Help needed - Improving lightbridge optic performance

[earth titan]

Apologies to everyone for being such a pain. More questions and I'm sure eventually, someones going to get fed up so I apologise for asking yet anopther one.

Well last night was the first real session with the 12" lightbridge. Definitely a step forward from the 150mm Skywatcher. Great cluster in Hercules was significantly more granular and I found the comet 103p Hartley – both despite significant bleaching from the moon and intermittent cloud cover.

While setting up for observing however, I aligned the telrad on the chimney pots a couple of streets away (about 200m) in the afternoon. They were all in focus with the 45 and 25mm eps but when it got down to the 6mm, I couldn't get a focus. I know the scope is for looking at the sky so focal distances in the foreground may be out but surely this level of issue will mean everything in the distance is going to be out of focus.

Got some BobsKnobs and stiffer springs on order to improve the ease and reliability of collimation for the primary as the ones currently fitted really aren’t man enough for the job as collimation drifted after a few minutes of using the scope and I noticed one of the screws has lot of play in.

Being flicking thourgh other forums and this one as well, and many posts about lightbridges with astygmatism. Anyway, being addicted to fiddling with things and always wishing to improve things at every turn, I’m going to turn my attention to the mounting of the primary and secondary mirrors over the next week or so to see if this improves matters.

Also want to ensure neither mirror is ‘pinched’ and both are free to expand and contract. I have read elsewhere, the mounting of the mirrors by GSO when they are built, leads to inherent astigmatism. With the light pollution, poor seeing etc. I need every edge I can get and it’s a good excuse to tweek and fiddle so:

I know the primary is stuck down, where as it should be supported but not stuck so I’m going to free off the clamps, remove the current adhesive and remount the mirror on some pads made from self adhesive cork tiles. Then I’ll remount the mirror with a business card thickness clearance on the clips to allow some float.

How about the secondary though? I have compared it to my Skywatcher and the Meade appears to be set in a recess (which I read is a possible problem - pinching the mirror). Anyone taken one of these out of the current mount and ‘freed it off’? How did you secure it? Is silicon sealant the way forward?

Thanks.

[Peter Drew]

Hello E T. What you have written makes perfect sence and mirrors many of the issues that others have raised and cured in recent posts. I suspect that the current ability to mass produce good quality optics means that it's unlikely that they will have inherent astigmatism. It is, however, very easy to introduce astigmatism, most usually by optics being squeezed by too much pressure. If, after you have inspected or remounted the primary mirror as you suggest and find evidence of astigmatism try turning the mirror and see if the astigmatism goes round with it, if it does then the primary is at fault, if it doesn't then it may be the secondary. Silicone sealant offers a good means of securing a secondary without undue pressure but be careful as to which type, some are not recommended for use on aluminium, which the backing plate is likely to be, and could fail in time.

[kidlands]

Very interesting post I look forward to the results earth titan, being a lightbridge owner myself and experiencing a possible similar situation.

steve

[albedo0.39]

Hi there. It's well known that the LB mirrors are unsung heroes (if you can read french, there is an awseome / detailed review of an LB-16's mirror that demonstrates almost unheard of perrformance..)

Anyhow, one of the known problems is as you mentioned, potential astigmatism caused by the 3 mirror clips being too tight onto the primary.

The LB-16 has a 27 point mirror cell - again, well above market expectation for a "cheap" scope and so provided the mirror is fixed well, then the 3 clips can be moved away from the mirror. Any traces of astigmatism caused by this, will be cured.

Good luck

Steve

[earth titan]

Okay. Here goes for the primary.

Removed mirror cell from rear optical assembly and turned mirror over onto a sheet of cardboard.

The next stage was to detach the current adhesive pads which I did with the careful use of dental floss (see below) as a saw.

Took about five minutes until the mirror was liberated from the cell - nowhere near as daunting as I thought it was going to be.

[earth titan]

Next stage was to clean the mirror of left over adhesive and clean.

Used a very sharp stanley knife blade as a scraper being very careful not to gouge the rear.

[earth titan]

I've found some single sided self adhesive felt pads of the type used to stop furniture scuffing surfaces. These should be fine.

As they are supplied they are quite thick so I have cut them down and attached one to each of the contact points for the mirror.

This should allow the mirror to expand / move etc. in the cell without wobbling all over the place.

[earth titan]

Replaced the mirror back in the cell, gave it a good clean and redid the centre circle just to be on the safe side.

I've looked at the mirror retaining clips and they are rubberised, so don't think I need to worry too much about them distorting the mirror. Therefore just nipped them up until there is a little resistance but they still move and the mirror will rotate in the cell.

Collimated everything and looked for slop or movement in the mirror assembly, of which there was none. Held collimation really well despite the mirror no longer being secured to the cell. Will replace the collimation screws and springs when the BobsKnobs arrive, but now ready for some clear skies to test.

I have decided to test this stage before I move onto the secondary, as I don't want to fiddle unnecessarily.

Only concern is the thickness of the pads I've added. They are substantially thicker than the adhesive applied by GSO in the factory, but if I struggle to get focus etc. I can also replace with a smaller pad. I have some wet suit neoprene which would serve if needed.

All in all a much simpler process than I had anticipated. Using the dental floss as a saw was a master stroke picked up from another forum. Can strongly reccomend to anyone else thinkinog of doing the same.

[earth titan]

As luck would have it, I was woken up by the call of nature at 4:00 this morning to see clear skies and no moon. As Orion was clearly visible from the bathroom window the temptation to grab the scope was too much.

Stuck it out on the viewing pad, kind of cooled it for 5 minutes, collimated it and shoved the 2" 26mm ep in.

Speechless.... M42 stunning and clear. Eventually worked my way through the eyepieces until I had the 6mm in. Sill a crisp image. Managed to get a good split and resolved the stars in the trapezium nicely.

Early days yet - need to try viewing after a long cool down, as this was a slung together session, but I would go so far to do a dance around the room and say 'fixed it'.

Whatever I did has made the images crisp and focusable. I was begining to get downhearted but not any longer.

[oldfruit]

Really good report, and of particular interest to me as i stripped my scope down last night and cleaned the mirrors. I really like the way you go through step by step and i can understand some of your reservations about taking mirrors out and re-mounting. I have a different scope but the nerves are the same for us all i think, also the basics for removing and replacing mirrors would be very similar to what you have done so not only have i found reading your post usefull, but i would immagine others would too,

[earth titan]

Bobs Knobs arrived today and have now been fitted.

I went for a full set.

When I got the scope the screws in the secondary were showing signs of getting mashed up and I could see they might become unusable over time. Although I don't intend to fiddle with the secondary too much, these do make adjustment very simple and I can strongly recommend them.

Fitment skill needed: 1/10

[earth titan]

Replaced the screws for the primary and, having identified a problem with the mirror slopping around under movement, I also fitted the stiffer springs.

All went very well and a nice simple replacement process.

Fitment skill: 2/10

All in all, a satisfying improvement.

Tested the time to collimate from scratch in the dark: 6 minutes.

This included positioning secondary using cheshire (which moved slightly on fitting screws), roughly aligned primary with laser then final alignment of primary using barlowed laser.

No dropping of screwdriver, in fact no tools required. Cool.

[Moonshane]

this is a great thread. keep it going! it will be good to see further mods as time goes by - there will doubtless be some

[earth titan]

Well I got bored and with nothing else to do, I stripped out the secondary (which I wasn't going to do) to remount it.

Removed it from the upper OTA easily enough and with a little easing with a fine screwdriver and the 'dental floss saw', out it popped from the holder.

[earth titan]

Next was to remove all of the sticky pads.

In total, the manufacturers had fitted the mirror in to position with:

3 sticky back pads

3 additional blobs of silicone

1 retaining clip on the front

They obviously didn't want it falling out

I scrpaed these off to make a clean holder.

[earth titan]

The holder seems to allow a bit of movement of the mirror so I don't think it is 'pinching' it because of a tight fitting.

Next I applied 3 small blobs of silicone sealant to the holder to allow a mount which allows a bit of movement and some grip.

The mirror was refitted, secured in place with the clip and reinstalled in the OTA.

Just need the clouds to clear now, so the modifications can be checked.

Next stage is flocking the OTA and then making a better shroud than the fabric one that came with the scope.

[earth titan]

Have now had an oppurtunity to test the completed modifications.

Star test showed excellent collimation with the barlowed laser set up, so pointed it at Deneb. Previously this had showed astygmatism in one direction.

The mods to the primary improved it somewhat (last test a few weeks ago) but now the secondary has been done I can report a much improved situation. Pin sharp!

Swung round to look at Jupiter which has always lacked detail in the past. Tonight however, GRS was visible and excellent detail. Could have been the seeing, but I have tried before in similar conditions and never seen such detail.

Looks as though the total modifications have made a great difference and improved the scope no end. Now a happy bunny!

:)

[simon84]

Excellent result, dont suppose you could post the link for Bobs Knobs, I do like the idea of the hand knobs on the secondary. Those philips screws dont fill me confidence at all.

Nice job with improving the secondary aswell, does the mirror sit in the holder completely or have you blackened the edge of the mirror?

Just to add but coming from a glazier, if you wet the surface of the glass before scraping with a stanley blade you reduce the chances of marking the glass.

[earth titan]

Link for knobs:

http://www.bobsknobs.com/

Purchased them from the US and they arrived good as gold after a few weeks. I also bought the stronger springs and these have helped tremendously.

Mirror still sits in the holder so haven't blackened the mirror edge but I might reduce the size of the holder. Perhaps another day...

Thanks for the tip about the mirror scraping. Hope not to have to do this very often.

Next stage is the shroud, then flocking then get the struts and other bits powder coated black.

[simon84]

Thankyou for the excellent thread, I'm ready to pull my 300P apart now....

Going to order some secondary knobs for sure. Glad to hear everything arrived safely.

Someone else mention the mirror edge, all you need is a black marker pen or a strip of flocking paper, I imagine the pen would be alot easier but it should offer a small amount of improvement in the contrast.

I also only use single edge razor blades rather than stanely blades, much sharper and really do get rid of any residue.

I think I know what I'll be doing tomorrow now.....

[gustavo_sanchez]

Wow... all those changes... I do not dare to tear apart my scope like that... I admire you guys! I ordered the secodary knobs and the upgraded springs, but I think that's about it... :S

Anyway, if any of you happen to travel one day to the Caribbean, I happily will let you mod my scope!

[earth titan]

Well if you would like me to come and mod your scope, all it would cost you is a plane ticket