130mm Solar Newtonian with Lunt CaK Mod -- 2nd results

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thesmiths
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130mm Solar Newtonian with Lunt CaK Mod -- 2nd results

Post by thesmiths »

In our previous posts we showed our initial work stripping the aluminium coatings off the primary and secondary mirrors of the SkyWatcher Heritage 130p. We also did a Lunt CaK mod by removing the magic yellow filter and combining it with a Baader K-line filter. Here we make some incremental improvements.

First of all, we determined previously that with the secondary mirror stripped of aluminium, there was not enough light for fast exposure imaging with a CaK filter (it was fine for white light though). So we ordered a new 35mm diagonal mirror from Orion Optics UK. As we mentioned before, the existing mirror was anyway a bit oversized at 40mm. Here is how the new diagonal looks (attached again with "Picture Hanging Strips"):
secondary
secondary
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We also discovered a nice trick to improve the back focus. We found that we can screw in the helical focuser from the inside (rather than from the outside). This required installing set screws (or grub screws as they're called in the UK) to hold the "eyepiece" as the normal screws don't fit on the inside. The filter stack screws directly into the T-mount thread of the ZWO 120MM camera. The filter stack is a Baader K-line, followed by the Lunt Ca-K filter, followed by a polarisation filter.

The polarisation filter (which also attenuates by 15%) is to try to control the Newton rings problem. I also removed the protective glass window on the ZWO camera. These efforts greatly improved the Newton rings problem but did not entirely eliminate it. Sam at ZWO suggests that the Aptina CMOS chips seem very prone to Newton rings when used with highly monochromatic light. I'm thinking that perhaps going back to the DMK cameras may be a better way forward for this application.

Another modification was to try to reduce light scattering from the primary mirror. The rear of the primary is frosted rather than polished. So 95% of the sunlight passes through the polished surface of the primary and hits the frosting. A significant portion of this light could be scattered back from the frosting, reflect off the secondary and into the "eyepiece" where it could reduce contrast. Here is an image of the frosting on the back of the primary mirror:
frosting
frosting
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To eliminate this scattering, I rubbed some microscope immersion oil on the back of the primary. This had a dramatic effect! The mirror immediately became totally transparent. The idea here is that the immersion oil matches the index of refraction of the glass and fills in all the tiny cavities in the unpolished glass surface. The oil is very viscous and forms a quite smooth layer because of surface tension. There is now almost no scattering, just the normal 5% reflection off the back side of the mirror. But since this is totally unfocused, its contribution at the image plane is small (I calculate no more than 0.2%). Here is how the telescope now looks from the back:
immersion oil
immersion oil
3283.jpg (88.88 KiB) Viewed 1741 times
So now around 90% of the light exits cleanly out the back. This actually makes it very easy to collimate the telescope (normally you can't see the orientation of the secondary from the back of the primary!). It also means that the secondary acts as an excellent Sol Finder when looking at the shadow of the telescope:
shadow
shadow
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We took some CaK images at F5 (650mm FL) and made a quick mosaic:
cak mosaic
cak mosaic
The seeing was not at all good so the result is not really demonstrating how well the instrument can perform. In theory, I believe that the telescope should not suffer from the type of aberrations that most refractors have near 393nm. The camera settings were: shutter speed 1ms, 30fps, gain 38. 25% of 300 frames stacked in AS2.3 and then wavelets with Regi6. Mosaic done with 4 panes with Panorama merge in Photoshop Elements.

It's not a brilliant result but also not that bad either. Future improvements will need to focus on the camera side of things and, of course, better seeing.
Last edited by thesmiths on Sun Jul 06, 2014 9:24 am, edited 1 time in total.


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swisswalter
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Re: Solar Newtonian Mod with CaK Mod -- 2nd results

Post by swisswalter »

Hi Douglas

it is so interesting to follow your project. Another great step forward


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Re: Solar Newtonian Mod with CaK Mod -- 2nd results

Post by marktownley »

A fantastic result! Looking forward to hearing and seeing more from this!


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Re: 130mm Solar Newtonian with Lunt CaK Mod -- 2nd results

Post by astroshot »

Great read. Thanks!


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