GSO 16" Truss Dob Owners- Discussion Thread

[glend]

I am starting this thread because of some issues that have popped up with my GSO 16" Truss dob. I know at least one other owner has had similiar issues and hopefully this will serve as a place where these things can be discussed and assistance provided.

First, I noticed today that Andrews Communications is advising on it's website that it in bringing in another shipment of the 16" dobs and the price has now dropped back to $1999 again.

Aperture $ value draws many people to the GSO 16 along with a pretty reasonable optics package for the $. Of course a budget price means that some other areas of the scope are probably either lacking in quality or in need to some owner work to improve the package performance.

It's a big heavy beast, obviously not as big or heavy as say a 20"+ dob but as a mass production unit it's near the edge in terms of ability to transport and manage. Despite breaking down into major components, transportation is exactly where my issues start:

I took my 16 up to Bretti last weekend, and carefully packed it vertically supported in my vehicle for the journey. Obviously collimation is a fact of life each time this scope travels, and on assembly at Bretti I found a problem that I had not encountered at home - lack of sufficient infocus on the shorter EPs. My 2" 30mm EP worked great as always, but I could not get anything below that to focus. You will be thinking the focal length is too long, and your right, when I moved the mirror up on its springs I managed to get focus again - but what changed from the home setup? Could the mirror have moved down during the trip - yes I may have left the locking knobs too loose.

I recollimated everything up at Bretti and the scope worked great from then on, but it was making some clunking noise towards the end of the last night which sounded like primary shifting to me. Yes, the GSO springs are probably in need of an upgrade. On the trip home the noise from the lower tube section got worse and it was obvious the mirror/carrier was moving around - while I did tighten up the spring bolts a couple of turns for the trip home it wasn't enough. When I checked at home I found that two of the three spring bolts had dropped out and the springs were in the bottom of the tube. There is no way of locking that heavy mirror from upward bump travel and it had worked the bolts loose on the bad road back from Bretti. Subsequent PM discussion with glenc revealed that he had an identical problem with his 16 on the way back home on a rough road from a weekend away. So it would seem that the 16 needs a primary locking system, and not just the locking support bolts because they only stop compression (they are not tapped into the carrier but only push against it) - its extension that's causing the problem. The answer may lie in through drilling and tapping a hole in the carrier to serve as an upward movement lock.

Regarding the infocus problem, here are my views. When the scope is packed by GSO the mirror seems to be to torqued down for shipment and thus way to the rear of optimal focal length position. Now as we all know, Andrews does not open the boxes or do any sort of setup on these scopes before they are warehouse shipped to the buyer. There is an obvious assumption that any 16 buyer knows dobs and how they should be setup, and that's ok as these are not usually something that a novice goes out to buy, but a sheet of paper reminding the purchaser to optimise the Focal length during collimation would be a good idea.

Related to this was a very loose secondary when I unpacked mine, ok again I know what to do about that, but it's potentially a big problem if not caught before assembly. This could be a shipping problem but it's something to watch out for when you open the top section box. Check it before you lift it from the box.

My 16 is now perfect again and giving me amazing views but it does require tuning work, don't expect to just open the box and put it together and find it's optimised. There is a learning curve but you will be better prepared for any future with this knowledge.

I know glenc has had his primary mirror (escape) issues when travelling but I would like to hear what other GSO 16 owners have to say as well.

I suppose this could be a GSO 16 user group thread, so let's hear from the other owners out there. I know there are a fair few because Andrews keeps selling these things by the container load. Maybe we can all get together one day at a dark site.:thumbsup:

[glenc]

Thanks Glen. I replaced the spring bolts on the primary with slightly longer ones today.
Unfortunately the new ones need a screwdriver to adjust them.
Where can I buy stronger springs?

[glend]

Most of the larger Bunning's Warehouse stores have compression springs of various sizes. I did a search and found one Aus guy that is using spring C-822 from Bunnings. Search on the Bunnings website and you can find the springs. That C-822 seems a little long to me (at 4" extended) but I guess when compressed it seemed to work for the guy I found in the search. Here is the link, page down to his entry (name is Duffy):

http://www.myastrospace.com/forums/forum/equipment-discussion/3534-16-gso-upgrade-advise

The Bunnings info:

http://www.bunnings.com.au/spring-compression-century-15-16x4in-c822_p3971210

Just be careful that you don't stack the spring (compress it so much that it becomes a solid coil), I guess it could be cut down by a couple of windings as well if needed.

I haven't tried these myself.

[glenc]

Thanks Glen

[astro_nutt]

Hi Glen
I had the same issues with my 12" dob and I changed the primary mirror adjustment springs with heavier ones. I cut them to the same length as the originals by using a pair of pincers and used my 4" vice to provide the pressure! A grind over the sharp bit then a coat of nail polish will keep it clean. I used the original springs to give the locking screws some resistance. Try some blue Teflon tape around the locking screw thread to stop it coming loose.
Cheers!

[glend]

After another night with lack of in-travel to achieve focus on the higher magnification eyepieces, I went ahead and replaced the stock springs and the adjustment bolts and locking bolts - on the basis that this will allow the primary to be moved upward enough to achieve good focus range of travel, and hold the mirror securely. I got tired of trying to find that sweet spot where the bolts still held the mirror carrier (without clunking around) and I could use my higher magnification EPs.

I chose the Century C-792 springs (from a Bunnings that stocks them) cut them in half and they still provided 10mm more un-compressed length (at 40m) and a stiffer spring overall, and the adjustment bolts were changed to M8*50mm (an extra 7mm of length), locking bolts to M8*50mm. These bolts were a test setup and they seem to work fine, with lots of range, so I will try to find some M8 star knobs of that length.

This problem needs to be addressed by GSO back at the factory.

Testing again tonight, will report back.


Update on last night's testing - the scope is now perfect. :eyepop:

Star test was great right through the EP range, and focus achieved on all EPs right to 300x (6mm). Tried it out on a range of DSOs and the collimation held true through all angles of altitude, including the Orion Neb at very low angle just above the trees, and this was a problem with the stock springs.

All good.

[glenc]

My son-in-law Tim had some 20 mm thick monopan left over after he built a caravan.
He cut two 800 mm diameter circles for the base of my GSO 16" and the scope is now 8 kg lighter.
It also rotates better, its very smooth.
The rest of the base is still the same.

Monopan 20 mm thick can support a weight of 380kg/m2
http://www.monopan.ca/about/

[glenc]

The springs on my 16" primary mirror are C-836 from Bunnings. I cut them in half.
The focus and collimation used to vary, now it is very good.
It is an excellent scope with monopan and the new springs.

[AG Hybrid]

This thread needs more Glen's.

On a side note there is a GSO 16" review on CC with recommendations of some changes that could be done to improve performance.

Also, upgrading the springs on my 12" allowed me to throw the locking screws.

Do you guys use mirror fans? How long does it take to cool your 16" mirrors? They are 1.8" thick are they not? Must take a while until they are at equilibrium.

[glenc]

I don't use my mirror fan.

[Satchmo]

At 1.8" thick crown glass ( 3 X higher coefficient of expansion to Pyrex ) the figure of the mirror is very unlikely to ever reach an equiliberated state. I have had reasonable results with plate up to 1.3" thick. This is essentially one area where there are no free lunches on large low cost scopes.

[glend]

I haven't reported back lately. The 16" GSO is performing great since the primary spring upgrade. I have done a few other things as well:

Installed string trusses (2) as per Dennis Steele's Dobstuff String kit. Instead of wire I used Dyneema rope (well 2mm dark grey Dyneema actually). The beauty of Dyneema is that it can be tied (with the right knot) and it is easy to work with, and does not reflect light the way a wire string would. With the string trusses in place the scope now holds collimation through the arc of its altitude range. I will try and get some photos up of the string trusses soon.

I made a new fan shroud out of shin EVA sheet and it totally seals the rear of the scope except for the square cutout for the fan air entrance. The air positively rushes out the top of the scope now.

I had the scope out at the Lake Chaffey Dam dark site and we had two really good and really dark (like SQM reading of 21.77 dark) nights. First night (no wind) and I had a great time with the Horsehead, and actually got another IIS member there to look and confirm the Horse (such was my disbelief at the clarity). Ran through all the nebulas up at that time. One night up there we had a fairly heavy breeze and my Astrozap shroud was acting like a sail and swinging the dob off target, much improved when I dropped the shroud down around the bottom section.

This past week I have been working on the planets: Jupiter, Saturn and Mars, and have had some very good 2am-4am sessions when the moon is down and the planets high ( a fringe benefit of being retired - my time is my own). I am also running a side by side planetary comparison with my new 102mm refractor riding on my iOptron goto mount. The refractor provides very crisp high contrast views of the planets but I can't push the magnification the way I can with the dob. The Dob wins in my view because the object just has so much more size. IN the refractor's favour, no collimation concerns and pin point stars all the time (no coma).

As far as the fan question is concerned, I usually run it when viewing but I didn't run it at Lake Chaffey as the scope was sitting out all the time (under it's cover and it was always at equilibrium as far as I could tell). At home it is kept in the garage (which is not heated or cooled) so it is as near equilibrium as I can make it). Mark if you can't say something nice why bother...frankly I don't care, I love my scope and what it's giving me. This was suppose to be a 16" GSO Dob owners discussion thread.

Will report back later.

[Gem]

I usually turn the fan on 2-3 hrs before taking it outside (my dob lives inside the house). I then leave it for another hour cooling outside before use. This is more of a routine of mine than exact science. I find it definitely gets better with cooling though. I also find the scope cover for the trusses has improved the performance.
No focusing issues for me though. Guess I was lucky. :)

[sn1987a]

For what it's worth I don't have the fan on my 16 Lightbridge any more. I stripped it off and chucked it. I find if I put the scope out around an hour or so before observing it cools down enough to use. I removed all the silicon and the sticky pads on the triangles the mirror just floats on bare metal. My 18 has no fan either and doesn't need one.

[glend]

Just a final comment on this equilibrium concern, this has been debated across the internet for years, and in astronomy glass material opinions are like navels - everybody has one. There is little difference between BK7 and Pyrex in practice, however, BK7 is generally acknowledged as being as better optical glass (more homogenious, which may not matter much when used as a mirror but does as a lense). All you have to do is google BK7 vs Pyrex and there is heaps to read on the subject, including an interesting IIS thread where Mark raised the that claim originally. There is also plenty written about GSO mirrors.

BTW the 16" mirror is 45.3mm thick.

Suffice to say, that given the 16" GSOs great fan there is little to be concerned about. If you want the ultimate in equililibrium tooling do the GSO fan mod covered in the the Projects & Articles section, and install a tube baffle to direct air over the surface of the mirror and shroud the rear.

[Satchmo]

For the benefit of the `owners thread ' it would need to be pointed out that replacing your 16" 1.75" crown glass stock mirror with a 1.6" Pyrex mirror has clearly payed you dividends in the areas of equilibrium of figure. :) ( sorry guys I thought it fair to point that out.)

[Satchmo]

I think it is reasonable to discuss the optical figure on these mirrors- without a discussion of that - discussion becomes distorted towards fan vs no fan and doesn't take in to consideration the original figure.

I think the Crown glass equilibrium issues make it harder to nail the figure during manufacture, as they do not really have time to leave mirrors sitting over night to fully equilibrate after polishing and handling in the final stages .

All three 16" GSO /Meade mirrors I measured came in at around 2/3 wave wavefront under-corrected . I suspect they leave them undercorrected to counteract the bending `bi-metallic strip' effect a mirror undergoes when there is temperature difference between the centre and the thicker edge.

This under-correction may help to give a sharper image earlier in the night but not later. Undercorrection is better than over-correction at the eyepiece, so if time and available material is an issue it is better to leave a mirror under-corrected.

[AG Hybrid]

Very interesting post.

But, we can all be honest here Mark. No body on this forum is going to argue with you about mirror quality and material suitability.

[glend]

Here is the information from Dennis Steele's Dobstuff page on how to go about putting string braces on a strut based scope.

http://www.dobstuff.com/string.htm

I utilised this design on my bracing, but used Dyeema 2mm string in place of the wire. The top section connnection attaches at the point where the stut bolts go into the rim and the bottom attaches to the steel tab on the rim exactly half way between the strut mount - I drilled a hole in the tab to tap in a bolt where the turnbuckle hook attaches.

As promised I will get some actual photos of my solution soon.

The reason for this mod is to stiffen up the structure and prevent collimation drift when the altitude is changed.

[sn1987a]

Ha!, well there you go that never occurred to me. Still money well spent. :thumbsup:

[glend]

So in summary, strut (and truss) dobs sometimes suffer from collimation drift when moved from zenith to horizon, and therefore most people collimate at 45 degrees to minimise drift when altitude changes. It is easy to check to see if your dob suffers from collimation drift, just put a Cheshire or laser in the focuser and move the scope from zenith to horizon and watch the collimation target. If you see drift off of perfect collimation then you have some flex in the structure.

This is possibly caused by a few things: 1. top section sag, 2. top section rotation due to focuser weight, and 3. mirror movement. The first two are addressed by this mod. When we put heavy items like coma correctors, 2" EPs, etc in the focuser the weight on the top section is increased to the point where the section is placed under rotational forces or sag when moved to low altitudes. The string brace provides two triangular braces to the top section, pulling in opposite directions, which creates a virtual truss configuration. The string braces stop rotation and sag by securing the top section to the lower tube top ring.

Here are the string brace photos I promised. Based on the Dobstuff article (linked to in previous posts), and materials sourced from Bunnings and ebay for the Dyneema 2mm string. The turnbuckles have heat shield slide overs to black them out. The Dyneema is attached to the top bracket with 2mm wire thimbles (just to prevent cutting by the bracket), it can be tied to the turnbuckle hoop with a double figure eight knot. Tensioning is by feel (although you can get sailing standing rigging tension measuring devices - they are expensive). Just make them equal and pluck them like guitar strings to get them the same. Don't make them so tight that they pull the scope out of alignment.

For those curious about the yellow hoop (its black on the inside), it's electrical cable snake material and it's an idea I picked up from Arthur A to keep the shroud from sagging into the field of view.