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Home / Discovery 17.5" Split Tube Dob
by Phil Smith 09/19/07 | Email Author

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Discovery 17.5 inch Split Tube Dobsonian

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.

First impressions

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.

Setting Up/Transporting

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.

Bearing movement

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.

Collimation

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.

Optics

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.

Modifications

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.

Overall

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.



Phillip Smith
Mount Gambier
Australia

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