Peter Drew

The pressure tuner on the Lunt50 that I had use of needed both hands to adjust the pressure, one to hold the scope steady and the other to turn the piston. If not, the effort to turn it moved the scope off target. 🙂

I have had good results with both tuning systems. The only criticism I have of the pressure tuning is the amount of effort needed to turn the adjustment barrel compared to the light finger touch of the tilt wheel. 🙂

Eek!, Scorpio Nigella! 😱

Yes. I have heard of this causing similar problems if the screws are too tight.

That would be an interesting option, I presume you propose tertiary flats? Catadioptric telescopes usually have large central obstructions so plenty of room to hide them. 🙂

I have built both SCT and Maksutov binoscopes, initially attracted by the compactness. However, the relatively high resultant magnification and restricted field of view, for me, detracted from the main benefit of binoviewers, the wide field potential. In this regard I feel that refractors and Newtonians are a better option. Another drawback of the catadioptric binoviewer is that considerable back focus is necessary to account for the additional optics to achieve translation to normal eye separation, although possible easily due to the moving mirror focusing, any mirror shift causes misalignment of the images. Adding a third party external focuser to overcome this just exacerbates the back focus problem. 🙂

I would be surprised if your secondary is pinched, most SCT secondaries are not constrained in their cells and usually bonded to the backing plate with an adhesive pad. If the optics were pinched at 80F they would be crushed below freezing point. What brand of SCT is it? I would suggest trying to release the tension of the screws holding the corrector ring. 🙂

I was a "low achiever" at school, 5 scraped "O" levels, including maths!. Anything that I have been eventually successful at has been self taught. 🙂

If I wasn't already there, the Astronomy Centre would be a Mecca for me. 🙂

I worry more about the MOT for my car. 🙂

I also have a magnetic tray. 🙂

My problem too! 🙂

I managed a couple of sessions with 35mm and 150mm apertures. Earlier rain showers had cooled and cleared the air so the seeing was quite good, the large aperture providing detailed closeups of the full disc features shown by the 35mm. 🙂

Precisely. The ellipse is still the most effective solution for manufacturing purposes. 🙂

Geometry suggests so. If you draw an axial centre line and converging beams from the primary, cutting through the light cone at 45* shows that the cone radius is greater below the centre line of the diagonal than that above the centre line. The "faster" the primary mirror is, the more this is exaggerated. 🙂

Superb animation, shows how much we miss observing visually in real time. 🙂

The light cone from the primary mirror gets larger as the cut off position of the secondary gets nearer to it. A secondary just large enough to accept the full aperture of the primary at the centre line of the secondary would be slightly oversize above the top half of the secondary and slightly undersize on the bottom half. Theoretically, a pear shaped secondary with wider portion towards the primary would correct this. Due to the mentioned difficulty of manufacturing such an awkward shape, secondaries are usually elliptical and slightly oversized, offsetting them also helps to address the issue. 🙂

The image shown by AstroKeith perfectly shows the issue. Although the radial offset of the focuser is relatively unimportant provided that the focuser is square to the tube, it's rather sloppy construction by the manufacturer as it means the buyer has to rotate the secondary to remedy it. ES is correct in their description of the secondary offset, the cone of light from the main mirror is wider at the lower half of the secondary than it is in the fop half. Ideally, a Newtonian secondary should be pear shaped but this would be more difficult to manufacture. 🙂

It's been hovering there most of today and is still there now at 18.40pm. 🙂

A polarising filter or a neutral density one to cut the brightness down can improve the contrast of surface details, specially filaments. The solar surface is generally bright enough to swamp the available contrast. 🙂

Difficult to tell colours from photos but in a good specimen of iron pyrite the internal structure usually looks a lot more golden. Hence the adage "fool's gold". 🙂

I made a small number of 8" F20 Schmidt-Cassegrains wih optics made by Jim Muirden. Jim hand figured the correctors by the ATM book 2 method that you mentioned. The secondary obstruction was no more than 25%. A user who contributed observations of Jupiter to the BAA planetary section was challenged as to how he could make such detailed observations with a 8" SCT assuming that it was a Celestron of that era. 🙂

Is this a current technique?, my understanding was that the SCT corrector plates were vacuum deformed against a master former and the exposed surface ground and polished flat. When released from the vacuum the plate assumed the correct profile for the corrector. 🙂

We have one that we use visually on a 16" SCT. It is very effective whilst the planets are low in declination, the AD can be completely dialled out. Not so sure as to its value on small apertures. 🙂

Although expensive, at least with the complete tube you get a new 5mm blocking filter as well. When the ITF in a PST fails, prime time for a mod to a larger aperture! 🙂