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Steve Holmes

A really great bit of lateral thinking there Grant, but I feel it doesn’t really move us forward. I’m sure that an 85mm reflector would indeed give a better image than a small refractor finder but the practical difficulties of mounting, and probably counter-weighting, such an item on the OTA of the sort of slightly up-market but still essentially “consumer” reflector a keen beginner is likely to be using (something like a Skywatcher 150?) hardly bear thinking about. Then of course, as the 85mm will almost certainly have been intended to be used as a “proper” (if small!) telescope rather than a finder, there is the question of FoV to consider. One might even have to mount a finder on it!! (and round we go again). As for “blobby coloured images”, I can assure you that those produced by Roger’s device are nicely round and white, right to the edge of the field. [P.S. I sincerely hope you didn’t mean “85mm refractor“, by the way – available, but not exactly at the budget end of the market!]

We did consider using Uranometria as a guide to star hopping, but I’m afraid the Bodleian didn’t seem keen to lend out their copy ….

Moving on to your second post Grant, I knew that Roger had mentioned “plate solving” so I checked back with him and he confirmed that, to put it succinctly, “been there, done that, didn’t get the T-shirt”. There were two basic problems. Firstly, rarely came up with a solution, even “after the event” let alone “live”, probably due to having captured an insufficient number of stars. Then, even if it had found a solution, the instruction to move X in RA and Y in Dec. does rather assume that these axes are correctly aligned and that the relevant setting circles are accurately calibrated and with a fine enough resolution and lack of back-lash to permit an accurate move. With a beginner’s telescope which has to be heaved outside and set up each time an observation is attempted, neither of these constraints is likely to be true. If they were, one could of course simply use the setting circles to point directly at the target of interest without having to go through the star-hopping process. As mentioned earlier, not such a problem with permanent installations but a frequent problem with beginners’ set-ups.

In Roger’s case, the reason he was needing to visually identify exactly where he had got to in the sky after each “slew” between target stars was precisely because he was aware that, although he had taken care to ensure that his equatorial was set up as accurately as possible and that he had turned the dials as precisely as he could, he was aware that the setup was usually just slightly “out” which meant that the end-point was often not quite where it was intended to be. This error could easily be corrected if only he could see sufficient stars to get a “fix” – not possible due to the small FoV of the main instrument and not possible through the optical finder due to low sensitivity. Hence the need for a wide FoV, high sensitivity, “electronic” finder.

And yes, “eyeballing” will certainly give a good starting point but what then? Looking along the tube will give an approximate aiming point but, assuming the target is invisible both to the naked eye and to the (optical) finder, this could be some distance away from the intended aiming point. Even if it’s fairly close, unless you know exactly where the telescope is pointing there is no way to know in which direction to move, let alone how far. As above, relying on the RA & Dec. circles may not be helpful, and “random search” (taking images each time through the main instrument) is unlikely to be effective for faint objects.

I suppose a summary of many of the points I have made in all the posts is “Welcome to the world of the beginner”! Everyone has to start somewhere, and that somewhere can be well down the scale of instrument sophistication. Beginners often have problems which “professional amateurs” don’t even think of, which can lead to blindness as to reasonable ways to solve those problems.