130mm Newtonian Mod + Lunt CaK Mod -- first results

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thesmiths
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130mm Newtonian Mod + Lunt CaK Mod -- first results

Post by thesmiths »

In our previous post "SkyWatcher Heritage 130p Solar Dob Mod" we showed our initial 540nm imaging with the Solar Newtonian. I think the results were promising. For "white light", the Solar Newtonian (as already shown by others) is an inexpensive and safe way to achieve large aperture solar viewing/imaging. It can obviously scale up to 8", 12" or more (although the occasions when you can actually get good seeing with more than 8" during the day is probably restricted).

But our real motivation for attempting this mod was driven by two earlier SolarChat comments:

"To me one of the easiest ways to see the results of spherical aberration is with our CaK images, quite simply the longer the native focal ratio the more contrast and sharper detail would seem to be visible. This is because scopes are often not designed to operate at 393nm, however f10 and upwards seems to give the best results." (Mark Townley)

"I preferred to make the SHG with mirrors to be free from the problems of chromatic aberrations and variations related to the wavelength." (J-J Poupeau, as quoted by Ken Harrison)

So the goal was to make a fast and moderately large aperture telescope to use for CaK imaging and also (at a later stage) in a spectrohelioscope (SHG). In theory, the Solar Newtonian should be unaffected by wavelength over an extremely broad spectrum. Here we present our first results of imaging near 393.4 nm (Calcium K line).

The first step was to try imaging with the Baader K-line filter. For this we used a DMK51 with the Baader K filter plus an Opticstar IR blocking filter. I am not 100% sure if the Baader K filter lets through much IR light or whether the Opticstar IR filter cuts off some 393nm light. But there was definitely a decrease in brightness with the addition of the IR filter. The camera was operating near its fastest shutter speed (1/5000) and minimum gain (142) so a little bit of extra attenuation was welcome (I didn't have any very low ND filters).
Baader K
Baader K
The Baader K image seemed good to me, again comparable in resolution to my 106mm F6.5 APO. But I did not do a side-by-side comparison and variations in seeing conditions are bound to dominate.

Next I set to work on the Lunt CaK mod. I carefully removed the small square yellow filter from the 1.25" eyepiece end of my B1200 Lunt CaK in-line module (camera lens spanner wrench required!). I then installed this into a Celestron 1.25" to T-Mount adapter I happened to have around using black foam board. The Celestron adapter is made to do eyepiece projection so other 1.25" accessories fit into it. I cut two pieces of foam board (using a special compass knife) 31.5 mm in diameter and with a 12.5 mm hole in the middle. The yellow Lunt filter was sandwiched between the two pieces of foam board and a blank 1.25" eyepiece cylinder was put on top with some compression and held by the retaining screw on the side of Celestron adapter.
Celestron adapter
Celestron adapter
3246.jpg (48.08 KiB) Viewed 2190 times
On the 1.25" front end I screwed on the Baader K filter and on the T-mount back end I screwed on a ZWO 120MM camera (it has a native T-mount). This fit nicely into the helical focuser of the Newtonian. It did require more back focus than a DMK camera with only 1.25" filters so I had to shorten the tube a little more than before (using the incredibly convenient Flextube feature). Small note: the ZWO camera, having its USB cable on the side (versus the back with the DMK) tended to rotate under the weight of the cable in the helical focuser unless some care was taken to relieve this weight.
Lunt K
Lunt K
Sun_07_59_57.jpg (233.3 KiB) Viewed 2190 times
Here is the first image taken with the CaK mod. The shutter speed was 20 ms, gain was minimum (0), image size 1280x960, 30fps. 300 frames with FireCapture 2.3, 25% selected by AS2.3, some minor wavelets in Regi6, a small amount of basic processing in Photoshop Elements 11. With higher gain (35), the shutter speed could be brought down to 5 ms, or even 2 ms (gain of 50, which is midrange). These are not especially fast shutter speeds, especially considering it is only at F5. This leads me to think that for CaK, using a normal fully silvered secondary mirror is probably better (which is then the equivalent of using a Herschel wedge -- Mark Townley was right again!).

If you look closely at the CaK image, you can just barely see the dreaded Newton rings. They are not so visible at F5, especially since this image has been reduced about 50% in size for presentation. But at F10, the Newton rings are very obvious and annoying. It seems to have to do with the particular combination of the ZWO camera and the filters in front of it. They did not appear when I used a DMK41 camera with the Lunt CaK/Baader K filters or when the ZWO camera was used with a combination of Baader Continuum/ND/IR filters. I think I know how to get rid of the rings but it will have to wait for another day.

One thing I did notice is that the ZWO 120MM appears to be considerably more sensitive than the DMK41 at 393 nm. The specifications of the chips do suggest a much better sensitivity in the blue for the ZWO camera. My preliminary comparison was that it was a massive 4 times more sensitive! The images, particularly at higher magnifications, were much better with the ZWO than the DMK, presumably because of the higher shutter speeds possible. I need to revisit this though to make sure I didn't make a mistake.

So my conclusion is that the Lunt CaK / Baader K-Line filter works well with a 130mm F5 Solar Newtonian. Is it better than a normal Lunt CaK module on a normal 4 inch refractor? That I can't say yet. It should have less optical aberration but it might also have more sensitivity to atmospheric turbulence (since it has an obstructed aperture). I really have not been able to adequately image yet at high magnification. So first I need to sort out the Newton rings and also to reduce the attenuation (with a mirrored diagonal). But this approach continues to look promising.


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Re: 130mm Newtonian Mod + Lunt CaK Mod -- first results

Post by Merlin66 »

Well done!
Excellently presented and detailed report.
The Newton Rings are also wavelength dependent, getting worse with monochromatic light.
Interesting to see the capabilities of the 120MM.
Keep up the good work...
Look forward to it's transition to a SHG. (You just to find an 8" mirror for the spectrograph....)


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Re: 130mm Newtonian Mod + Lunt CaK Mod -- first results

Post by swisswalter »

Hi Douglas

wonderful first light with the CaK wave range


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Re: 130mm Newtonian Mod + Lunt CaK Mod -- first results

Post by marktownley »

Loving these experiments Douglas! A very fine first light, i'm looking forward to more!


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Re: 130mm Newtonian Mod + Lunt CaK Mod -- first results

Post by Derek Klepp »

Thanks for the reveiw.


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