Skywatcher 90T Maksutov spotter (1 Viewer)

Meaning to try one of these for ages, although I have had several other 90mm Maksutovs.

Not really tried on birds, but just tested the optics on a dull rainy day.

Initially I used the supplied 25mm eyepiece, probably a Plossl, but switched to the Vixen 8-24mm zoom.

The main thing is that the field is small.

I know that the quoted focal length of 1250mm increases if one uses the supplied 90 deg star diagonal.
I saw a measure of 1483mm for the probably identical Orion 90mm.

I used a Televue 45 deg erect image prism as it is more comfortable. Surprisingly this is uncoated. I would think the focal length would increase even more, but roughly the eyepiece gives 60x to 180x, maybe slightly more.

Although the image is dim I see no false colour at any magnification, but I have to wait for a sunny day.
Using it through double glazing seems fine, maybe not perfect at 180x but good.
The problem is that my eyes are beginning to show artifacts with the 0.5mm exit pupil, which were fine when younger.

The field at 180x is about 20 arcminutes.

The Slik 88 tripod is steady, although one has to aim a bit high as locking the head brings the scope down a bit.

Very encouraging.

However, when unpacking the very well packed multibox supplied packing, and opening the waterproof carrying case I noticed mildew between the waterproof top and case. Although the case is nice with several pockets and supplied with the scope, I got rid of it even after cleaning. Any sign of moisture effects for me are quickly dealt with.
The optics seem fine. The corrector is multicoated and the mirrors bright. The central obstruction is about 25%, which is good, but I'll measure it.

The weight of the 90T scope including the Televue 45 deg prism, the Vixen 8 to 24mm zoom eyepiece and the Slik 88 stud attached is 1693g, just under 60 ounces.

This includes the metal bracket for the finder, which I could detach as I am not using a finder.

A Philips screwdriver is supplied for attaching and detaching things. Also 2 eyepieces, a red dot finder, the 90 deg diagonal, and the case, the latter I chucked.

There are 5 standard tripod threads on the base, so one can get the balance correct.
Also supplied is a T ring attachment, I think for cameras etc.

The eyepiece locking knobs get in the way of the focuser, when trying to detach the eyepiece tube so detaching is difficult. I think you have to remove the eyepiece locking bolts.
The eyepiece holder may be a bit tight for certain slightly oversized eyepieces, such as the Russian ones? It would not surprise me if some eyepieces don't fit, but most will I think.

The magnification range could be about 63x to 190x but I am not going to try to measure this accurately.
This is for viewing at 120m or about 130 yds.

The Skywatcher 90T is suitable for planetary and lunar observations and long distance terrestrial views.

The field of view, a maximum of a little over 1 degree is probably not suitable for birdwatchers.
Low magnifications are not possible.

A refracting spotting scope able to take low powers and wide fields is better, unless for distant views.

Some complain of lack of contrast with Maksutovs.

A 6 inch aperture Maksutov, however, used at long distance would take some beating.

But they are not waterproof, except maybe some military versions.

Summary of long story.

02.00 UT this morning.
Moon 1.5 days before full. Mountains and small craters seen well near terminator. Best magnification 115x.

07.30 UT.
In shadow crow and blackbird disappointing. Flat image, contrast dull, no bite to image, birds seen against brightening sky.
Chimney pot in sunshine bright at 65x, higher magnifications possible.

12.04 UT 65x bright 135x still good. Bright sunshine. No birds but bugs seen flying around at 120m.

11.50 UT wind 240 deg 8kn visibility 50km clouds few 1,800ft scattered 20,000ft 10C TDP 6C 1027.5 hPa.

So up to now a good astro scope for lunar and planetary, but not so good for birds.

But.

Success.
14.00 UT beautiful day very bright sunshine, Sun low uncomfortably bright.
Bl..dy enormous crow sitting on chimney pot at 124m. Managed to get scope on it at 190x.
The bird almost filled whole field. Never seen such an enormous looking bird in a scope.
125x best magnification. Bird flew off after 1 minute and I saw it take off and fly a bit.

So if the Sun is very bright then 125x is very good. At average light levels scope doesn't have bite and a refracting good scope is much better I think, but did not set up one to compare.

6inch f/10 Maksutov is great terrestrially at 95x, but needs separate secondary.
Most Maksutovs too high power.
With 6 inch Maksutov you can use 2 inch eyepieces for wider field and an f/10 can be excellent.

Problems are temperature gradient sensitive.

The secondary spot diameter is about 28mm, which is about 31% central obstruction.
This is about average for a 90mm Maksutov.
The same design revealed by Maksutov in 1941 was also 'invented' about the same time by Bouwers, Penning, Gabor and maybe Vaisala.

Normally a central obstruction of 25% or less is needed to achieve fine results, but this usually only occurs in larger instruments.
Photographic units often go up to 44% obstruction.
This central obstruction reduces contrast.

Although I have not yet tested the Skywatcher 90T on planets, the performance is broadly similar to a Questar 3.5 inch.
The Questar has the great advantage of an inbuilt finder and inbuilt flip barlow.
Mine is ex gov. and works quite well.
Maksutovs are used much by the military, but I don't know if they use birding spotting scopes.

For £126 delivered new the 90T is a bargain. Friends have bought excellent secondhand ones for £50.