(Small) Frac attack on Mars

[Stu]

I didn’t have the energy (or permission 🤣) to be up late last night, so I just thought I would have a little refractor comparison on Mars instead of putting anything bigger out.

I had thought about Vixen FL102S vs TAL 100r, something I must do at some point soon, but the TAL has a Losmandy dovetail on it for some reason, so I opted instead for the Telementor and Scopetech 80mm f15.

The Telementor has an 840mm focal length, 63mm aperture (f13.3) and I fitted it with a Baader Zeiss T2 prism and 2 to 4mm Nag zoom. The Scopetech has a 1200mm focal length, 80mm aperture (f15)  and I used a Tak 1.25” prism and the 3 to 6mm Nag zoom. As near as I could get to a level playing field.

These combinations gave almost identical magnification ranges, x210 to x420 for the Telementor and x200 to x400 for the Scopetech. I stuck to the lower ranges for obvious reasons; tiny exit pupils being the main one, 0.3mm in the Telementor at x200.

Initially views were pretty underwhelming in both scopes, little or no detail and quite wobbly. After dinner I went out again for about 30mins with better results. In the Telementor I got a fairly well defined disk with little or no CA. Syrtis Major was clear and positioned fairly centrally. There was some evidence of lightening in the detail to the South but not much. I could see a little of the haze around the North Pole, but could only suspect the South Pole, more imagined than seen!! Floaters were very evident; it wasn’t a particularly easy viewing but still quite surprising for such a small scope.

With the 80mm, the disk itself looked cleaner and more sharply defined. Syrtis Major was easier to see and showed that little bit more subtle detail. Whilst I would not say I could clearly see Hellas, the brightening in that area was much more obvious. Northern polar haze was clearer and the South Polar cap was easy to see, bright white at times. Floaters at the same mags were less obtrusive due to the slightly larger exit pupil.

I did up the power at times and the 80mm sustained it more easily than the 63mm as you would expect. Tracking on the manual mount with only a 50 degree afov was a little lively too! With Mars a little higher and perhaps on a better night I may have seen more, there was loads of dew about, so the eyepieces were misting up at times but the objectives stayed clear. A fun and interesting little experiment in the relationship between aperture and resolution none the less.

[JeremyS]

Super report @Stu

[chiltonstar]

More interesting stuff Stu, and nice pics. I am often amazed at how good the view with my little ED80 (Orion) is of the planets - visually it seems better than it could possibly be for the small aperture. It certainly shows though that a small frac is a very good choice for a beginner/youngster who is more interested in lunar and planetary views, despite the obvious appeal of say a 200mm dob.

Chris

[Stu]

39 minutes ago, chiltonstar said:

More interesting stuff Stu, and nice pics. I am often amazed at how good the view with my little ED80 (Orion) is of the planets - visually it seems better than it could possibly be for the small aperture. It certainly shows though that a small frac is a very good choice for a beginner/youngster who is more interested in lunar and planetary views, despite the obvious appeal of say a 200mm dob.

Chris

Thanks Chris. One other object I did observe was the Double Double. It was a classic illustration of airy disk size vs aperture.

Both scopes split the stars quite easily, but the stars were clearly tighter in the 80mm vs the 63mm, showing the smaller airy disk size in the larger scope, so the split was definitely easier.

[RobertI]

Nice shoot out Stu. Oh my - that Scopetech is a long scope! I know what you mean about the large airy discs in small scopes - although I split the double double in my 66mm at high power, the airy discs were huge!

[Stu]

3 hours ago, RobertI said:

Nice shoot out Stu. Oh my - that Scopetech is a long scope! I know what you mean about the large airy discs in small scopes - although I split the double double in my 66mm at high power, the airy discs were huge!

Thanks Robert. Yes, it’s a long beastie but pretty light and easy to handle and mount.

I do enjoy the star views in these small scopes, possibly because the large airy disks hide a multitude of sins, and so aesthetically they looks very nice ie big, clean airy disk with a nice neat diffraction ring. These are visible quite easily, whereas with a larger scope you need much higher power to see the smaller airy disk, and it is often disrupted by poor seeing or tube currents.