Discovery 17.5 inch Split Tube
Basic but Good
I am no way associated with Discovery or any
other telescope manufacture or reseller.
I have been involved with visual observing, ATMing and some astrophotography
now for twelve years. I have owned
numerous Dobs that I have built myself and owned various other types of
reflectors, refractors and cassergrains.
I have used a very wide range of telescopes, from small 60mm refractors
up to a 25 inch cassergrains.
Why a Discovery Dobsonian.
My plan was to order the optics about 16 to 18 inches
and build a Dob. But due to a
career change, resulting in moving 3 times in the past 3 years and likely to
moving again at least two more time in the next year or so and no wokshop for
this time. I decided to cheat and
buy a pre built Dob for now until a workshop was available to myself again
(When the moving is over) then I would build my own Truss structure for the
optics. I did look long and hard
at the premium truss manufactures, no need to build later, but I do like solid
tube Dobs, I enjoy how impressive a solid tube Dob looks. The down side to solid tube Dobs of
course is the impressive effort they take to transport.
First delivery, the telescope came in four very
large boxes, one each for each half of the tube, one for the base and one for
the mirror. Each box was filled
with the expanding foam which stuck like glue to the boxes, the tube boxes
where destroyed while getting the contents out, but well packed. Unpacking the telescope and assembling
it took 20 minutes.
Boy, this is big, this is a nicely made Dob and
boy oh boy this is big. Having often
used truss Dobs in this size and allot bigger but never a solid tube in this
size, nothing prepared me for the size of this thing. The first item that went through the mind was maybe I should
have ordered the Truss Model. The
initial shock at the size of the scope did go quickly. Above is a photo of my wife standing
next to the Dob, she is 5 foot and 5 inches tall. The ATMer in me could not help seeing a few simple modifications
that will be done. Overall, this
appears to be a very basic but nice, well built Dob, it is not off Obsession build
quality but the quality of build is good. The base is made from 23mm plywood and finished off with
clear vanish coat. The tube is the
standard cardboard Formatube, painted and nicely blackened in side, all bolts
that protrude into the tube have been blackened out too. The bearings are 8 inches in diameter
and nicely made, they attached to the tube to brackets that the position of the
bearings can be adjusted for balance. The bearings sit on Teflon pads, the bottom bearing sits on six
Teflon pads. The focuser is a JMI
2 inch, a nice Novak style spider and secondary holder.
Two grab handles on the bearings and two handles
on the upper section of the tube are well placed and very handy to have. The bottom half of the tube is 50.5
inches in length and the top half is 40 inches in length.
Side Bearing with the grab handle
protruding from the bearing
One of three connectors joining the
two halves of the tube assembly
This telescope is nothing fancy it is just a
basic nicely built and finished Dob. The three connectors to join the two halves of the Dob and
have proved to be up to the task.
Time to install the main mirror, The mirror came
in a cardboard box filled with expanding foam, I did not like the look off this.
It is an okay way to pack a mirror
but when that mirror has to travel halfway around the world, a bit better packing
would be appreciated. A inspection
of the box showed no external damage to the box, in fact not even a scuff mark on
the outside was visible. Upon opening
the box and carefully removing enough of the foam to allow easy removal of the
mirror the mirror which was lifted by the cell out of the box up on to the
workbench, the mirror was shipped installed in the primary mirror cell wrapped in
plastic. Everything looked fine,
placing hands under the back of the mirror to move it, a piece of the mirror come
away, not good.
Damaged mirror in Plastic Wrapping
Pieces that fell away
Damage and showing
basic mirror cell
This is a two-inch thick mirror
so a little bit of damage like this should be okay, the mirror should still be
useable. But two concerns. First is two stress fractures, one can
be easily seen in the above photo, that will be a problem in the future if not
now. The next is there was no
damage to the box or packing. Was
the mirror packed with the damage done already? Thank god for insurance. Now the long wait for the replacement mirror to be made and
shipped with the insurance claim to process.
Okay even with a damaged mirror
lets see what this telescope is like.
I have now been out under the stars over a dozen times with this telescope.
Even with the split tube, it is
no easy task to setup. This is not
a truss tube so bulkiness is an issue.
Setup and transport by one is possible but two people would be wise, a
set of wheelbarrow handles and wheels installed on the base is even wiser. Transporting the telescope is a task, a
medium size 4WD station wagon with back seats folded forward fits the scope all
in with not allot of room to spare.
This is not an easily transported scope and a reasonable size station
wagon or van is required.
I have been very unimpressed with
the moment on most small commercial Dobs.
Installing a spring to add load to the altitude bearing is cheating and
a sure sign that the bearings will have a sticky movement. If built correctly all that should be
needed is a counter weight for balance on for swapping between eyepieces, finders
etc. A Dob mount is a simple thing,
very easy to get right or wrong when building. The movement in both axes of this telescope is very good,
there is always room for small improvement. This is not an Obsession Dob and to expect it to move like
one would be unfair, but it is pleasing to use. So much so that there has been no thought of modifying the
bearings. The movement is good
enough that no thought is given to move this telescope across the sky, no
sticking and same effort to get the scope moving and then to keep it moving, it
stops dead when the pushing stops, this is what a Dob mount should be like and
Discovery delivered. There was bit
of vibration at first, the simple and easy fix was some corner bracing to the
base which is a common fix done to this telescope, several different examples
of this in photos can be seen at the Discovery Telescopes Yahoo group.
I am very fussy when it comes to
collimation, a well collimation scope is very important to me and I notice very
quickly if the collimation is not good.
The Secondary holds collimation well, the primary needs to re collimated
several times through the night, usually 2 – 4 times in a 8 to 10 hour
observing session. The primary
mirror cell is a standard good basic design and does the job, the primary
mirror cell will be replaced/modified, not because it is bad, I just do not
want to re-collimate as much as I do now, room for improvement here. Other people may overlook this but I cannot.
My eyepiece collection is very
good, a full set of OU Orthos and MK-70 Kongs, some Tak LE’s, Naglers, Pentax
and other good eyepieces sit in the collection, all used during star testing.
Let me start by saying this is a
damaged mirror, I did not expect textbook star images, but this is what was
seen. First though was moving the
mirror forward an inch so focus could be achieve with high power
eyepieces. Achieving focus with 16mm
to 40mm eyepieces was okay but there was not enough in focus for 5 to 12mm
eyepieces. Three holes for the
mirror cell bolts where drilled in the tube one inch forward of the current
ones. Measuring the focal length
the focal ratio was found to be F/4.87, which I considered an acceptable
variation, but Discovery should have mounted the mirror in the right position. Talking to the owner of the other 17.5 ordered
at the same time as this one found he had the same problem, he moved his mirror
1.5 inches forward. Once the mirror
was moved forward the optics appeared to be excellent, there is a little, got
to look for it, coma on the very edge when using a high power, eyepieces, apart
from that the main mirror has delivered near text book images, what more can
one want. Hope the replacement
mirror is as good. When star
testing the optics Airy disks are real close to being textbook again except for
a very small area that corresponds to the where the damage is on the mirror is
and then it is only just noticeable.
Visually it is hard to fault this set of optics despite the damage to
the main mirror.
First thing was to buy four 90-degree
metal bookshelf brackets that where used to brace the corners of the base, this
removed vibrations. In addition,
the mirror cell has been moved 1 inch up the tube, Also wheelbarrow handles
with wheels where added for rolling the scope out of the shed for
observing. A Telrad and the
process of considering what is to be done to the main mirror cell or replace it
as to hold collumation better. As
stated before the mirror cell does the job, it supports and holds the mirror
well. Cool down time has not been
an issue at this time of year, expect it will be during summer so then installing
cooling fans for the optics will be considered.
The wait was worth it, hope the
wait for the replacement mirror will be too.
This is a simple solid tube Dob,
not a Obsession Dob so do not expect the Quality build to be Obsession
standard. What is important is
“Functionally of the Telescope” which is very good, a couple of very minor
issues that are easily fixed or they could be overlooked if one didn’t want to
do a thing to the telescope, except for having to move the mirror forward, this
should not have to be done. It has
to be said again, this telescope is huge.
The split tube is very handy
making transporting possible, (note that the word easy is not used here),
without the split tube, transporting this scope would be impossible for me, I
would also expect most people would be unable to transport a non-split tube
telescope of this size. The Dob
movement is good and the optics despite the shipping damage are excellent. Eyepieces that are proving popular in
this telescope are a 5mm XM Pentax, 7.5mm Tak LE, 9 & 12mm Nagler, 25 and
40mm UO MK-70 Kongs. This whole
telescope cost the same as a mirror from some mirrors manufactures so it is a
value for money telescope. I will
be building a truss structure sooner than later due to make it easier to
transport if my brother will allow me a corner in his workshop.
If I had to describe this
telescope in one sentence I would say, it is a basic, good, honest, very very
large heavy Dob.