Hi this is Graham from Jenham's Astro if you've seen a few of videos on
my youtube channel know that I'm a big fan of this telescope the Celestron C90
which is a 90 millimeter Maksutov real bargain very versatile for imaging the
moon and the planets tonight I'm going to try something different going to use it to
see if we can resolve some double stars so I've chosen a very famous example
called the double-double which is Epsilon Lyrae which is pretty well overhead in the
middle of the summer close to the very bright star Vega so on paper the
resolution of this scope if you look at the aperture and the focal length the
Dawes limit resolution is just over one arc second and Epsilon Lyrae is made of
a pair of stars which if you have enough resolving power each of them itself
resolves into two stars four stars and all and the separation of those two
pairs is just over 2 arc seconds so on paper at least if the seeing is good
this telescope should allow us to resolve a little bit of dark sky in
between the elements of epsilon lyrae so how we going to do it so we're going to
put the basic scope on to an equatorial mount you see here that I've attached my
failure finder which is a Telrad finder and I've used some tape to secure
it to the side of the tube as I find us the best way of making sure the
alignment stays between the finder and the main telescope in order to improve
the image scale we're going to use a barlow lens I've chosen of two times
Barlow and then finally going to connect a digital SLR so this is a Canon
digital SLR and we're going to just play around with some different exposures and
see whether or not we can resolve the famous double double into four stars ok
let's give it a go.
okay we're ready to go one more thing I'm going to use one of these which is a
Bahtinov mask a diffraction mask which is a focusing aid the reason we're going
to need this is I know from experience that if we use this telescope
and the Canon camera the live view screen is really only any good to see bright stars
so Epsilon Lyrae is magnitude 5 and six approximately and we're going to struggle
to see those images on the live view screen unless the focus is good so what
we're going to do is going to point the scope at the bright star Vega use the
mask to achieve a fine focus on Vega and then move the telescope to the target to
epsilon lyrae hopefully we'll should acquire it then on the live view screen
and we will achieve an accurate focus
ok so we centralized the bright star Vega in the live view screen this is on
the in the zoom mode and I'm going to place the bahtinov mask in front of
the objective ok so that's a characteristic picture that you get when
you have one of these masks over the objective you've got a series of lines
radiating out from the object you need to use a pretty bright star like this
and then we can see what happens when we focus one side or other of what you
think is the best focus position so that's moving one side if we go quite a
way out of focus we can actually see an image of the diffraction mask and then
coming back into focus and out the other side so it's quite a fine adjustment and
then at this position we're trying to make very small changes until the three
main lines radiating out approximately separated at approximately equal angles
i think that's about the best we can do and if you remove the mask i think
that's a good focus will leave that position locked in and then we'll move
the telescope on to epsilon lyrae
okay so there you have it after a bit of moving to and fro to centralize the
double star in the live view if you find it you can see the two components
epsilon lyrae 1 and 2 and opposite sides of the live view field I
think it's clear that you can see a little bit of dark sky in between each
of the pairs so that we've clearly split this into a double-double. It shows that
the C90 once again for the money it's a bargain its optical performance at least
of this particular one is it's very good and it's allowing us to see what is a
challenging object clearly resolved so all that remains to do is to try and
take a few exposures of this frame and then see whether we get a good still
image after a few varying exposures ok I'll give that a go
okay so this is a result after a bit of trial and error this is an exposure
taken iso 400 with a manual exposure of just over one second I think you can see
that each of the components is as well recorded with a bit of dark sky in
between each to show you that it is clearly a double double
so that's it. Epsilon Lyrae job done