Ca K telescope

Post by **marktownley** » Mon Jan 20, 2014 11:10 pm

[quote][quote]Simpliest Fraunhoffer refractors of smaller apertures and very long F/D, at least F/10 and also they need to be severely UNDERcorrected for green light.

Hi Valery, is there a 'list' anywhere that would tell us the telescopes that were undercorrected for green light?

Hi Mark,

There is no such list of telescopes undercorrected in a green light. But I can tell you, that MOST of chinese refractors are under corrected. So, they have better, than normal refractors, correction in a very deep violet.
As more your refractor is under corrected in green, as better for CaK.
Some years ago I bought such severe under corrected (in green!) objective 120mm F=1000mm. It has very very good correction in a CaK line.

Thanks Valery. I have 4 different fracs 40mm f10, 70mm f6, 100mm f10, 127mm f9 and all 4 perform really quite differently at CaK wavelengths. The f10 scopes are by far the best by some considerable way. The next steps in my ongoing CaK experiment is to mount my CaK filters in a collimated beam - in similar fashion to a internal etalon Ha scope.

sullij1 · Post by **sullij1** » Tue Jan 21, 2014 2:35 am

What Valery is talking about, said a different way is - An Achromats normal design focuses the red and blue light as the chief rays or rays that are most in focus at the imaging plain. Thus, as Cak is dominant in these blue/red = violet wave length, they are better for CaK imaging. The inference the more uncorrected the green the better corrected the blue/red.

Savy Tonto?

The grossness of the under correction will manifest as the ugly violet fringe on achromatic images.

(horrible for astrophotos, Da Bomb for CaK.)

Here is the wiki definition and ray traces. the basic idea is in the first paragraph. Hope that helps.

http://en.wikipedia.org/wiki/Achromatic_lens

Correct Valery?

Post by **solarchat** » Tue Jan 21, 2014 5:16 am

There is no blue or green in Calcium K. It is one, very small wavelength of violet only at 393nm. It is not a combination of blue and green source light. Lenses focus light of specific wavelengths, they do not operate on the RGB combinations that computer displays do...right?

swisswalter · Post by **swisswalter** » Tue Jan 21, 2014 5:19 am

Hi Chaps

don't beat me too hard. I do not see the point here. As long as we look only at a very narrow band, why should we care about other rays way outside of the target band. Don't we cut of green and red when monitoring at the CaK II lines

Post by **marktownley** » Tue Jan 21, 2014 6:53 am

What Valery and myself are going on about here is spherical aberration: this is where the outside (perimiter) of an objective lens brings light to a slightly different focus than the centre of the lens at a given wavelength. This is not chromatic aberration where red, green and blue does not come to the same focal point in a simple achromat; as Stephen says chromatic aberration is irrelevant when imaging / viewing in narrowband. With spherical aberration most telescopes are designed to perform well at red and green end of the spectrum, at the expense of the blue - especially so at 393nm. With regards to CaK, spherical aberration will manifest itself as a slight softening to the image, slightly lacking in contrast, you may well find when imaging the shadows end of the histogram is lifted slightly as a result. Just because a scope is an expensive triplet does not mean it will necessarily be free of spherical aberration at CaK wavelengths, and is why a simple achromat will make a killer CaK scope when stopped down (outside edge of the lens is not being used)...

Valery · Post by **Valery** » Tue Jan 21, 2014 7:07 am

Hi here!

Let me clarify this subject a bit more. I hope that now you will have a clue.

ANY achromatic refractor can be corrected for two wave length in the sense of chromatism correction - in other words only two wave lengths can be put in one focus. All other wave lengths will be focused on lesser of larger distance from this focal point.

However any doublet achromat and any doublet ED or non ED apochromat and any ED or non ED triplet (with thin space between the lenses) can be corrected for Spherical Aberration (SA) only for ONE SINGLE wave length. Normally in the visual refractors designers choosing 555nm or 546nm or 587nm (green to yellow) wave length to be corrected for SA. All other wave lengths will not be perfectly corrected for SA.
Also such a short wave length as 393nm usually even not considered when one designing a visual refractor.
OK, if a given refractor achromat or apochromat is corrected for, say, 555nm, then all shorter wave lengths will be less or more overcorrected and all longer wave lengths will be less or more under corrected.
Overcorrection means that outer zones of objective have longer focus than central zones.
At a 393nm such objectives will be really severely overcorrected - more than for 1 wave length. So, the image will be soft by any means. To fix this overcorrection we need to add undercorrection. If a given objective is corrected for SA in a green light, then to be corrected for SA at 393nm, it need to obtain a severe portion of undercorrection.

If you have a visual refractor with good correction for SA in a green-yellow then we can reliably state that this objective will work poorly at 393nm. ANY properly corrected visual refractor will work poorly at 393nm. All of them will work poorly. How poorly is depends of the objective design. As smaller objective is and as longer it's focal distance (as slower the F/D) as better it will work at 393nm.
So, how to choose the best variant of standard refractors available on the market? Few simplest rules:

1. This must be a small and long focus refractor. Say, 80mm F/11.
2. This refractor should have undercorrection in a green-yellow spectral range.

For bigger and faster objectives their under correction in a green light must be really heavily undercorrected, really unusable for visual observing. However such a telescope will work better than others at 393nm.

Valery · Post by **Valery** » Tue Jan 21, 2014 7:24 am

The next steps in my ongoing CaK experiment is to mount my CaK filters in a collimated beam - in similar fashion to a internal etalon Ha scope.

It is VERY important that the telescope will be designed straight through. Otherwise you can't warranty what will be a SA correction and coma correction at the final focal plane.

Few advices if you will use standard accessories available on the market.

1. The main objective aperture must be as smaller as possible - just enough for your task and have undercorrection in green.
2. The collimator negative lens must be as weak as possible.
3. The refocusing objective must be as slow F/D as possible and have undercorrection in green.

Valery.

Post by **Montana** » Tue Jan 21, 2014 10:11 am

Thanks guys

In English please, is the Celestron 102 OK? (not that we can find one) I can't find any of the specs mentioned above in any brochure of any telescope. I only found one reference on Cloudy Nights which states the Celestron being a Fraunhofer type design

Most of Mark's telescope suggestions just state the telescope is a refractor

Alexandra

AndyM · Post by **AndyM** » Tue Jan 21, 2014 5:48 pm

Hi Alexandra,

this scope will work - a similar one worked with me quiet well. Maybe not perfect, since the guys are right about the optical issues caused by the exotic wave length, but it will work. You will find a nice phenomena when focusing: there will be a "point of best contrast" and another with "best sharpnes", but they will never be together. But, it's not that big deal, because contrast can be done by software

And yes, the Celestron 102/1000 is a Fraunhofer. Fell free to send me an email or PM, if you need a selected one.

swisswalter · Post by **swisswalter** » Tue Jan 21, 2014 7:20 pm

Hi Mark, Hi Valery

thank you very much for the details. I did not take into account the SA. You are right. I can now follow better your arguments.

sullij1 · Post by **sullij1** » Wed Jan 22, 2014 2:14 am

Sorry Stephen I over simplified. Yes you are right as is Walter. Only one wave. I see Mark and Valery's point.

http://en.wikipedia.org/wiki/Spherical_aberration

To me AndyM's comment would be a better solution than stopping down, it seems to me stopping down will rob light that will serve to preserve detail.

sullij1 · Post by **sullij1** » Wed Jan 22, 2014 3:02 am

Sorry Alexandra, I did not mean to complicate the decision for you and hubby.

Post by **marktownley** » Wed Jan 22, 2014 6:55 am

You will find a nice phenomena when focusing: there will be a "point of best contrast" and another with "best sharpnes", but they will never be together. But, it's not that big deal, because contrast can be done by software

What an eloquent way of putting it Andy, I never thought to describe it this way but it is so true!

TheHubby · Post by **TheHubby** » Fri Jan 24, 2014 7:31 pm

I have made a start!!!

It fits the Tec140 and was ordered on Tuesday from Teleskop Express and arrived today. Not bad at all except I paid £1420 which is a lot of money and works out at the astronomical amount at today's rate of $2,286. How come it is made in the US but we get 4 day delivery yet it takes months in the US? It is something to do with ripping off the Europeans I am sure and we fall for it every time....

There is a downside to ordering from Germany....

I've not a clue what I am meant to do with it and what not to do with it but I hope I don't blind myself in the process. My schoolboy translation does not include the word "dammit" but I have been saying that quite a lot in the past 2 weeks trying to find a solution.

All I need now is to nick the Tec140 off Alexandra and give it a go,,,

No chance with that lot this weekend. Never mind my scope and mount are on order and should take 2 weeks. That should be enough time for the weather to go all sunny on us.

Thanks everyone.

The Hubby

Attached files

brianb11213 · Post by **brianb11213** » Fri Jan 24, 2014 8:09 pm

There is a downside to ordering from Germany....

I've not a clue what I am meant to do with it and what not to do with it but I hope I don't blind myself in the process.
No chance of blinding yourself. I take it you know that there is little chance of actually seeing anything in CaK light, it's really only useful for imaging.

Stuff the long fat arm up the scope's focuser & stuff a camera with a 1.25" nosepiece fitted into the eyepiece hole. If you want to use a barlow I suggest you place between the diagonal & the camera. The eyepiece well unscrews (T threaded) & can be replaced with a 2" version if required ...

... so far as the German text is concerned, it really doesn't say anything you really need to know. I have this theory about Germans that they can't work anything without extensive instructions ... my Lunt CaK module came (from a UK distributor) with no instructions at all & I haven't found any disadvantage from this.

Keep fingerprints off the optical elements & don't clean the shiny surface at the front end of the module with anything other than a camera lens type brush/blower.

With the sun being so low at the moment (heavy atmospheric absorbtion of shorter wavelengths) it should be fine with the full aperture of the TEC 140. But restricting the aperture to 100mm is strongly reccomended when the sun is shining high in the sky (110 mm works OK for me but the sun's altitude never gets to 60 degrees here even at the summer solstice!)

Post by **marktownley** » Fri Jan 24, 2014 10:06 pm

Mos definitely fun up the Cheshire

You do realise such a purchase has made it cloudy for the next 16 centuries here

Watch out for the cloud breaks

swisswalter · Post by **swisswalter** » Fri Jan 24, 2014 11:54 pm

Hi Hubby

congratulations on the new toy. Have sun fun and post pics as many as you can. The Manual really does not tell you anything you would not know. The only restriction is that is should only be used with openings up to 4" . Your model is for scopes up to 1620 mm FL

Derek Klepp · Post by **Derek Klepp** » Sat Jan 25, 2014 12:04 am

My 1200 came with a fingerprint on it so at least yours is clean.Down the track I can see the TEC mounted with an aperture Iris as Pedro uses on his scopes and the in house debate as to which wavelength should take priorty of the day.Look foward to the first pics.

Pedro · Post by **Pedro** » Sat Jan 25, 2014 12:13 am

Derek is right, you should use your TEC140 @ F/10. This means reducing the aperture to 980 mm

Pedro

sullij1 · Post by **sullij1** » Sat Jan 25, 2014 4:20 am

What mount did you decide on?

swisswalter · Post by **swisswalter** » Sat Jan 25, 2014 4:57 pm

....and the in house debate as to which wavelength should take priorty of the day.Look foward to the first pics.

me too

Post by **Montana** » Sun Jan 26, 2014 11:49 am

Thanks guys, I do keep trying to tell him that even if it wasn't raining we can't test it till mid February anyway.

Brian, some good points there :thanx:

Joe, the iOptron, it will be something different, we didn't want a duplicate EQ.

Alexandra

Post by **marktownley** » Sun Feb 23, 2014 1:40 pm

Any news on the arrival of the scope for this project yet?

TheHubby · Post by **TheHubby** » Sun Feb 23, 2014 5:40 pm

Well you already know we have the Cak module.

The IOptron pro was collected from our Stockport shop yesterday after their call this week (2 weeks late);

: WP_20140223_16_52_46_Pro[1].jpg (1.62 MiB) Viewed 6202 times

but the dosey shop assistant ordered a Celestron 127 rather than the 102 which now has to go back and be swapped, so no, I am not operational yet.

Chomping at the bit to get started

Post by **marktownley** » Sun Feb 23, 2014 7:49 pm

Looks nice! That is why today was gray and rainy then!

You've got the beginnings of a telescope forest starting in that conservatory

grimble_cornet · Post by **grimble_cornet** » Wed Apr 16, 2014 4:50 pm

After drooling over images posted by Mark, Alexandra and others, I decided to buy a Lunt CaK 60 telescope.
I quite like the idea of a dedicated scope which I can use mounted side by side with an Ha or white light scope and I'm pretty happy with my first images.

However, I also want to use the CaK 1200 module with my Equinox 80 and even my Equinox 120 for greater resolution.
This is where the problem starts!
The two Equinox scopes have identical draw tubes and focusers - both have a very small step about 2 inches in. It is very small but just enough to stop the CaK module going past it which leaves about half of the black tube sticking out! Not surprisingly this makes it impossible to reach focus either visually or with a camera. Don't really fancy reaming out the Equinox tube so what else can I try?
My ASI 120 camera has a T-thread and I can screw this directly to the CaK prism if I unscrew the eyepiece holder. This allows me to reach focus and the Equinox 80 does seem to give slightly better images than the Lunt 60 unit. However, I can see no obvious way to use a Barlow or Powermate in this optical train so the resolution I can record is limited.
I assume that the black tube with its silver filters at each end is acting as an energy rejection filter? I wonder if I could remove this and use the remainder of the CaK unit with a Baader Ceramic Wedge or even a Solarfilm filter? Not sure if I can find an adaptor to replace the black filter tube with a plain 2" tube?
Any advice from those members who have travelled this road before would be most welcome.

Mike

swisswalter · Post by **swisswalter** » Wed Apr 16, 2014 6:41 pm

Hi Mike

using a herschel wedge, followed by a K-line filter, followed by the LUNT CaK filter, followed by an UV/IR cut from beloptik works perfect if you have enough inward travel of the focuser

grimble_cornet · Post by **grimble_cornet** » Thu Apr 17, 2014 11:02 am

Thanks Walter but I'm now confused!
I had assumed that the black tube was just reducing the sunlight intensity and that all of the CaK components were in the prism?
Your answer suggests that some of them (K line filter ?) are in the tube and I would have to provide alternative ones if I removed the tube and replaced it with a Herschel wedge?

Post by **marktownley** » Thu Apr 17, 2014 11:36 am

Hi Mike.

I can't offer anything much here i'm afraid; some focusers do have these steps, and they aren't any good with the Lunt CaK. Short of reducing the tube length of your OTAs, getting different focusers for them or cutting off the end section of the focuser tube with the notch that's about it. Great opening images btw!

Mark

grimble_cornet · Post by **grimble_cornet** » Thu Apr 17, 2014 5:10 pm

Thanks Mark - I was afraid that might be the case

I may try to take the Equinox tube apart and see how easy it would be to remove the step or...... following the discussions here, consider a long focal length achromat.

Post by **marktownley** » Thu Apr 17, 2014 5:11 pm

see if you can pick up a tal100R

sullij1 · Post by **sullij1** » Wed Apr 23, 2014 1:31 am

Sorry, Just saw this. Yep you need the 2 filters contained in the black tube. The module will not function without them.

As for the baffle in the focuser, See if you can find a piece of plumbing pipe slightly smaller than the focuser diameter but larger than the baffle with enough length to leave you with 6 inch sticking out of the focuser end. Remove the focuser tube from the focuser. With the pipe inserted in the focuser and against the baffel, strike the pipe with a hammer until you have knocked the baffel out of the focuser tube. The baffle is glued or spot welded to the inside of the focuser and it will not take too much of a blow to knock it free.

I have done this in the past.

swisswalter · Post by **swisswalter** » Wed Apr 23, 2014 4:55 am

Hi Joe

the golden tube from a PST has just about the right diameter ;-)

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