andrew s

Yes. In theory the target is not relevant. However, optical quality will matter as will scattered and stray light. Regards Andrew

If it were me I would start as in the top Altair diagram but rotate the whole instrument so that the levers (A) were at right angles to the observed dispersion. Then open them up to remove it. In addition if it's not obvious with a planet try a bright nearby star were it should show more clearly. Regards Andrew

Both sets of instructions assume the parallactic angle is vertical which is only true at the meridian. But trial and error will fix most things! Interested to hear how you get on @johninderby Regards Andrew

In theory it is possible to calculate the parallactic angle and the degree of correction needed given your location and that of the planet. Then you should be able to just dial it in! However, I have not seen it done. Maybe a project for someone. Regards Andrew

If you mean can it correct for the achromatism of the refractor no it can't. Other than that it will work. Regards Andrew

The atmospheric dispersion is along the parallactic angle and is only vertical at the meridian. The bubble level may be misleading if your observations deviate too far from there. As above the bubble won't work with a Newtonian or with a star diagonal if not in the right orientation. Regards Andrew

Thanks @Don Pensack I might give some a try. I have a Takahashi Mewlon 180 and at f = 2160mm I don't need that short a focal length eyepiece. Indeed many modern planetary eyepieces seem targeted at smaller aperture and focal length apos. Regards Andrew

Your welcome. Regards Andrew

Thanks, I often use the telescopeOptics.net site which is liked in the CN discussion. It's very good I find. What I especially liked about the first paper I posted was the modeling of the complete system, telescope eyepiece and eye I had not seen that before. It also explained that the eyepiece aberations were derived by reversing the light as that had not been clear to me, although I had suspected it. Incidently do you know anything about the Siebert Optics Monocentric IDs? Regards Andrew

You were warned on entry. No refunds given. Regards Andrew

The shorter OED has 1769 "To turn (a thing) round upon it's own axis, or without shifting it from its place; also loosely to swing round." It does not give an origin for the word . Given this it fits rather well. Regards Andrew

Pardon, "another scope", plus the othe six or so! Regards Andrew

I think the BAA variable star data base is freely accessible. I thought the AAVSO was as well but maybe just to plot. If you get stuck ask @JeremyS for help as he is the director of the BAA Variable star section. Regards Andrew

Merriam Webster has "to turn something about a fixed point that is usually an axis". May be originally nautical as they give a ships spar as well as a telescope as examples. Regards Andrew

Very nice work. Regards Andrew

I think all such test should be done double blind. Don't foresee any problems with that do you? 😉 Regards Andrew

Can you provide a more accurate reference at least to the key points of science if you feel there are errors in this area. Personally I am less concerned with the history. Regards Andrew

Yes pity it's fallen over. Regards Andrew

Sorry mirrors only for me (eyepieces excepted) Regards Andrew

Ok all phase 1 components arrived and working. Clearly not got this phone pictures down yet. However, you get the idea? Regards Andrew

Mirror @Dantooine. Regards Andrew

The final part of my back to basic system (just who am I kidding). Regards Andrew

Looks grand. I am glad the oak went as it just did not fit in with the retro look. Good work. Regards Andrew

I have been trying to get better acquainted with eyepieces, the eye etc having been putting together a visual system. Several recent discussions have included eyepiece schematics and aberration diagrams. To better understand these I have Googled about and found two papers that may be of some interest, The first one covers the modelling the combination of telescope, eyepiece and eye! I found it explained several points that had perplexed me especially about the eyepiece aberrations and spot diagrams, how they are done and what they mean. http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECE_s.pdf The second is the parent article to the first and covers the evolution of the eyepiece. https://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECES.pdf Regards Andrew

Interesting, at about the max altitude of Saturn last night the difference in the red and blue dispersion is about 2 arc secs. I would think this would be resolvable with a 120mm scope with a classical resolution of about 1 arc sec in good seeing (0.6 for the 200mm). Just musing no real insight. Regards Andrew