17-DEC-2007
Initial results - severe astigmatism
This was how images looked in my Ca K PST originally. It shows bad astigmatism. Obviously the word 'focus' is used loosely here ! I didn't know if the astigmatism was caused by the 40mm objective, the pentaprism (focus mechanism in the PSTs), or the blocking filter (BF), but my first step was to check and see if the pentaprism was out of alignment. It was : it was 8° out
19-DEC-2007
Realigning the Ca K PST pentaprism
To see if I could resolve the astigmatism issue I opened the PST, found the pentaprism out of orthogonality and 'squared it up'. However unfortunately this did not result in any improvement. The next step would be to remove the pentaprism. This was done and a hole bored in the back of the PST 'Black Box' and a helical 1¼" focuser (from Hutech) added plus 1¼" extension piece. At this stage I continued to use the original Ca K blocking filter (BF).
26-APR-2012
Straight through design, - no pentaprism.
Here's the modified Ca K PST stripped of it's pentaprism (which caused the astigmatism whether it was 'square' or not). In order to change the focal plane the gold tube was cut and rethreaded. On the back of the 'black box' is a 1¼" helical focuser from Hutech. The CaK filter is inside the black box and behind the focuser are two adapters and then the original 5mm Ca K BF.
26-APR-2012
Ca K module using the Ca K's original PST BF
The long term plan was to always use the Ca K filter as a module that could be swapped around scopes, and I knew that in the long run I'd be wanting to use it with scopes of greater aperture than 40mm, more likely around 100mm. So I reused an existing aluminium adapter (the Ha & Ca K PSTs have the same thread) Part 'C' in the photo is the CaK filter, then there's an aly adapter and part 'E' is the original blocking filter from the Ca K PST. Unfortunately this BF only has a diameter of 5mm which is as large as it needs to be for the original 400mm FL PST, so it works, but vignettes the image at focal lengths longer than 545mm.
14-MAY-2012
Ca K module waiting for the ERF
I was now waiting for an energy rejection filter (ERF) to reduce the heat load. This was especially importatnt as I planned to use the module in scopes of around 100mm aperture, so the heat load was going to be much greater than when it was in the 40mm f10 original configuration.
Baader UV & IR rejection filter as an ERF ?
I already had a Baader UV & IR rejection filter. Unfortunately as you can see from the diagram it cuts off the light at around 405nm, so that's not going to be very good for passing light at the Ca K wavelength of 393nm. As usual Mr Baader was spot on. I checked it and there was no light passed at 393nm. So I searched for another UV rejection filter that wasn't quite so aggressive as the Baader filter.
IDAS 'UIBAR' filter as an ERF ?
As you can see from the spectral response curve the IDAS UIBAR filter didn't cut off light till the wavelength got below 375nm. This was just what I wanted. I ordered one from Hutech on 3 Jun. It was OOS but that was OK I was prepared to wait. The 'UIBAR' filter was shipped on 13 Jun. It was now mid winter and I had to wait a few days to get suitable conditions. Eventually the sun and some time off coincided and I tested out the filter....nothing...no signal at all on the camera, no matter what shutter time I set. Major disapointment. I mulled this over and a few days later sent an e-mail to Ted at Hutech explaining the situation. I received a reply the next day :
"I'm so sorry, it looks like type III was sent to you.We will arrange type IV for you ASAP."
Type III, type IV ???? none of this differentiation was on the website or price list. Oh well all is not lost then. I'll send back my type III and await type IV. These notations aren't marked on the filters by the way, they're marked only 'IDAS UIBAR'
The Type IV was shipped on 10 Jul and I received it on 19 Jul
08-JAN-2013
Showing the Ca K filter module with the IDAS type IV UIBAR filter as the energy rejection filter
The image shows the Ca K filter module with the IDAS type IV UIBAR filter as the energy rejection filter.
15-JUN-2012
20120613 22 40 hrs UT CaK 63 8402HS0000.jpg
This image was taken using the Ca K filter module being used with 63mm 840mm FL objective. The filter module still used the original 5mm blocking filter (BF) and so it resulted in severe vignetting as the solar disk would have been 7.7mm in diameter (840/109). You can see the results with the darkened corners of the image. I tried to minimise this with disk placement, and in theory flat fielding could have solved it, though it was pretty severe vignetting for that. I wanted a larger BF and first of all tried the Omega 393nm filter as a BF.
22-JAN-2013
Test image with the IDAS type IV UIBAR as the ERF, and the original PST BF
The type IV IDAS UIBAR filter worked as an ERF, presumably behaving as the specifications of the spectral response curve showed on the Hutech website. At this stage I was still using the original 5mm diameter blocking filter from the original PST and so the vignetting at the corners was severe. Time to look for a replacement BF.
Omega Optical 394nm filter as a blocking filter
Looking at the spectral response curve this Omega Optical filter centred at 394nm should work well as a blocking filter and replacement for the 5mm PST BF. With a diameter of 25mm it wouldn't present any vignetting problems and it was a reasonable price at US$34.50 + post. I mounted the filter in an empty 1¼" filter cell and tried it out..............
Image taken with the PST blocking filter
First up the PST BF, again it shows vignetting
