Nik271

There is an easy way to test for spherical aberration using a 30% central obtruction mask (cut out a 30mm paper disc and attach it to the front of the dew shield with some thin strip of cellotape for example). Aim at Polaris with a medium power EP, focus. Now defocus equal amounts of about 3mm either side and compare the doughnuts. Does the central hole look similar in the doughnut? If not - you have a spherical aberration. Another way is with a Ronchi eyepice, which costs about £40 from FLO.

Mars will be smaller than recently but will get very high in the sky at opposition. Hopefully the winter weather will be better than the last one ...

I could see Armstrong easily in my 100 ED F/7 yesterday. Looked briefly for the other two but no success. I find them hard in anything less than my 7 inch Mak.

It is sunny now but with a LOT of wind. I manged a quick look in Ha. There is lots of activity:

But unlike the Askar 185 this has a good chance of being very well corrected. For a scope of this size and with 7 lens elements arriving in collimation is a key issue I think.

I use my Quark visually and I can definitely see there is uneven illumination of the field. In solar chat forums there is a lot of discussion of uneven illumination, both of Quarks and of Lunts, so my guess is that it is a very common issue with these filters.

Obstructed optics can split slightly closer pairs of double stars, in the special case when the unobstructed optic presents touching central discs, like this: (Excuse my terrible drawings, it's for illustration puspose only) Unobstructed aperture: With 30% obstruction the central disc is a but smaller at the expense of the larger first ring, which becomes fatter and brighter, like this: This is very useful for pairs of equal stars at the Raleigh limit. Of course obstructed aperture is more often a liability e.g. for unequal stars where the dim companion is exactly on the first diffraction ring. Observers of double stars in the 18 and 19 century were aware of this and I have seen entry in Herschel's catalogue where he recommends adding an obstructon to the refractor for some close pairs.

Amalthea is never more than 30'' away from the disc of Jupiter so glare from the planet is a big problem. Needs a big mirror with very low light scatter. Or perhaps a top quality very large refractor.

No, there is also spherochromatism, that is spherical aberration which differs from colour to colour. So for example if green light is perfectly focused the blue may not come to a single focus point. The central part of the lens will have one focus point in blue and the outer part will have a slightly different focal point in blue. Minimising this aberration is the key distinction between a budget refractor and a premium one.

Mars is small but very bright, it may appear colourless initially under low magnification becauseof this excessive brightness. I sometimes experience this with Jupiter using a large scope. If you allow more time for your eye to adapt you should start to see red hues. Otherwise something is wrong with the scope.

I've only seen it with my 180mm Mak, trying it with the smaller 120mm Mak might be pushing it a little. When observing the Eskimo nebula recently I saw a nearby 12.7 magnitide star with my small Mak which is 5 years old and has two mirror surfaces which (I hope) were at 95% reflexivity when new and a corrector plate. So I expect a 130mm refractor should have no problem on a 13 magnitude object as long as the transparency is good. Oops forgot the quasar is a bit further south, gets to at most 40 degrees in altitude from the UK. This will make it tricky with smaller apertures. Definitely a fun one to try!

I made several quick simulations with Aberrator: Tegmine AB with 100mm unobstructed aperture: And with 128mm aperture: I also simulated what will Tegmine look if it was exactly on the Raleigh limit (1.36'' for 100mm aperture): The theory shows that pairs of almost equal stars will be resolved at the Dawes limit but in order to be split they need to be at the Raleigh limit (or further).

Last evening I had a very nice session with my 127 Skymax. I dug out my first 'proper' eyepieces 20mm and 9mm Svbony redlines from the case. They work remarkably well in the Mak, 9mm is great for double stars and planets with magnification of x170 and 20mm is good for DSOs with a moderate x75. The night was a bit misty but the transparency at the zenith and west was very good. I had no trouble seeing M1 (the Crab), M97 (the Owl), M35 with its neighbour NGC 2158, The Eskimo NCG 2392. They showed a bit of detail, noticeably more than with my 100mm refractor, so I think it's a case where aperture wins. I saw a very dim star making a triangle with the Eskimo and the nearby HD59087, when I looked it up in Stellarium it gave its magnitude as 12.7. I'm pleased with my little 120mm Mak, the mirrors must be in very good condition after 5 years of regular use. On double stars I used the 9mm Ep for a tour of my spring favourites: Theta Aur, Delta Gem (Wasat), Castor, Algieba and Iota Leo low down in the east. The seeing was good and all displayed perfect airy discst except Iota Leo which was sufferring from some turbulence. This was a test of my old Svbony eyepieces, which I don't use very much now. I have moved to Hyperions and even one Morpheus, but in honesty for a Mak the budget Svbony is pretty good. And they are very small and light. If I have to fly with hand luggage to a dark location guess which ones I will take 😉

On top of the central obstruction you have 6 diffraction spikes, perhaps they create the hairy look (when they are too dim to be easily seen as spikes)? Are you comparing the two scopes at the same magnification? Seeing starts to mess the star images a lot at high mags. Also if the Mewlon is not completely cooled down the in focus star image will not be very clean. I see this in my 180 Skymax often. The doughnut being off center indicates a slight miscollimation, the question is how much. A picture through the EP will be the best way to measure it.

Just saw this very informative youtube video of a talk on the 1824 Fraunhofer refractor mount. We seem to take the german equatorial mount as 'obvious' nowadays. It took quite a lot of trial and error to find the right solution.