16" Lightbridge woes.

[epk]

This is my first post here after just reading for a long while.

Last night I read a post...and of course I can't find it today...about problems with image sharpness In 16" Lightbridges.

I upgraded from 12" to 16" some time ago, and I've the same problem.

My collimation...verified both by laser and Cheshire...seems to be spot on, yet I just can't get sharp images, despite using Televue eyepieces, long cool down times etc.

When observing a couple of nights ago I noticed tht my friend was getting far better resolution in his 10" Skywatcher Dob with both Baader and my Televue eyepieces, and it's been bugging me since.

Like the other poster I've noticed this also with binaries..a bit smeary rather than pinpoints, and even coma correction hasn't helped.

Anyone got any suggestions?

[hal2000]

Did you have these problems when you had the 12"? If not, then the obvious solution is to ask the person who bought the 12" if they would like to swap the 12" for the 16", and let that person worry about it. Of course, the person who bought the 12" might then just sell the 16" and put the money towards a Celestron Nexstar8 instead. Hope this helps :-)

[Doc]

This sounds like coma you are seeing to me.

Remember that your 16" is F4.5 so very hard on eyepieces. What Focal ratio is your friends 10" Dob?

Mine suffers from Coma especially on the outer third towards the edge and it does make splitting doubles pretty hard going. My TAL 4" refractor is better at splitting doubles.

Another thing it could be is antagonism, have you checked your mirror clips are not to tight?

[epk]

Stevie, You're a scream!

The other dob is an f 4.8, so no big difference there.

It's not really coma, just a lack of sharpness and resolution.

It's fine with bigger eyepieces, and my Paracorr goes a pretty good job right across the field, but once it gets down to using my Nagler 11mm and 7mm, getting a sharp focus is problematic.

[Doc]

Are you using a light shroud as I found that adds contrast to your views.

[epk]

No Mick. It isn't even a contrast issue. It simply isn't sharp enough to discern the level of detail I should be getting with the equipment. It's particularly noticeable with views of Saturn and, foe example, Epsilon Lyrae which I've never yet split.

I'll try the mirror grip tightness if I can do a star test,

[Doc]

In my 16" Lightbridge splitting The double double is pretty straight forward. On good nights of seeing I can easily see the cassini gap at the end of the rings even though the rings are edge on. These were all seen with moderate eyepieces, yours are Naglers so should be sharper.

I'm scratching my head as to what it could be.

[JAO]

I seem to recall a thread some time ago about mirror clamps on the 16 LB being tightened up too much, probably for shipping purposes? As far as I can recall from the posts this led to the mirror being pinched causing a degree of astigmatism. I am not an LB owner so I may have got this wrong but there was a thread about this theme some months ago. I am sure LB owners on the group may recall better than me.

Adrian.

[Andrew*]

You may have been reading a thread by me: http://stargazerslounge.com/equipment-discussion/69737-16-dobsonian-underperforming.html

I still haven't got to the bottom of this. Unlike coma it gets worse with higher magnifications, not wider fields. I can't see fine details beyond 150x or so.

I have got collimation spot on and still the problem has been there. I have loosened the mirror clips and the aberration has persisted. I have used the best eyepieces out there but the blobby stars just don't leave me alone!

I have heard of owners having astigmatic secondaries but I don't know how to confirm this is the case with mine.

I don't know my aberrations that well, but from the description it sounds like spherical aberration...

Andrew

[epk]

Andrew....that sounds exactly like the problem that I have.

It must have been your earlier post that I read.

Could we be looking at a manufacturing mirror flaw?

[Astronut]

Jarbi had an astigmatic secondary on his 16" LB. I asked him what led to suspect he had an astigmatic secondary in this thread.

I've got niggling doubts about the secondary on my 12" skywatcher dob as I too get get blobby flaring stars, especially at higher mags, despite using a Televue Paracorr, x2 Powermate, 13mm Ethos and perfect collimation with a Catseye system. Details on Saturn, for instance, could be better eg. Cassini Division, I think, should be visible (despite the rings being almost edge-on).

Also, I can split the double-double at x265 (paracorr + powermate + ethos) but it's not a clean split ie. the blobby stars don't allow me to see black between each double. I'm fully aware that a dob is not ideal for doubles but I've read so many reviews of my 'scope saying "the double-double was cleanly split" or words similar that it raises doubts in my mind for my particular optics.

I have a ronchi grating from Orion Optics UK and as far as I can tell I'm getting a good diffraction pattern from the main mirror but the secondary is in the light path during this test - go figure

[epk]

Just read that whole thread of Andrew's.

One thing I can rule out is collimation. It's spot on, both with Cheshire and laser....I'm using the Hotech laser that is self centring and which has no slop at all.

Astigmatism may be an answer, so I'll have a read up on that as well as checking the mirror clips.

[mdstuart]

Hmm well I have a 10 inch and I can cleanly split the double double. What about your own eyes. I have glasses and if I do not wear them with binoculars I cannot get stars to spots but I can with the glasses on...??

Mark

[epk]

Well, I'd imagine my eyes would have produced the same image whatever scope I use. They don't. I can get pinpoint images, but they degrade when the magnification is stepped up.

My Panoptic 35mm and 27mm give me incredible views of clusters. Bring the higher mags in and it all goes woozy in the Lightbridge.

[mdstuart]

Really odd, ultimately you perhaps need another experienced pair of eyes to look at what you are seeing with another 16 inch next to yours to compare I suppose, How frustrating.

[Doc]

epk how do you rate your Hotech laser collimator? Is it one of the best available in your opinion. I have the Baader laser collimator and am not 100% sure of the accuracy as like yours it does not elimminate slop in the focuser.

[epk]

The Hotech is superb.

None of the usual laser collimators will eliminate it, but in 2" mode it is rock solid, and in my focuser there is negligible movement, unlike the 1.25" collimators I've used before.

It is wonderfully designed, like the green laser pointer/red light/white light combo he also produces....made to look like a Star Wars light sabre!

Eamonn

[Doc]

Cheers for that I think I might trade in my Baader and buy one of these.

I'm planing on a Nagler line up one day. It will be a long wait as money is a tad tight and maybe will allow one good nagler a year. Would you consider the parracor much better then the baader MPCC?

[Astronut]

The Paracorr is adjustable for different eyepieces. As far as I know the baader MPCC isn't but I haven't seen or used one.

Hmm well I have a 10 inch and I can cleanly split the double double. What about your own eyes. I have glasses and if I do not wear them with binoculars I cannot get stars to spots but I can with the glasses on...??

Mark

I've tried with and without my glasses. I have slight astigmatism (0.5 dioptres) in both eyes and the glasses help a little with the Pentax XW focussing (I see short vertical lines perpendicular to each other either side of focus on stars - stars are tiny crosses at focus without my glasses) but that has a much bigger exit pupil than the Ethos. There's no difference in the view through the Ethos with/without glasses. I prefer to observe without my glasses.

Your post has put more doubt in my mind re. my 'scope optics. When you say the doubles are a clean split I'm assuming you mean you can see black between the split stars, correct?

[epk]

The MPCC has to be screwed onto each eyepiece for use, whereas the Paracorr can be left in the focuser.

A sliding scale on it adjusts it for whatever eyepiece is in place, locking it as well.

I'm very happy with the Paracorr, and it also has a slight Barlowing effect, magnifying by 15%.

Mind you, add it to a 35mm Panoptic and it looks as big as an artillery round.

I also observe without glasses, and prefer it that way, and I've no difficulty getting stars as points.

[calibos]

Howie Glatters Holographic lasers and blug are the last word in laser collimators. Have never heard of one arriving out of collimation nor ever going out of collimation after years of use. That can't be said for the Hotech. Its also said that the self centering hotech is moot when one doesn't have a self centering EP. ie there no point collimating the scope to the centre of the drawtube where the hotech would collimate to when you then go and put in your EP which without a self centering mechanism gets pushed to the side by your thumbscrew. Rember, one isn't trying to collimate the scope to the centre of the drawtube but the centre of the EP.

The glatters cost a lot but are worth every penny for piece of mind that your laser is in collimation and it gives me near autocollimator accuracy at night without flashlights in about 30 seconds flat. The BLUG is a barlowed 45º target that fits in the end of the drawtube. You collimate the secondary with the 1mm apeture stop in place for a highly accurate tiny dot instead of an oval blob like other lasers, so its easy to see when the laser dot is perfectly centered in the primary donut. Then one inserts the blug and can collimate the primary from the back of the scope in seconds by looking at the target between your trusses from the back of the scope by the collimation knobs.

While it is said that the barlowed laser collimation method equates to a good cheshire collimation, in practice it appears to be even better. Every person who has both a Glatter and an Autocollimator like the catseye system say that they only now rarely double check with their autocollimator because they found that the autocollimator always seemed to confirm that the glatter had already perfectly collimated the scope and the catseye triangles were already perfectly stacked.

[epk]

I'd a look at the Glatter in OPT and yes, it was both very expensive and well built. I'm happy with the Hotech as a good compromise between quality and price.

As I use a Cheshire as well I'm also happy that my collimation is pretty good to begin with, as both methods confirm each other.

I think the Ronchi grating is what I'll be looking at next, as I think I'd much better resolution with my previous 12" dob.

[calibos]

I hope no one felt I was denigrating the Hotech. After the Glatter, Farpoint, Astrosystems and possibly Kendrick in the expensive premium laser end, the Hotech is nudging in there. Great value, and after the ultra pemiums probably the best built and least likely to go out of collimation. I just feel the self centering mechanism is a bit moot and Mick asked whether it was the best available. Its not, that title goes to the Glatter but the Hotech is indeed great value and up there with the best.

[albedo0.39]

This is a most interesting thread - may I comment from my limited experience ?

I've really gotten to know the collimation of my 16-LB a lot lately and 2 days ago managed a little trick to get the collimation (with cheshire) even closer to "excellent". It involves standing a long way away from the hole in the EP so that you can start to narrow in your view of the crosshairs. As you know, when your eye is close to the peep hole, then it is possible (by moving your head) to adjust the reflected image.

I found that I could get perfect "in and out of collimation" just by moving my eye a little to the left and right. Now, I stand right back (2 feet or more ?) so that the axis of the X and the peep hole are totally concentric. I then allow my eyes to drift focus to the reflected image and align on that.

You will need a lot of light to see the reflection.

Result ? A much different setting c/w normal methods

Viewing result ? - Well, when I first collimated, I could not split E Lyra - at all, but the stars were sharp. However, 2 nights ago, with a 17mm "cooking" Plossl, I could easily split the double double and seeing was a lot worse as well.

Second

The optics have recently re-coated by Galvoptics - perhaps this helps ?

Third

I have an excellent report from the previous owner (he did a reasonably detailed write up on the LB-16 for a local astronomy club and it makes for facinating reading - If you would like a copy of the article please PM me with your e-mail address and I'll forward). He noted poor quality images and found it to be the clamps on the mirror were tight and causing astigmatism. He loosened them so that they now sit ~1mm above the mirror. Much improved images

Fourth

Have you tried collimating by winding the lens cell almost fully down and backing off each screw by ~ 1.5 turns? This seems to ensure that collimation is held very tightly irrespective of telescope attitude. Works for me.

Good luck and let us all know what else you might find. I for one, am now very very happy with my LB and I just cannot wait to try out some EP's with better quality

Rgds

Steve

[Doc]

Cheers guys I'll have a look at Glatter as well. But the Hotech is a good price.

Anyway both I would imagine are better then my Baader.

Calibos if I did go for the Glatter what would I need.

The Laser, Barlow or Blug or all three.