Pushing the Limit

[Polar Bear]

I managed first light with my modified / restored 70mm F15 refractor last night.

Firstly I would like to say I have decided the direction my observing is taking and I am only interested in visual at the moment, predominantly the moon and planets.

Obvious target to test it out on was Jupiter.

Using a Vixen 7.5mm Plossl to give me 150x the views were very sharp with no CA but as I discovered even at this magnification I could only just make out the banding and not the GRS.

Is there an alternative than going down the route of a big dob ?

Any experience of high mag visual would be of great help to me for choosing my next scope.

Paul

[GazOC]

Hi Paul,

A common line of thought is that 100mm (4" inches) is where refractors start getting really interesting for planetary observations. You have a lot of options in the 4-5" refractor or 6"-8 reflector/ SCT/ Mak ranges that would give good results.

[Capricorn]

You will have had a magnification of 140x so I would have thought that it would show a reasonable amount of detail. I have seen bands at 60x on a small refractor.

How about drop back to a 10mm and try 100x. You could be at or beyond the max for a 70mm achro lens. I read it all the time that max mag = 2x dia in mm, but in refractors there are achro's, ED and APO triplets. They are not all equal on performance otherwise we wouldn't need to pay up for an APO Triplet, we would just buy a cheaper achro and get the same performance from it.

At 100x I would say you will get a better image and one that is a bit brighter.

The title is Pushing the Limit, you may be beyond the limit for the scope.

[Polar Bear]

Thanks for the input.

I may be expecting a little too much from 70mm as you say and on further measurement it seems to be around F13 - F14 so magnification is probably a more realistic 120x . Looks like I will need a 5" refractor (maybe a Helios 120mm / 1000 ) to obtain the views I am after.

Paul