1st October 2020 Increased protective filter standoff.

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After what seemed like an age [12 days] the SVBony M48 threaded extensions finally arrived from eBay [UK.]

It was soon obvious that the tubular rings were oversized [for 2" push fit] and would not fit through my 2" back plate. 

So I had to remove the back plate from the 6" refractor. A couple of minutes of light sanding of the bore in the lathe solved the clearance problem. I smoothed the ends of the pinch screws while I was at it. They had been marking the Baader Click Lock clamp's 2" spigot.

Then I could replace the back plate and took a couple of pictures of the complete set-up.

The total distance from the filters to the focal plane is now 42cm. Well within the 50cm maximum to avoid vignetting. 

The protective filters are to save the etalon, following filters [Lunt B1200a BF, Maier ITF & PST IBF] and  the ZWO '174 camera from being cooked. The focused sunlight from the 150mm aperture is still hot after the full aperture D-ERF. 

The cost and difficulty of replacing the entire H-alpha filtering system would be a disaster. Not to mention the price of the camera. I had already cooked my ZWO 120MC with my original 6" PST modified refractor. It has a dappled screen effect burnt into the sensor.

The protective filters are a 48mm, red, Baader H-a 35nM and a Beloptik KG3, UV/IR cut. As can be seen from the top image they now sit well forwards of the PST etalon up inside the main telescope tube. Where the beam from the objective is well spread out. Rather than the sun's image being a small, fiercely concentrated spot.

It is overcast and windy today. 60/58F. So little chance of a live trial on the sun so far. Just brief "teaser" glimpses of brightness between thick clouds. Which are travelling fast from the south east. 

I have the drives running but haven't even been able to find the sun's image in the camera yet. The sky is so dark at times that it looks like rain is possible.

A short clearing allowed me to Sync on the Sun and have a look in White Light. 20mm for 50x in the 90mm Vixen with Lunt wedge showed no surface detail except for limb darkening. There were longer periods of sunshine but the seeing was very poor. Having to work the processing too hard which is leaving everything spotty.

Later I realized that my etalon and filter stack had the proportions all wrong. Which could well explain the poor spread of brightness across the image. By inserting the focuser, after the etalon, the 200mm inside focus had been thrown awry. The spacing should remain accurate to within a millimetre or two.  Though the original PST focuses before the camera and after the etalon. So the spacing changes. Though usually only involving a change of eyepiece. The PST optics were never intended for the extreme magnifications of a 6" aperture PST mod with four times the focal length of the PST! Maintaining the f/10 requirement makes sense but little else does.

I had to do a quick rebuild without the focuser. A shorter spacer in front of the etalon ensured the correct spacing. I managed to focus by push-pull on the 2" spigots. There was more even lighting across the frame but the seeing conditions are still dreadful!

When adding or removing GPCs/Barlows these throw the focus further out. So, in my fuzzy logic, it means the etalon should move outwards too.  Otherwise it falls outside the required 200mm inside the focal plane. Which has just been moved, by up to 25mm, by the chosen GPC/Barlow. 

So, where do I put the focusing adjustment required to bring the camera into focus? My best guess is before the etalon. Then the entire assembly moves in unison with the changing focal plane. Which is something I have been denying myself from the very start of my H-alpha PST mods. I could fit the 2" helical focuser before the etalon but this would require mods to the backplate. Or sawing off a length of main tube. I could remove the clamping ring to gain some space. Reversal of the backplate would put the camping inside the tube. Not ideal! 

I have two Vixen 2" R&P focusers. A curved base, short reflector type and a much longer refractor version. Fitting either of these to the back plate would certainly give me focusing. I also have a Chinese knock-off of a 2" SCT Crayford but it really is complete crap. It won't hold or even move under load regardless of adjustment. The PST etalon has a 2" spigot in the front which means a 2" focuser with clamping is needed.  

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