16" dobsonian underperforming?

[Andrew*]

I am not convinced my 16" is doing as good as it can.

At low power, stars are very small, but at anything over 100x, stars are blobby and very careful focusing doesn't result in a tiny star. This means even "easy" doubles are out of reach. Epsilon Lyrae barely shows elongation. This is under good seeing and high altitude. Last night for example I observed the doube double in my three scopes. The ED80 gave the cleanest split of the three, but still there wasn't black space between the components. The 16" came out worst.

Also deep sky objects are not significantly brighter than the 8". It's as though there a lack of contrast. It's difficult to quantify this, but I feel the two things could be connected. I also don't believe the less-than-perfect skies are to blame. The Milky Way was obvious last night, and M43 was only barely visible, even with the help of filters.

Possible problems:

1. Collimation - it's not far out at all. Visually with a film cannister collimator it's as good as it can be, but I haven't done a successful star test yet. Could the last few mm of collimation result in poor images? Well, from my experience of my 8", no. Perhaps the f/4.5 is just that much more critical about collimation than the f/5...

2. Spherical aberration from somewhere? I don't know, but as far as I know the symtoms are very similar - not getting a clean focus at high power.

3. (REMOVED)

4. unclean primary. I doubt it - there is some dust on it of course, but I think it's impact would be minimal.

5. Badly figured primary? I just wouldn't know.

Does anyone have any thoughts?

cheers,

Andrew

[Astronut]

The only thing I can suggest is starting from scratch with collimation. It can be a pain I know but at f/4.5 I would spend as much time as possible getting it right. I've had better success with collimation with my f/5 after getting a Catseye kit but you may not be willing to spend that amount of cash on collimation gear. A more inexpensive option would be a standard cheshire - this would still give more precise collimation than a home made cap made from a film canister although I've found it next to impossible to see the outer edge of the secondary in a 2" focuser with 1.25" adaptor attached using a standard 1.25" cheshire - a cap is more suitable for this step.

I've found doing a star test is next to impossible for me. There always seems to be too much turbulence up there to see a decent defocused airy disc and it's quite small anyway because I can only reach a max magnification of about x230 with my eyepiece collection.

Just a thought and you've probably thought about this and checked it but is the primary mounted in it's cell properly ie. not pinched or held too tightly?

As for lack of contrast it could possibly be the result of stray light entering the light path - do you have a truss shroud?

Best of luck in your endeavors.

[Sam]

Andrew, Astronut is bang on. It's a whole different ball game collimating a big scope - it took me ages to get mine right and even now it still takes quite a while before each session. Good luck!

[barkis]

Andrew, I'm with Astronut on the collimation. At f4.5, there is little, or no tolerance, It has to be spot on mate. Also, having such a large objective also means any atmosphere anomalies will be so much worse in a light bucket, so good seeing is another prime requirement. I remember well the frustrations we had with our 16" f6 scope in the BAS observatory in Carlisle. The southern part of the sky had the local Technical college in the foreground. It was a flat roofed building which was occupied sometimes late into the evening catering for various courses. As a result, the warmth rising up from the roof destroyed any hopes of the scope performing to it's capability. I know you probably don't have any issues like that, but I was pointing out the damage poor seeing can do to a large instrument. I would give it more time, having got the Colli spot on. Use a Barlow, or Imagemate to increase the focal length for doubles. don't depend on High power eyepieces. Theoretically, the resolving power on that instrument should crack doubles with ease, but only if all the other parameters are in place too. Have you checked that the mirror is comfortable in it's cell?

Not pinched in any way. I wouldn't know if there is lateral adjusters that may be a bit tight, or even the retaining clips holding the mirror from tipping forward. Hardly likely, but a thought anyway.

As far as getting the mirror tested, I have a Foucault tester, but it is not really suitable for testing a 16". I could probably determine if it was under, or over corrected, or if it was suffering from spherical aberration at all.

I somehow doubt if the problem is the mirror. The other areas need to be addressed first.

Good luck mate. Hope it works out.

Ron.

[brympton]

Andrew,

Got to agree with the last two (now three - must learn to type faster) replies. I had similar symptoms to those you describe in your first paragraph with a SW 250px until I borrowed (then bought) an AE Cheshire. Not an expensive item given the investment you have already made in the Lightbridge. My new OTA is f4.5 also and I thought it may be worth investing in the CATSEYE system - and I'm pleased to say it is. I used to have a laser collimator which was only really useful for primary alignment (fine provided that the secondary is adjusted properly). I still spend a ridiculous amount of time on secondary alignment but it's always worth it

[ringz]

Andrew,

You could try an aperture mask to reduce the effective aperture to something smaller to see if it's the seeing causing you problems or something else.

--

Martyn

[Andrew*]

Hi guys, and thanks a lot for your help.

I will do as you suggested and get smarter with my collimating. I admit I'm usually a bit hasty with it and it's probably showing. I've just emailed a guy on AB&S about a cheshire eyepiece so we'll see if it comes through.

I had one before, but I went and sold it for a Baader laser.

Andrew

[Merlin66]

I've build and used fast reflectors (<f5) from 13" up to 29".

If the primary mirror is OK, the 9 times out of ten any problems are associated with seeing conditions and collimation.

One of the best things you can buy for a "fast" dobbie is a Ronchi grating; not only will this show the collimation but also the state of the image when observing. generally straight, even and clear lines close to focus on a star will give good to excellent images.

[Casus belli]

Silly question I know but did you seat the truss rods correctly? I once set up and noticed results much like yours and it was due to one truss rod not being seated correctly which threw the collimination waaaaayyyyy off.

[Astronut]

I've build and used fast reflectors (<f5) from 13" up to 29".

If the primary mirror is OK, the 9 times out of ten any problems are associated with seeing conditions and collimation.

One of the best things you can buy for a "fast" dobbie is a Ronchi grating; not only will this show the collimation but also the state of the image when observing. generally straight, even and clear lines close to focus on a star will give good to excellent images.

I'm unsure how a ronchi grating would help with collimation. I got one of these from OO a while back (a very fine 250 lines/inch) and it was only to allay any worries I had about the mirror. If the lines are straight and parallel and don't change angle as you shift from one side of focus to the other then the mirror is figured correctly. I rotated the ronchi at all angles to check for curvature and such across more than one diameter and the results appeared to be the same.

I was worried about mirror astigmatism which prompted me to get mine but I'm quite happy with the test. If there is an angle shift of the lines when going from one side of focus to the other I can't detect any although an expert eye might see something. I've only used it once and I'd like an opportunity to try again, weather permitting. After using it the first time, I concluded the astigmatism I was seeing in my low power eyepiece was down to my eyes and a check on my last prescription from my optician revealed 0.5 dioptre astigmatism in each eye which is pretty mild but noticable with a large exit pupil eyepiece.

http://www.orionoptics.co.uk/OPTICS/telescopetestpag.html

[Merlin66]

That's fine, but the Ronchi can do much more than that.

Close to focus, you should be able to get two or three broad lines across the star image; if the edges of the dark bands are smooth, parallel and straight to the edge; you may have to watch and wait while the atmosphere steadies, then the focussed image should be pretty good. At focus, the image should darken evenly all over and present a soft grey uniform disk; atmospherics and tube currents will show up as dark wavey lines ( but you probably know that already!)

If the optics pass this test, I'd be VERY surprised if a 16" can't EASILY split and make good Ohhhh Ahhhh images of the double double......

[Doc]

Hi Andrew.

I think my collimation is slightly out in mine as well. it makes the stars very slightly oblong looking and even though I can split the double double it's not a large clear gap between them.

I also use a collimation cap and looking through it the donut is bang in the middle where it should be. But being a large dob I think the secondry needs to be offset slightly and I don't know if mine and maybe yours is set correct.

I'm getting a baader laser collimator in a few weeks so will try setting it up so it's spot on.

Other thing I've found is the scope takes at least a couple of hours to cool down. I've seen smoke like trails coming from a defocused star on mine. After it's cooled it's a lot better.

Having never looked through a big dob before I have no comparisons to make. Objects are big enough, contrast could be better but I don't know by how much.

You have much darker skies then me as well so your view should be better.

[Milamber]

So then - are all you people aware of the mirror cell issues with early Lightbridges? Many of them - and the larger the mirror the more sensitive they are to the problem - were shipped with the mirror clamps way too tight. I would suggest that the very first thing you do before trying to collimate something that may be uncollimatable (to any degree worth looking through) is to undo the screws around the base of the mirror box and lift the tube off. The three rubber-padded steel clips that hold the mirror have a couple of screws each that will need to be undone and then re-tightened allowing ther clamps to sit slightly looser. It is these three clamps that, pressing on the edge of the mirror, will make things very odd especially if you are a double person rather than a DSO type. If you look at them you will see that the shape of the clamp forces it into the mirror surface as it is tightened... so be careful or you may end up back where you started.

Once you have this sorted then collimate and go try again... you may be *very* surprised.

Arthur

[Astronut]

I've been lead to believe the collimation springs in the primary cell were too weak on early Lightbridges - bobs knobs have replacements. This was back when 16" lightbridges weren't available so it may not be a problem on the 16" Lightbridge.

Might be worth checking these out while you've got the cell out.

[Andrew*]

I will check the clips and springs. I believe this is a newer LightBridge - it has a dual speed crayford as standard, which I think the older ones didn't...

Perhaps someone with know-how at Kielder would be willing to take a look at it for/with me. Any offers?

Andrew

[zero]

Only newer lightbridges have dual speed, and there are no cell issues associated with the 16" that I've come across. I'm sure it's just collimation. Contrary to popular opinion, the Chinese produce consistently better newt mirrors than anyone (in my opinion), so I'm positive it's not an optical defect. It could be that the clamps are tight also, but that wouldn't cause the problem, it just wouldn't help it.

Stick with it, as once sorted it'll be fantastic. I have the original prototype that Meade sent to all the trade shows around the world prior to it's release. I collimated it before Astrofest last year, bought it back, used it, taken it to Hertmonceaux etc.. and it's still a cracker -though it's looking tired.

Hope you get it sorted.

Steve

[Milamber]

Well, I can name a few people that have had this issue - people that really *do* know what they are talking about wrt optics. No matter, maybe they are wrong then?

Arthur

[Andrew Robertson]

Andrew,

As Arthur mentions the problem is most likely pinched optics due to the main mirror being held too tightly by the 3 clamps. I have a 16" lightbridge (it's about 18 months old now) and it displayed astigmatism when I first bought it and I hate astigmatism more than any other optical defect. As such I was unable to resolve close doubles such as epsilon Lyrae properly. Closer inspection revealed the 3 clamps to be tight on the primary mirror. These are saftey clamps and should not be tight against the mirror. Raise them such that the cork gasket is about a millimeter above the surface. After doing this to mine using x300 on Epsilon Lyrae you could drive a bus thro' the gap! Central star in M57 has been observed several times with this scope which would not have been achieved with the aforesaid pinched optics.

Andy.

[zero]

Hi Arthur,

I haven't come across this issue, and with all the lightbridges sold, and running the returns dept for the Meade importer I normally get to hear of such things. I wasn't suggesting you (or they) were wrong for a minute, or calling into question anyones optical knowledge. I was simply saying that I haven't come accross it as a 'known' issue (rusted PST's as an example). You clearly have, and from people that know what they're on about, so my apologies. A pinched primary wouldn't normally be the primary cause of the lack of resolution that's being experienced, as it has all the hallmarks of a collimation issue. It would, as has been pointed out however induce astigmatism and prevent accurate collimation and focus. So in that respect you could say that a tight cell could well be part of the issue.

I realize that I have a long way to go before I really *do* know about optics, but then surely that's half the fun. and it's a good job there are plenty of people that know far more than me.

Anyway, I wasn't out to disscredit your post or opinion in the slightest, and I'm sorry if that's the way it read -I'm sure you know me better than that.

ATB

Steve

[Milamber]

... well, my post *now* looks a little abrupt so sorry for any naughtiness implied Steve - I didn't mean anything (you know me as well :mrgreen: )

I was actually under the impression that Andrew (Robertson) had mentioned the issue to "the importers" once we had it sorted... maybe something got lost somewhere?

Arthur

PS - finally started looking at the PM1 again

[zero]

piccies of the pm1 required as a matter of urgency!!

Nope, I didn't hear about it, but then it's possible he phoned TH and got Alan, in which case I'd never of heard of it.

[mdstuart]

I have collimated my 6 inch scope and it was a real challenge. Eventually I was sure I had it right by defocusing and focusing on a bright star at a low and high power. Even so lyra double is still a bit challenging until the scope cools...Best advice is probably get someone who has done it before a few times to have a look rather than getting more and more frustrated and possibly breaking something....

[Andrew Robertson]

Nope, I didn't hear about it, but then it's possible he phoned TH and got Alan, in which case I'd never of heard of it.

I did speak to someone at TH but can't remember who for sure now - I was just letting them know. It wasn't a problem for me, we sorted it and it's not my main scope just a weekend away one and also to take to Norwich AS on a club night. Also I didn't get it for doubles, they're just a good test of the optics, just can't accept astigmatism. The thing with Astigmatism is that you can never achieve proper focus. It takes 5 mins to set up and 5 mins to collimate. Normally align the secondary with cheshire and/or laser. Then collimate primary on Polaris. Mine is not that old and came with a dual speed focusser.

Andy.