The Dark Star Dobsonian Telescope
I bought my dob in the summer of 1999
and have enjoyed it immensely. It came with all
the right bit's that a good dobsonian is supposed to such as a metal Rack and Pinion
focusser, blackened internals, good quality bearings and a very solid base. The actual
tube itself is thick PVC (or some kind of plastic) tubing painted in a blue Hammerite
finish which has proven very hardwearing. Collimation is via a 3 point cell adjusted with
wing nuts. There's a good gap between mirror and tube so cooldown times aren't too bad.
My main gripe has always been finding those elusive deep sky objects. The real problem is that even when you are pointing in the right area of sky, a general scan of the area isn't usually enough to pick up a very faint object in a heavily light polluted South London sky. Here's where a computerised SCT would be a real help but unfortunately, I don't have the funds for one of these, I'm not sure I'm going to get into Astro-imaging so I don't need the high performance tracking at the moment and I hate being ripped off by what appears to be expensive pricing compared to that paid in the US (see http://www.vu67.freeserve.co.uk/protest.html) so I needed an alternative.
Whilst perusing the July 2000 edition of Astronomy Now I started reading an article by Steve Jones about computerising your Dobsonian telescope. The thrust of it was the making and fitting of setting circles to the telescope and then using a program running on a Series 3* Psion organiser to give Altitude and Azimuth values for various sky objects to 'dial in'. After reading the article I was convinced that I had found the help I needed to find these DSO's with my dob so I put together some makeshift setting circles and had a go. I made these using the program Steve makes available to make the markings but they were only out of paper. Never the less, I dialled in M51 the Whirlpool Galaxy and there it was! Bigger brighter and clearer that ever before! Well actually it was still the murky smudge it always appears as through my scope but it had proven that it was worth me getting my rusty saw out and manufacturing something a little more permanent. The picture below shows both the Altitude and Azimuth scales I finally managed to fabricate. What isn't clear on these pictures is that I've used clear plastic from a bubble pack to make the indicator which looks very professional (I copied the idea after seeing pictures on the StarTrak web site). There are far better pictures together with full instructions for making and installing the circles at http://www.startrak.co.uk
I was so impressed with StarTrak that I wrote to Astronomy Now to congratulate both them and Steve on such a good article and was very happy when it was printed in the August edition. I had also been conversing with Steve via email about Startrack and making the circles and one thing that I had realised was that the Azimuth circle was liable to damage when the rocker box was lying on it's side such as when transporting in the boot of a car. He told me about a 'fender' arrangement that he had used to protect the circle so I fitted one myself as can be seen below:
I really feel that Alt Azimuth circles for a dob are the most useful things to have and should be fitted to dobsonian scopes as a matter of course. StarTrak is a marvelous program that enables constantly updated Alt-Azimuth (and RA/Dec) values of most night sky objects to be made available in a portable manner enabling easy use in the field without resorting to a relatively bulky and power hungry notebook.
* Steve Jones has recently released an EPOC32 version of StarTrak
90mm Maksutov Cassegrain
This (not very good) picture shows my 90mm Mak Cassegrain sourced from Orion Optics. It came supplied on the GEM mount shown on the left but this was pretty shaky so I managed to find an old Gitzo tripod from Mr Cads in West Croydon. It's probably 40 years old and cost a fortune but built like a tank and has totally transformed the performance of the scope. I'm hoping to take them on holiday in Aug 01 where the skies should be nice and dark. I wrote a short review of this scope and posted it on the Yahoo group telescopesuk. It's copied below.
Overview
Firstly I would like to point out that although I’ve been into astronomy for several years now I am certainly no expert – particularly with optics. I’ve also never written a review of a scope before so bear with me.
I currently have a Dark Star 8.75” dobsonian but have been looking to purchase a smaller telescope for holidays and general out and about viewing for some time. I had been looking at the 80mm rich field refractors available from Helios and such like but noticed on the telescopeuk group that Orion Optics were taking delivery of some small 90mm Mak scopes that were thought to be good value. I emailed Barry who was very helpful and eventually ordered one. Unfortunately I was really after the Alt/Azimuth tripod mounted version for portability but all they had left were the EQ mounted ones so I settled for one of those as I thought an introduction to this kind of mount would be useful. Cost supplied on the EQ mount next day delivered was £369.
The
package turned up after a couple of days and all seemed well protected. My first
examination was of the OTA which looked very well made in it’s rugged rubber
armouring. I then set about putting together the tripod and mount. This was
pretty straightforward and on first impressions the mount seemed very solid
although at this price point I wasn’t expecting precision engineering. The
Tripod had a pop-rivet that had come adrift of the centre support but I was able
to put a new one in without any bother. The mount is fitted with slow motion
controls and setting circles. The movement of both axis was initially a little
stiff but with the counter weight in the correct position and after much
twiddling it seems to be much smoother now.
The
scope was provided with an 8x21 finder with a three point adjustment and a 26mm
plossl eyepiece of (I assume) far eastern source. I already have one of these
that looks remarkably similar and which gives great views. Also in the box is an
erect image prism which screws into the back of the scope. This can be used as
an alternative to the top eyepiece holder – switching between the two is via a
flip mirror mechanism built into the back of the scope. The focusing knob is
very smooth in operation.
As
per usual with a purchase of this type, the weather has been atrocious recently
and I had to satisfy myself with an image of the front door on a house about 500
meters away for a few days which looked very clear.
Finally
after missing out on a clear night Friday because I had commitments I couldn’t
get out of (believe me I tried!) I was treated to a couple of hours on Tuesday
night of clear skies. After leaving the OTA outside for an hour I finally
plonked the mount in the general direction of Polaris (I had already set my
latitude) and started viewing. These GEM mounts take a bit of getting used to
for a dobsonian owner but if I released the axis bolts I was able to twist it
all around as required. First target was Jupiter. The sky was still pretty
bright and the image was quite detailed. Using the supplied 26mm I was able to
make out both bands easily and next tried a 17mm.
I
tried the 10mm with a 2x Barlow (240x) but it looked pretty mushy (probably no
worse than the dob though) and being a bit short of viewable planets at the
current time, I tried a few star tests. I’ve never been very good at
interpreting these but lets just say it looks OK – definitely different in and
out of focus but not spikey. Stars tend to focus quickly into sharp points with
all the eyepieces, although at 240x the Airy(sp?) disk was evident, and I was
generally very pleased.
I
tried a star test with the supplied diagonal but this didn’t seem so good. I
thought that by bypassing the flip mirror I may get better alignment of the
optics – maybe the diagonal supplied is not too good (not surprisingly!).
On
this point and collimation, there doesn’t seem to be any easy way to collimate
the scope. I have a laser collimator but I’ve no idea if they are suitable for
this kind of instrument. When you insert it in the top eyepiece holder (using
the flip mirror), the laser seems to bounce out of the scope partially missing
the secondary mirror. With the diagonal, this is better although as shown above
this doesn’t seem to give such good images. I emailed Barry regarding user
collimation and he said given the optical configuration and aperture he thought
that collimation facilities would be superfluous. Bearing in mind the image
quality is good as is I am inclined to agree with him.
Anyway,
clouds dictated that I wouldn’t get a chance to try any DSO’s before it got
really dark so I had to leave my first session at that.
I
got a second chance to evaluate the scope on Thursday evening. Around 10:00pm I
had the OTA cooling and looked out and all was clear. I grabbed my April copy of
Popular Astronomer that had an article on spring doubles and printed off a
couple of relevant sky charts from Chris Marriots Sky
Map Pro.
First
on my list was an easy Regulus in Leo. It was at this point that I discovered
that the finder was just not big enough. Even bright stars such as this were
difficult to locate and centralise. No problem with splitting Regulus through
the scope once I found it though. Next came Gamma Leonis at 4” separation.
This has been a problem in the past with my Newt – I could always tell they
were a double but the two very similar stars were difficult to separate cleanly.
In the Mak it was crystal clear even at x120. At x240 they were very distinct
although starting to get a little blurry. I next turned to Cor Caroli in Canes
Venatici. This pretty double was easy at 19”. Reading through the PA text, it
was mentioned that a 60mm refractor would not be able to split Castor in Gemini.
I had looked at this in SMP and estimated the separation to be approx. 2”. I
turned the scope to Gemini and viewed it at x120. It looked pretty single to me
so I popped in the Barlow with the 10mm and it split quite cleanly. The Airy
disks were interfering a little but it was definitely what I would call a clean
split.
Finally
I turned the scope to Polaris. I tried at x240 and the Airy disk was quite
pronounced and looked fairly concentric.
This
time out I was much happier with the performance of this little scope. I have
only previously had experience with reflectors and so the resolution was quite
astounding to me. I haven’t had an opportunity to try for any DSO’s and
generally the views are quite a bit dimmer than my 8 ¾” so I don’t think it
will be very suitable but for double stars and planets it appears excellent.
Strangely, I seemed to get on better with the mount this time – maybe I’m
getting the hang of it.
Conclusions
Generally I am very pleased with my purchase and I feel it was good value. I think that given the option I would definitely skip the EQ mount and purchase it either as an OTA only for use with a better mount or on the Tilt/Pan head tripod option.
I’m
am however quite keen to know if the scope could perform even better and one
thing that might be worth trying is a quality star diagonal using the screw
mount at the back of the scope. Unfortunately, I don’t have one of these and
buying one would probably cost a good proportion of the price of the scope
itself!
One
thing that I will examine is the use of a camera tripod to try and improve the
stability and portability.
I
was looking to upgrade my old dob and go for one of Orion’s larger reflectors
but I’m so taken with the convenience of a compact scope that I may look more
seriously at the OMC140 although I would quite fancy a go at imaging and maybe
the larger, faster reflectors would be more suitable for capturing those faint
fuzzies. Perhaps I need both!
Gavin
Moore
20th
April 2001