Crazy eyepiece, crazy magnification, crazy little planet!

[Ags]

With Mars looming larger in the morning sky, I thought I would invest in an eyepiece to give me higher magnification. My 6mm gives me 220x which is already very high for for a 102mm scope, and this magnification is only usable on Saturn, the Moon and doubles.

So I did not want to spend a fortune on a new EP, given that it was only for silly magnification. So I picked up an SR4 EP for 10 euros... SR4s are famous for being the rubbish EPs included with department store scopes. But i tried my grandson's SR4 on Saturn earlier in the year and the view was not bad so I thought it was worth the price of a SaN magazine. The EP yields 330x magnification in my scope.

This morning I got up early for my first view of Mars for the season - my first proper view of Mars for over twenty years in fact. The scope was outside all night so it was definitely cooled. The 6mm TMB clone showed a small disc so I put in the SR4 and I was quite surprised. I could see a neat little disc, slightly gibbous in shape, with a whitish region on its northern end, where the limb meets the terminator. I presume that was a polar cap, and the cap's color appeared more pink than white. The planet's surface mostly seemed featureless aside from the cap but there were occasional hints of something.

The view was bright enough because Mars has a surface brightness 3 or 4 times higher than Jupiter, so it can take a bit more magnification.

The view was good enough on-axis. Off-axis showed chromatic aberations around Mars and the image became very soft. I confirmed this by also having a quick look at nearby Algeiba. On axis the SR4 gave a nice split of the double, showing clear sky between the components and good diffraction rings, while off-axis it was hard to split the stars. If the SR4 has a 30 degree AFOV then I would say only half that is actually usable.

The other problem with the SR4 is that it is so low-profile my head bumps into my red dot finder. I can always solve this problem by buying a x3 barlow to raise the EP a bit - this would yield 1000x magnification! :-)

[John]

Interesting report. I've got one of those SR (Special Ramsden ?) 4mm's knocking about. I used to use it with my old Tasco 60mm refractor to give 200x - only used it on binary stars though. As you say, on axis, not bad, off axis - ugh !. If they stopped down the FoV to just show the good part you would have something like a 10 degree FoV eyepiece. Nice to have some fun with something that does not cost much though

[rwilkey]

Syncronised Ramsden, I believe John. All well and good if it is usable, I wouldn't consider buying one though to be honest. too much magnification for me.

[Ags]

I thought SR stands for Symmetrical Ramsden, the EP is composed of two identical (symmetrical) lenses. If I'm not mistaken the SR was the very first multi-element EP design, so it is the mother of all Naglers!

[rwilkey]

Ags, you're probably right and I am wrong!

[John]

I'm happy to go with either of your suggestions

Whatever it is, the name is a bit of a mouthful for what is a very simple eyepiece design !.

[Ags]

I suppose at an exit pupil of 0.31mm I am theoretically in the realm of 'empty magnification'. In fact I would say I start to see empty magnification at 150x on the moon (small crater margins appear unnaturally smoothed).

But the greater scale provided by higher magnification does help reduce the glare of the planet and also enable the eye to pick out slight changes in shading, like the polar caps.