70mm refractor thoughts

[Cosmic Geoff]

One of the other subsections here had the owner of a 70x700mm Bresser refractor grumbling about the views he got with his telescope. This prompted me to get my 70mm out on 28 Dec and have a look through it. 
I should explain that this is a vintage Ross terrestrial telescope, originally sold by Charles Frank as an astronomical telescope with an equatorial mount. I didn't get the mount and surveyor-style tripod, as the son of the late owner wanted a lot for them compared with the scope. The mount looked like it needed a rebuild anyway. That was probably a mistake as I later  spent nearly £200 providing an adequate mount for a very long telescope weighing around 5Kg. It has a 33mm dia (note that figure) Huygenian eyepiece and an erector lens set.  This eyepiece slides out (one can feel the air pressure sucking it back in) and can be replaced by a 31.7mm dia astro eyepiece wedged in with a bit of card. With the erector set left in place the effective focal length is ridiculously long (over 3 metres), and even a 25mm astro eyepiece gives a high magnification.

Many decades ago, a 3" brass refractor was what amateur astronomers aspired to. So how does it perform? On a previous outing I could see Airy disks with it. On the recent outing I got, with poor seeing, Venus as a half-disc with some yellow aberration at the bottom, Mars as a clean reddish disc, both close doubles of epsilon1 and epsilon2 Lyrae resolved, Gamma Ari well split, and Beta Cyg split easily and and looking good. Under these conditions a much bigger telescope would not have done strikingly better (as I proved a few nights later with my 200p trained on Mars and Venus).  My Mak would not do much better with the original eyepieces.

It's a shame that newbies may not get the chance to see what a 70mm aperture can do before they are urged to get a big aperture scope regardless of mounting type. Unfortunately some of the 70x700 mm refractors on offer will be made to be cheap rather than delivering maximum performance. How much would one have to pay today for a 70mm f10 'frac to guarantee premium performance? Rather a lot, I suspect, but the mounting can be lighter.

Of course, a larger aperture scores on nebulae, galaxies and star clusters, where it makes a big difference to the spectacle. 

 

 

 

[mikeDnight]

A lovely post Geoff! 

I'd suspect that most modern 70mm achromats are pretty good optically and the eyepieces too. I've seen many a substandard diagonal though and wouldn't be surprised if many scopes are condemned because of that.

I suppose it depends on what the observer expects to see through a small instrument that could determine if he likes the scope or not. I remember being disappointed that I couldn't see bright spiral galaxies through my 60mm Astral back in 1981, after all they were shown on the box. Once I realized my scope had limitations, I settled into observing the objects it could see well and was perfectly happy. ?

I think you're right about newcomers to the hobby being urged to buy bigger telescopes too soon, before they have any real idea what the scope they already have can do. I've often felt that many smallish scopes are never given a chance and that the opinion about their true capabilities is grossly underestimated and distorted, usually by those who speak with great authority about something they know little about. It's my experience that small telescopes often deliver well beyond what their aperture class might lead someone to assume. 

Mike

[iPeace]

I started out at 70mm and I still enjoy using it immensely. It's a more modern design, an f/6 ED APO with a lightweight carbon fiber tube and for its aperture it does very well indeed. This may be helped by the use of high quality eyepieces, which don't come cheap, but as stated above it requires much less in the way of mounting.

When it comes to enjoyment by aperture, reclining on a comfy deck chair with a nice pair of binoculars is hard to beat.

[Peter Drew]

I have used many entry level F10/11 60mm and 70mm aperture refractors. In general, the objectives are surprisingly good, the eyepieces although of minimal quality, perform reasonably well at that focal ratio. Unfortunately that is where it usually ends, the mount, tripod and other accessories just don't do the telescopes justice. Given a good mount and decent eyepieces, these apertures can be very rewarding.