Discovery Truss 17.5"

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Brand and Model:Discovery Truss 17.5"
Price ($USD):$3399.00
Attributes: un-checked Go-To un-checked PEC
Aperture:445mm (17.5")
f Ratio:f/5
Focal Length:2222mm
Electric Power:none
Weight (lbs):125 lbs.
Dimensions (w/h/d):
Description:Discovery's Truss Page

Vote Highlights Vote
Discovery Truss 17.5"
I have had my 17.5TD since May 2001, but have only recently had much opportunity to use it. For anyone who is interested,

here are my comments (long... sorry)...

1 - Structure. The scope woodwork is mostly apple-ply, and is bulky and heavy; one-person setup is not advisable, unless you

are strong and active - being 7ft tall would also help. Pointed fully upright, it is almost 8 feet high, and i need a ladder

when using it.
That said, it is very solid, with no vibration problems - compared with the mount on my Starfinder 10EQ, the Discovery is

built like a bridge pylon, and holds its collimation well (another review mentions flexing of the tubes, pulling the

collimation out - *all* nuts and bolts in the truss assembly need to be very tight, and the bearings need a small amount of

lubrication). Speaking of solid, i have accidentally tipped the optical assembly out of the rocker box twice (before i

installed counterweights) and the UTA did not break. Yet!

2 - Fit and finish. I see a complaint about finish in an earlier review. I had no problems - everything appears square, and

fits together well. The finish is not furniture grade, but that is not why i bought it. I refinished some of the mirror box,

(due to mods i made to the woodwork), using 1000-grit and polyurethane. The scope looks good - nothing to be shy about.

3 - Shroud - the shroud is made from a goretex-like fabric. It is baggy and a bit funky, not cool-looking like the

Starmasters. But it sheds water well - where i live it needs to. Care is needed - if you do not pull the shroud right down

to the mirror box, the shroud might shed dew onto the primary.

4 - Transport - when moving the scope round, the wheelbarrow handles are essential; i can not imagine using the scope

without these. They attach/detach in seconds. To transport it to other sites a van is required. With ramps.

5 - Alt-az movement - i applied Amorall lightly to the alt and az bearings. Movement is a tad light in altitude, ie, it can

"take off" if you move it too fast then let go. Azimuth movement is about right, but a bit stiff near Dob's hole.

6 - How well does it work? Naturally, careful collimation is vital - I start with a Helix laser, and fine-tune with the

Techtron auto-collimator. On a "clear" night (a relative term, since i live in the US Pacific north-west) i can usually

focus cleanly to about 300x (7mm EP), and have a few times been able to focus at 555x (4mm EP). ie, my viewing is

"sky-limited". Since we have nothing approaching clear or dark skies, i can not comment on the scope's abilities on the

faint fuzzies. Messiers are easy.


Wheelbarrow handles - essential
Ladder - essential
Telrad - much easier to use than a telescopic finder
Good collimation tools - essential
Dew-busting system - essential where i live. I can leave the scope out all night without dew problems
Counterweights - essential with 2" EP's
Better collimation knobs - desirable; i used nice Aluminum knobs from Reid Tool Supply - this sure helps

Fans - install your own and save a few bucks
DSC's - I have em but never use em. Had to slightly modify the mirror box to clear the az encoder.
Flocking - i installed Protostar fabric in the UTA and on top of the mirror box. Also baffled under the mirror box.

Would i buy another TD? Yup, but probably a 15". (i am 58, and my joints ache)

Bob Bennett, Maple Vly, WA

Overall Rating: 9
Mount:9 Ease of Use:7 Value:10
Weight: 20 (Notable Vote)
By: Anonymous (
Link to this vote:

Discovery Truss 17.5"
This scope does not come close to a Starmaster or Obsession Telescope in Quality of construction or optical performance.

Overall Rating: 7
Optics:7 Mount:6 Ease of Use:7 Value:5
Weight: 10 (Trustworthy Vote)
By: Staruser
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>This scope does not come close to a  Starmaster or Obsession Telescope in Quality of construction or optical performance.

Or price...

What do you expect for about half the price?  A StarMaster 18 inch is $6395 without options, and an Obsession 18 inch is $5295 without options (prices based on what is listed on their respective web sites).  Discovery 17.5 inch lists for $3395 without options.

I don't think the StarMaster and Obsession are twice as good as the Discovery optically - any optical difference is barely noticeable, and most people probably could not notice ANY difference. 

The differences in quality of construction and ease of use are more noticeable, and probably what somebody considering these telescopes should consider most when determining if the extra cost for the StarMaster and Obsession is justified.

Don't get me wrong, the StarMaster and Obsessions are nicer (and better) telescopes, but that doesn't make the Discovery a bad telescope, particularly if the price is considered.
Honestly have you ever looked through a Discovery 17.5".  I really doubt it because unless you eyes have some integrated optical testing sensors in them it would be near impossible to tell the difference between 1/4 -1/10RMS @5280Angstroms visually.  However, it is a trivial matter to analyze a given optical system per a star test.  I will agree with you on the quality of the construction but totally disagree with the optical performance.  It is a sad to think that most amateur astronomers assume that when a given mirror has some special wavefront test conducted that it deems it superior, without checking it themselves.  I have made 9 mirrors from 8" to 18" and believe that Discovery mirrors are as good as most others ie Nova.  
>This scope does not come close to a  Starmaster or Obsession Telescope in Quality of construction or optical performance.

Discovery Truss 17.5"
A solid performing telescope. Mine is a little too smooth and requires a little effort to balance. Great optics, and very good mechanically.

A slightly lower ease of use factor, simply because this is a big scope, and requires effort to set up. It's not difficult, just big!

Overall Rating: 9
Optics:10 Mount:9 Ease of Use:8 Value:10
Weight: 10 (Trustworthy Vote)
By: rleyland
Link to this vote:

Discovery Truss 17.5"
I’ve had my scope just over a month now and I must say that it’s the greatest scope I have ever used. I actually drove out to Oceanside from West Phoenix to pick it up 241 days after placing the order. When I first saw it, I was amazed at just how BIG it really was. They wrapped it up in plastic (saran wrap) and helped me load it into my SUV, it all seemed to fit with the back seats laid down. The mirror box is advertised to be 75 pounds but there really isn’t any easy way to lift it solo, so a little assistance is always welcome. Most of the time I have to do it myself, but I don’t know why my back hurts…
First light was on a breezy night and this scope doesn’t handle well in the wind even with the sail (light shroud) off. During brief periods of calm, I star hopped around some familiar points and was quite amazed. I could actually see some red in the Orion Nebula and I’ve never seen it so bright as are all my favorite objects. It moves so very smoothly in every direction unless near zenith (the dob hole) as I would expect. I spent a considerable amount of time star testing and the optics are as good or better than any I’ve ever seen. Stars focus to fine points, but brighter stars display the expected diffraction spikes due to the spider vanes. I put this scope through every test I know and even learned a few new ones, and it passes wonderfully.
I had no trouble splitting Rigel and could easily split Zeta Orionis at 277 x which was quite difficult with my 8 inch. Since that first windy night, I have had some calm and very good nights to observe and I have never been disappointed, I was awed by M-51 at 277 x, It looks as good as a photo. I do tend to get a lot more visitors at the bigger star parties and I hear “Oh Wow” pretty often (even from jaded astronomers) as they look through the eyepiece. I have never seen another 17.5 Discovery before mine and most people; even long time Obsession owners are very impressed.
Overall I have to give Discovery very high praise for their product, but I do feel there is room for improvement. As listed:
1. Discovery should not advertise 90 – 120 days for delivery.
2. The woodwork is very nice, but is slightly rough in a few spots.
3. The center spot was off center by nearly half an inch.
4. The secondary mirror was way too low and needed to be corrected.
5. The primary mirror was off center in the mirror box.
6. The light shroud fits far too loosely and I have to use spring clips to tighten it.
7. The upper nuts on the trusses need to be extremely tight or the cage sags and it has to be collimated a few times a night.
I all fairness the people at Discovery were great and I’m sure they would have corrected the little quirks if I would have waited a few more days. This is a lot of scope for what I paid for it and I would recommend it to anyone who is serious about astronomy. Just don’t expect to get it on time.

Overall Rating: 9
Optics:10 Mount:10 Ease of Use:9 Value:10
Weight: 9 (Veritable Vote)
By: Anonymous (
Link to this vote:

Discovery Truss 17.5"
For those who only think Obsessions or Starmaster are the only thing good enough for you, think twice before you badmouth or stereotype products. This scope works damn good and so if the woodwork is not good enough for the modern artesian, I don't care, neither should anyone else, unless fine woodwork is your forte. The optics are excellent and really our eyes can't percieve the difference between 1/8 -- 1/10 wave quality anyway, only by advance wavefront analysis, so why spend the extra money to gain that so called perfect mirror that you visually can not enjoy. I agree certain scientific measurements such a photometry and ccd level work would dictate the necessary means of finer resolution tolerences. I saved over 1,000 dollars that was reinvested in Naglers, since all components in a given telescope are important and a neglect in one area will compromise the quality overall. Finally, collimation learn it, live it and you will love the views. However no cure yet for atmospheric conditions. Finally, there were some minor tweaks to fine tune my scope but the real result is to be gained when searching faint dso's, trying to exhaust the finite numbers of objects possible in the Uranometria 2000.0, but progress has been slower than expected due the real appreciation of natures beauty.

Joe Belden

Overall Rating: 9
Optics:9 Mount:7 Ease of Use:10 Value:10
Weight: 5 (Veritable Vote)
By: jjb4469
Link to this vote:

Discovery Truss 17.5"
The optics on my Discovery 17.5" Truss Tube Dob are excellent. It was like seeing the sky all over again. I had no problem seeing the Central Star in M-57 with so so skies. (SF Bay Area. I saw M-56 resolved for the first time. Seeing M-13, Wow! My main gripe is the Woodwork and finish. I had to refinish it myself. Other than that, No complaints. I figure I saved over $2300 over the cost of a Starmaster or Obsession. I know that it is tall pointed to the Zenith, but I got bold as the night went on using only a 2 step stepladder. I am 5'9, But that was not a problem. No problems tracking also.

Overall Rating: 8
Weight: 4 (Unreliable Vote)
By: Anonymous (
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Discovery Truss 17.5"
Very good optics.
Great mount - smooth and solid.
Swings to close to azimuth bolt for DSCs, had to route a little off the edge of the rocker box to accomodate encoders.

Overall Rating: 9
Weight: 3 (Unreliable Vote)
By: Anonymous (
Link to this vote:

Discovery Truss 17.5"
Woodwork seems to be OK, and optics average and acceptable. Movement was slightly above average. Everything else left much to be desired. Mirror cell could have been much better; three edge supports with 6 silicone blobs as supports underneath. Amazingly it seems to work. When packed and shipped, the mirror had metal dust on the surface that could have done significant damage. Secondary mirror had a couple of small scratches and a blob of some kind of glue on the surface. Fortunately it cleaned up OK. Truss tube design is prone to a little more shifting than other types. More frequent secondary collimation. Poor communication from Discovery, and 7 month devivery time, made the experience frustrating from the start.

Overall Rating: 6
Weight: 3 (Unreliable Vote)
By: Anonymous (
Link to this vote:

Discovery Truss 17.5"
I remember my excitement when I saw my first Discovery 17.5" Truss dob sitting shrouldless next to the other dobs at a February star party. "My oh my is this thing big!", was all I could say when I saw the scope. The Discovery, being an f/5, made all the other dobs look SMALL by comparison. A gazillon questions went through my head - "How are the optics?", "How will it handle?" went through my head.

Jupiter was high in the sky so I asked the owner if we could take a look (we also looked at M42, Saturn, M35 and a few other objects). Optically, the scope was excellent - plenty of low contrast detail on Jupiter, M42 shows detail beyond what I am used to seeing. The optics on the scope were well above the average for what I am used to seeing in dobs - I would guess in the 1/4 to 1/6 wave range. If ranking the scope by optics alone, I would give it a "9" rating.

Since the scope is an f/5, collimation was not that difficult to achive (no super fine tuning like some f/4 scopes I have seen). A couple of tweeks with a laser collimator and you were ready to go. I did notice that in the short viewing sessing that I had with the scope, we had to re-collimate 3 times! Each time the secondary was off enough to warrant so-so images.

Also, since the scope is an f/5 it is T-A-L-L and requires a pretty large ladder to get to the top - a scary proposition in the dark. If you are clumsy (like I am) being 5 or 6 feet off the ground left me nervous and ready to come down - I did not concentrate on viewing as much as I was concentrating on not falling (to be fair, the owner of the scope had a shakey ladder - if a solid one was supplied I would have felt better).

The most disappointing feature of the scope was the difficulty I had trying to move the scope or track an object in the field of view. The scope is in no way an Obsession or Starmaster in terms of ease of movement. I often found myself plunging past the object since the mount DID NOT WANT TO MOVE in any direction (alt or az) especially near the zenith.

It was at this point that I realized why the scope was always off in its collimation - so much tugging and pulling was required to track an object that the L-O-N-G truss poles were flexing - causing difficulty in tracking AND throwing the collimation off. I think that this problem is built into the design of the scope since it is so tall (f/5) and hopefully it can be fixed. If I were to rate the mechanics of the scope, I would give it a 5.

I hope that I don't sound overly negatative but it is my belief that scopes should be easy to use and not require constant attention just to view through them. What is the point of great images if collimation is constantly required? What is the point of great optics if have to fight the scope to track an object? I have always had a bias towards easy to use scopes (other than large aperture, isn't that what dobs are all about?)

To be fair, I think that some of the problems with sticktion in tracking might be able to be solved with better teflon possibly or if one is handy and knows how to "improve" on such things.

In summary, the scope has great optics but was somewhat a chore to use. Mechanically it left something to be desired. In fact, the mechanics alone swayed me away from purchasing one. To be fair, the mechaincs of this scope could be due to this one unit and not to the the scopes in general (this was one of the very first truss dobs to come out from Discovery). If one is handy, I think that most of the problems MIGHT be able to be solved. However, it is my belief that it is a design flaw (the scope is too tall) and has too much flex built in. If these problems can be solved I would give it a solid "9" for performance.

Overall Rating: 7
Weight: 1 (Unreliable Vote)
By: Anonymous (
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