hi this is Graham from Jenham's Astro. A while ago I made a video review of the
current chinese-made celestron c90 telescope. As you know it's a 90
millimeter maksukov which sells for around 150 pounds or 180 to 200 dollars
I thought it was well made and a bargain for the money there's quite a lot of
interest in the video a few of you record your experiences with the
original c90 which celestron made in the US for around 20 years from the 70s
through the 90s. This older c90 has a mixed reputation shall we say so I
thought I'd make a comparison video of the old model versus the new so for an
old model on an auction site it cost me 80 pounds which is about $120 as you can
see it's a black tube model although many of these were made with the famous Celestron
orange tube so I'm going to compare four aspects the optical spec
the mechanical build performs performance of the two scopes against
daytime objects, the performance against Astro objects the comparison is
just for these two scopes but I try to mention a few of the good bad and ugly
points I picked up along the way through reading the online forums there's a lot
of resource online for the C 90 and you think you're going to buy when I
strongly recommend you have a look at some of those websites and forums but
having said that for the kind of prices you can pick one of these up you're not
taking a huge financial risk if you just go and buy one okay let's get started
so both of the scopes and 90 millimeter maksutovs, so you should expect a high
magnification with a smallish field of view it's about one and a half degrees
so that's a couple of moons across with a little bit to spare on the side you'll
see that later the old model has 1,000 millimeters focal length giving it an F
11 focal ratio it's written on the side in a rather special 1970 style font
the new version squeezes an extra 250 millimeters of focal length inside the
tube giving a focal ratio of around F 13.9 if you stand the two tubes next to
each other you can see that the new model is markedly longer than the old one
the old model uses the old Japanese size 0.965 inch accessories
it came with two Kellner eyepieces eighteen millimeters and 30 millimeters
and a rather tiny star diagonal again 0.965 inches all
feels very solidly built these accessories but if you want to use any
of your current kit you can have to buy some kind of adapter so you can use a
simple adapter like this to go between the two sizes of barrel or you can
search online for a thing called a "large accessory ring" which will allow you to
convert the old scope to use newer accessories. Well as you might expect the
the new scope is all one and a quarter inch and it comes with one Plossl
eyepiece, an erecting prism and an 8x21 finder which compares with
the 5 x24 the old model. As I said in the original video I don't think very much
of the finder but the other accessories are okay certainly get you started, you if
you've got some other eyepieces though I'm sure you will use those in place of
the ones that come with the current C90 so the new scope optically people play
around with it most probably to try and improve the contrast so they open the
tube to put various sort of baffling and flocking into the tube to improve that
whereas the old model is most likely that it will be opened up in order to
improve on its famous focusing mechanism
OK so the mention of focusing brings us on to the mechanical comparison
between the two scopes so the old scope it really thinks it's a camera lens you
focus it by turning this ring and that adjusts the separation between the
optical elements inside the tube. The trouble is really that the screw
lubricating this mechanism the screw thread can become stiff with time and
then it's very hard to achieve a smooth focus you can solve this you can take a
couple of screws out at the bottom of the tube split it into the two halves and
reapply some grease into the thread that's relatively straightforward maybe
some lithium grease and that restores a smoother operation to the scope having
said that I think the focus really is the strangest most problematic aspect of
the old C90's design came with a standard photo tripod thread into this
unit here so you could operate it as a spotting scope on a photo tripod just as
you can with a new model the thing is at higher magnification this a combination
of a fairly shaky mount and the focusing mechanism it's really hard to achieve a
fine focus using this old scope it's quite frustrating to do that so
definitely if you're going to try and use with these things I'd suggest you
put a more modern dovetail on it's quite straightforward and use bolt on where
the camera thread is on the original unit put it on to a rigid modern alt-az
mount, lubricate the screw and off you go then
you'll have a tolerable focusing mechanism I won't say it is a fine focus but
you can you can make use of it
okay if we compare that to the current c90 so here we have the focusing knob on
the rear cell in a traditional position for a cat scope and that generally works
better than the old c90 I guess that's pretty clear no real problems with that
I say perhaps it's alright it's a quite a small rear cell on the c90 so the only
problem can be if you've got accessories your camera as you will have attached
here at the back then there isn't a huge amount of room to get things in to
adjust the focus but it's a small point not a big problem
comparing the two tubes well they are both combination metal and plastic feel both
feel solidly built and quite pleasing both of them sort of tactile feel of
both of them nice well put together optical instruments of course as I said
before this is simply a comparison the unit I've got there's a lot of people
with mixed experience of the c90 build quality maybe quality control but I say
this unit I'm pleasantly surprised by how well it's put together okay
other comparisons I find the old units finder easier to use than the current
unit I'm not a big fan of this finder at all but either way it's not a problem
you could attach a Telrad to the unit and solve that problem easily the new unit
obviously coming with a standard dovetail should mention that so okay the
old model came with a screw-in dew shield just screws onto the front of the the
cell whereas the the new model doesn't come with any dewshield and as
we'll see there's quite a lot of comment about the contrast you can achieve with
the new scope perhaps not being the best perhaps not it's not its best feature
something you can improve on if you're willing to do
work on the scope. okay it's time to have a look through these scopes and see how
we can get on with some imaging.
So this is the the new C90 model the
current model focusing on that flower in a garden as you can see and now I'm
adjusting the focusing knob in and out
it's quite smooth you can see I hope that the object isn't moving a great
deal mounted on a Vixen Porta mount so it's good stable mount and you can judge
yourself the level of contrast you got with this scope so that's the new model
okay so this is the old C90 for comparison straight away looking at the same target
you can probably see the field of view it's greater with a scope as you'd
expect because it has a different focal ratio okay if I adjust the focuser you
can see then on a stable mount there's going to be some amount of movement of
the scope because you're turning the tube but it's it's a tolerable amount of
movement during the focus again you can adjust to focus on a object further
away quite easily and then back into focus somewhere around there so this is
the image with no dew shield and then if I put the dewshield in place for
comparison here it comes so with the dew shield and without I
think you can see there's a good improvement on in the contrast if we use
the dewshield and there it is again
okay
okay so just to finish up the data on comparison here are two still shots both
50th of a second through the two scopes the new C90 on the left and the old
model on the right obviously the main difference is the field of view secondly
I guess you can make your own judgment about the relative contrast of the two
images but for me I take the old scope with the dewshield it's probably
slightly better contrast.
okay so to start our comparison of astro imaging
for the C90s, I took a shot of the moon a few days ago the image on the left
with the new c 90 and the image on the right with the old scope i guess the
obvious difference once again is the image scale other than that the hue is
slightly different the new scope gives more monochrome image everything else
between these two shots is identical so they're about 1/80th of a second ISO 800
just with a Canon DSLR focus no problem with either C 90 here the moon is big
and bright obviously and it was quite easy to achieve the focus although I
would say easier with the new unit .
okay not much more the same
okay so this is the old C 90 we're looking at Sirius which is probably quite low in the skies
about 20 degrees above the horizon looking at on live view through the
canon eos on 10 times magnification i guess you can see this is pretty well as
close to in focus as i can get the image there's a bit of color fringing and now
we'll can have a look at the effect of turning the focus from
okay so turning one side of focus here we go I guess you can see the
diffraction pattern it's perhaps not completely central concentric but that's
what we've got and then you can see as I focus the scope it moves the star from
the field so I can move it back with the slow motion and then we'll come back
into focus somewhere around there and then we'll go the other side
okay that's the pattern on the other side and then again the image is moving
as I rotate the tube with the focus ring I guess that is pretty well it's as close as
i can get it back in focus okay so that's the old c90
okay so this is the new C90 once again I have Sirius in focus there's not much
sign of false-color there to my eye less than the old C90, if we turn the
focusing knob one side of focus that's the pattern that we're getting I say
there's definitely a more concentric series of rings it's just in the
collimation on this scope is better than that on the the old unit that I've got
and then not much movement of the image as we focus back I think to there that's
the smallest image I can get and then go in the other side focus quite a
different pattern
they're once again concentric quite a lot of movement in image which I guess
is an atmospheric effect and we come back finally into focus okay
that's the new c90
okay so just to conclude here's a snapshot taken from the previous two
videos for the old and the new C 90s of an out focus diffraction pattern from
the star Sirius I'm sure the optical experts out there
can comment and tell me what these patterns mean for the optical quality of
the two telescopes
okay so will tests complete where does that leave us?
we'll have to say that the new unit is winner of the test primarily because it's
focus and makes it much easier to use the old c90 it's also a great little
unit in many ways it set the f/11 focal ratio make it easier to use in some
respects than the f/14 unit but I think it is let down by its focusing mechanism
which introduces vibration which is really the last thing you want
combination with a high powered telescope like a maksutov, but still this size a
unit this size the design great for web cam imaging or the planets photographing
the ISS for example if you want to do some other tasks maybe like looking at
the lunar highlands a high power or looking at birds maybe you want to look
at a different type of scope maybe a small apochromat. So in summary I think
the old C90's reputation for being a bad telescope isn't really deserved
certainly there are a few issues with it and I'm sure that the new units quality
control means that it'll be a lot more consistent production with the new c90
but still a lot to recommend both scopes I hope you've enjoyed this
non-scientific comparison between the two models and
that if you've got a c90 in the cupboard you dig it out and get some use out of
it ok thanks for watching
you