 = 750. X = 750 / 5 = 150; so a refocusing lens with a 150 mm FL would yield an EFL of 750 mm.

The issue that will now come up is the B1200 CaK filter diameter versus the placement of the collimator lens in the objective's converging beam and possible vignetting (as well as a smaller Jacquinot spot). Just a with H alpha etalons, it might be better to go with a telecentric lens system and use a focal reducer...

[Bob Yoesle]

Hi Apollo,

Your lens system is neither a telecentric nor a collimator. But it has the same basic layout of of one which uses a negative lens or lens group for producing collimated rays, followed by a positive lens or lens group, which produces telecentric ray bundles.

You are using telecentric lens diagrams intended for industrial process to make incorrect extrapolations about how this applies to your Barlow lens with a following positive biconvex lens for an object lying at infinity.

All you really are doing with the 2 x Barlow is presenting you filter(s) with a longer f ratio, or closer to parallel light cones. They are most likely not resulting in collimated bundles of light, as this would again require the coincident foci of your negative Barlow lens and the objective. Nor are they telecentric. If the negative collimator lens of the typical Lunt or Coronado internal etalon telescope lies at a different distance other than its FL from the objectives focal plane, it to ceases to be a collimator lens and just becomes an negative amplification lens. Therefore this in not the same as the diagrams you posted.

Instead, you're presenting your filter(s) with a 2 x longer f ratio (FL 1200 mm > EFL 2400 mm, which likely results in a narrower FWHM aka bandpass - and of course there would be no change in wavelength) via the Barlow. Even if the positive biconvex lens had a telecentric output (highly unlikely), all you are using it for is a short focus focal reduction lens. And that is why your EFL is shorter than either the 1200 mm of the objective, the EFL or 2400 mm of the 2 x Barlow. Your positive lens is acting as a telecompressor - not a telecentric lens.

It's that simple.

For a more in-depth look at how a true collimator/refocuser is quite the same as the traditional astronomical telecentric lens system, see Peter Zetner's excellent work here:

TelecentricLens2cd crp.jpg (280.1 KiB) Viewed 492 times

https://pbase.com/p_zetner/opticsdesign1

[MapleRidge]

Hi Raf...

A very interesting post with the added CaK blocker

I need to get the club CaK module back and try to DS this way myself.

Brian

[Bob Yoesle]

... the collimated bundle, forms a parallel image at the center of the aperture. The terminology is called "tandem operation" in the optics world. and industrial vision schematics is no different from telescope vision schematics; albeit one is macro and one is telephoto. Lens operation becomes very different when multiple groups are incorporated outside of he standard pcx singlet + pcv combo. We are dealing with a focal point in this situation.

An achromat objective + the barlow creates a new "positive lens group" with a new focal point (the focal point being the image of the sun cast onto the aperture), the barlow also alters the focal point operation of the 100mm element ; from what i see with my eye it has reduced down to just 25mm (the size of my extension).

The design on the apochromatic triplet barlow brings all points of light from the objective to the same focal point at the center of the aperture in the center of the barlow, and when the DCX lens is focused on the center of this aperture point of focus it produces "tandem system operation", . Emphasis added.

Sorry Apollo, but since you have chosen to put this up on a public forum I feel obligated to let folks know your posts regarding macro telecentric lens systems applied to what you are doing with a Barlow lens (regardless of the number of elements) and a single element telecompressor lens are most surely inaccurate: While the triple element Barlow might coincidentally be reducing aberration at 394 mn, there is no way to know without a real-word optical element ray-trace analysis. Additionally, there is no such thing as a "parallel image;" "telescope vision" optics design results are different from "industrial vision schematics;" there is no way to tell that your triple element Barlow and positive singlet "produces 'tandem [telecentric] system operation';" nor would it be applicable to what you are doing for the placement of the CaK filter inside the Barlow lens housing where you appear to assume you are getting a collimated output, followed by the singlet telecompressor lens - which therefore is totally irrelevant to the performance of the filter, and is also likely not a telecentric output.

Without knowing the actual lens prescriptions and their placement in the optical system you can not make ANY statements regarding whether or not the Barlow-telecompressor system you are describing creates ANY collimated rays or telecentric rays at any point in the optical system.

Again, if your filter is performing better, it is most likely due to operating at a twice as long focal ratio, and perhaps due to additional aberration correction at 394 nm by the multi element Barlow. The positive singlet is doing exactly what one would expect - reducing the EFL to what you are observing. Neither the front Barlow lens elements (with objective?), nor the rear positive telecompressor are acting as telecentrics, and therefore it is not a "tandem [telecentric] system operation." And it is highly improbable that the Barlow is producing a collimated output to the refocusing telecompressor lens.

All you seem to be attempting to do is create a collimator for your CaK filter by using a multi-element Barlow lens instead of a simple single concave lens, and calling the combination of the objective with this collimator lens a "telecentric" - with the Sun lying at infinity as the object. The Barlow will not act as a collimator unless it is fixed with respect to the objective focal plane at a specific distance which outputs truly collimated axial rays.

I know little about lens theory, and maybe someone with actual prescription knowledge can figure it out;

This is an accurate statement. While we all appreciate your endeavors to seek "new" solutions to CaK and H alpha filter implementations, given the self-admitted lack of knowledge about optical theory, unless one can produce a valid ray-trace that shows what is actually happening with your specific optics, one should not be making any specific optical system claims or conclusions. This is the exact type of unfounded speculation and misinformation we should be striving to discourage, not encourage.

"The wise know too well their weakness to assume infallibility; and he who knows most knows best how little he knows." Thomas Jefferson.

[marktownley]

Lets get back to the OP thread please.