First Use II

Wavytone · #1 14-09-2008, 11:44 AM

After a long break from amateur astronomy (21 years) and having bought my first new OTA (after 35 years of ATM) I had a decent look through it last night, despite a nearly full moon, rather poor seeing and intermittent high haze. The scope is a Chinese 180mm f/15 Maksutov sold as a "Black Diamond" by Orion though probably rebadged with other names too, basically it's a slightly modified version of the previous Skywatcher of the same aperture. The change consists of a 2" back, though the perforation in the back is quite small meaning that while the useful field is more than 30mm, it will not fill the field of a 2" barrel, and it also shipped with:

- 2" back (at last compatible with other SCT fittings)
- 2" diagonal and 2"-1.25" adapter,
- 2" no-name brand wide-field eyepiece,
- 50 mm finder.

As you can see from the magnifications below this scope is all about magnfication and lots of it, quite modest eyepieces should serve fine views. Calulations suggest an un-vignetted field of view of 40 arc-minutes is possible and I have an NLVW30mm eyepiece on its way for this reason.

I had it on an Orion SkyView AZ mount (same as the SkyTee and others), connected via a Vixen dovetail (more about that later), and for eyepieces a Vixen LV10mm (270X), LV15 (180X), LVW22 (123X) and a Unitron 60mm Kellner (45X) which provides all the field possible in a 2" barrel as it has no field stop, to test the FOV.

With rather poor seeing it wasn't possible to glimpse a decent Air disk even for a split second so its abilities at high power on double stars or the planets remain unknown for the moment, however the defocussed star images suggest the optics are good. Despite the rotten seeing the 10mm was giving better views of Jupiter than I ever got from my old C-8 (or any C-8 I can recall).

The lunar image from this scope is stunning - the LVW22 provided a razor sharp view of the lunar limb edge to edge in the eyepiece, and the 15 and 10mm likewise for closer looks. In the LVW22 the moon is a tad larger than the FOV and it should fit neatly in the 30mm NLVW. The 60mm Unitron Kellner confirmed the full field of view available is about 40-45 arc minutes, though vignetting means the scope cannot fill a 2" barrel edge to edge.

The mount and the Vixen dovetail are a bit of a mixed bag. Firstly there were some quality issues concerning a dovetail bolt that arrived bent (I can straighten that one day) and several tiny lockscrews that were loose - before putting any valuable optics on top these all had to be checked - the sound of glass corrector hitting concrete is not something I ever want to hear.

While this scope will sit stably on the Orion SkyView AZ mount without showing signs of tipping over, I am not impressed by the Vixen dovetail arrangement and overall I think people are using this on scopes that are far too big and heavy - including this scope.

On this scope the dovetail is attached to the optical tube by two small bolts through the tube, which is quite thin aluminium. When attached to the mount the tube assembly quite visibly sags several mm and the optical tube is visibly distorted around the bolt nearest the corrector. Flipping the tube assembly over, the tube assembly flops the other way and the change of strain produces audible movement of that bolt in the tube.

This flexure also suggests that anyone hoping to use this Orion mount to carry two scopes in parallel alignment is doomed to be disappointed, the flexure will ensure any alignment doesn't last.

The motion in the axes is not on par with other isostatic mounts I've used - in particular an old mount I made many years ago out of the remains of a large Miller fluid-head for a 35mm movie camera. In azimuth I found no need for the clutch and it rotated freely without noticeable stiction. In altitude there was stiction to the extent that fine adjustments in altitude with the Maksutov at 270X (with the 10mm) were fairly tricky.

As for the tripod - it's a keeper, I liked it; quite sturdy and stiff, easy to unpack and set up in seconds. No octopuses or bagpipes to wrestle and it will clearly carry a sizeable scope comfortably and safely.

In conclusion:

a) the Mak optics look promising from the first night though I'll have to wait till the end of September for an opportunity really test it in good skies in the country.

b) The useful range of eyepieces for this scope is 10mm up to the Vixen NLVW30, and at f/15 it's not fussy - there's no need for Naglers and more modest eyepieces with 4-5 elements elements and good coatings should give sharp views.

c) The Orion SkyView AZ mount is not really a sensible choice with this OTA - it is really suited to one or two lightweight refractors in the range 80-100mm and up to a useful magification around 100X (as it happens I have such a refractor on the way). Anyone hoping to use magnifications well beyond 100X will find this mount too frustrating to aim, and while it is technically feasible to put a 6" or larger Maksutov on it, I don't think its a good idea long term and it could lead to the ugly sound of a telescope hitting the ground. As the dovetail itself appears too undersized, I don't think there is much gained by trying to put the Mak on an EQ5 or EQ6 style of mount either, as these used the same small dovetail and don't solve the flexure problem where the dovetail attaches to the tube.

Consequently I've concluded a more sensible alt-az mount for the 180mm Maksutov is a rocker-box sitting on top of this tripod, and making one is now a priority for me. I'm also considering having a simple fork made from aluminium plates bolted together at a later date.

dannat (Daniel) · #2 14-09-2008, 07:59 PM

Nice report, any pics? - maybe you could review the mount for the reviews section.

Terry B · #3 14-09-2008, 08:54 PM

Nice report.
With the problem of the dovetail I would have thought that getting some rings and mounting them on the dovetail would be much easier than making a fork.

Wavytone · #4 14-09-2008, 11:03 PM

Mmm... leave it to next month to finish it, I'll have the refractor and should have had some time under decent skies by then.

Wavytone · #5 17-09-2008, 04:47 PM

Decisions, decisions... Starting to think about:

(a) Buying a pair of tube rings for the Mak from Parallax, and either bolting the Vixen dovetail to these, or stepping up to a Losmandy plate.

(b) Ordering a DM-4 or possibly the DM-6 from www.discmounts.com and adding an Argo Navis.

(c) Having the pieces made locally for a simple altaz fork. I know its going to cost...

(d) In the meantime making a rocker-box mount that fits on top of the tripod.

Apart from the DM-4 and DM-6, I'm somewhat surprised at the lack of decent altaz mounts suitable for larger scopes and especially the total lack of any that can accurately mount two decent sized scopes in parallel...