Hope you are all well and getting clear skies during these tough times. I have a GSO 830 10" F5 reflector with a Moonlite focuser. For collimating I use the Howie Glatter laser collimator. The collimation process I use is as follows:
I attach the barlow adapter to the howie glatter and put it into the focuser, it projects a tight red dot onto the primary mirror. After adjusting the screws on the secondary to put the dot perfectly centered in the dot on the mirror, I take the howie glatter out, remove the barlow and put it into the "tublug" (https://www.collimator.com/barlowedc...ion/125-tublug) so I can adjust the primary mirror.
All looks great for both primary and secondary mirrors with the laser dot remaining perfectly on the primary dot and shining perfectly through the tublug, I unlock the RA clutch and rotate the counterweight bar to be horizontal.. the red laser dot then moves onto the edge of the alignment dot. If I rotate it 180 degrees the opposite way so the counterweights are facing east, it isn't *as* bad...
My question is, what could be causing this movement when I rotate? I have a Farpoint Dovetail Universal Plate for the main mounting to my NEQ6 Pro, then the 2 rings that come with the GSO830, then on top the standard dovetail that came with the GSO830 to stiffen it all up. I've tried tightening my primary mirror cap screws that secure it in, made sure my adjustment knobs are all tight, checked all my bolts on the dovetail.. I'm at a loss? Has anyone else managed to sort out collimation getting out of whack when rotating the scope?
Sorry for the long post, but I just don't know what else I can do because it is affecting my DSO images (stars are becoming lines or commas)
I have uploaded a video I took on my phone whilst trying to hold it against the scope and looking at the dot.. excuse the shakyness of it but you can see the laser dot moving as I rotate it about the RA axis - https://www.youtube.com/watch?v=zsDg9fet9cc
The issue would seem to be the secondary mirror is not held securely in place. It would be the only reason for the wandering red dot.
Two causes for this:
1, Could be because the sec. collimation screws are not all pushing against the secondary holder.
or
2, There is too much play in the thread holding the sec screws in place, the spider hub, so when the scope is moved about, the play in these threads is allowing the secondary to move about under its own weight.
What does not help this second point is that these collimation screws have a sharp round tip that pushes against a soft plastic secondary holder housing. Of if is is metal, it is soft aluminium compared to the steel screws. In either case the holder housing is being gouged by the collimation screws, forming pits. These pits will make the collimation process less stable for the secondary, and allows the secondary to move about under its own weight because these pits allow the collimation screws to also move because of the play in the spider hub.
If it was a loose primary mirror, the movement of the dot would be more of a jolt than a gradual shift.
Ok awesome, that's a great place to start. I gotta take the primary mirror out again tonight to loosen the screws as I have pinched optics (was testing if the primary was the cause). I'll check out the secondary screws tonight and make sure they're all firmly in place - the spider vanes I did up very tight too with the multi grips. I'll let you know how I go
Be careful not to over tighten things with OTA's. Using multi-grips is over kill, and can cause distortion in the tube because of the excessive pressure on the spider. This may in turn affect the position of the focuser, leaving it not square. If you've used multi-grips, you have over tightened the spider! This will also put undue pressure on the small screws that hold the vanes to the spider hub, and can distort the vanes too. Firm finger/hand pressure is all that you need to hold the spider securely in place.
To prevent gouging of the secondary mirror holder by the secondary adjustment screws cut two circular plastic washers from a 2L milk jug and place them between the mirror holder and the spider.
By using the soft plastic, all that is being done is protect the secondary housing, but not changing the issue created by a gouged surface and the play in the spider hub. Instead it is making things worse because the gouged plastic will slowly deform under the pressure from the collimation screws and the play in the spider hub. Ultimately it is a problem that needs more drastic solutions or the unstable secondary assembly will not be solved.
Years ago I had a Southern Cross 12 inch dob with a plastic secondary holder. I found a large metal washer the same diameter as the secondary holder and superglued it to the holder so that the adjustment screws contacted the washer and not the plastic secondary holder.
Tonight I pulled the scope apart and took out the primary mirror cell as well as the secondary spider so all that was left was the focuser and tube rings + dovetails. After measuring the distance from the top of the scope to the center of the focuser, I put a small mark on the opposite side of the inside tube. I put the howie glatter in and it hit the small cross I made perfectly centered which was great, I knew my focuser was nice and square...
As a test I put the tube back onto my mount and then rotated it manually once again.. to my surprise the laser dot moved 3-4mm from horizon to horizon So it looks like my GSO830 is suffering from tube flexure around the focuser. What do you guys recommend doing next to try and solve this?
As far as the movement of the laser dot when you move the tube from horizontal to vertical and past, have you checked that the laser is not moving in the drawtube. May only be .01mm slop in the fitting but that can account for the dot moving if the collimator is not clamped firmly.
The machining tolerences of mass produced astro goodies sometimes leaves a bit to be desired. The laser collimators I've got , admittedly cheapish ones, have a sloppy fit in my newt focusers and I shimmed the barrels with some tape to make a firmer fit with less play. Might be worth investigating.
Hmm yeah there definitely is a bit of movement when I don't have the 3 thumb screws tightened up on the moonlite. When I do it up, it does *appear* to be quite snug, but maybe because the Howie Glatter is so damn heavy, it could be still having some slight tilt. I'm about to head out to the observatory again and try moving the tube right down so that the first ring is snug against the focuser for a test. If there's no more movement there I'll probably end up ordering some more rings. If there is, I'll try shimming the collimator.
Sorry to hijack, but what's the solution to the issue of secondary adjuster screws gouging the secondary. Mine has a metal plate at the back of the secondary. But I can still see some marks on the plate from those screws. Smooth filling the ends of the screws?
Chris
Quote:
Originally Posted by mental4astro
Hi Mike,
The issue would seem to be the secondary mirror is not held securely in place. It would be the only reason for the wandering red dot.
Two causes for this:
1, Could be because the sec. collimation screws are not all pushing against the secondary holder.
or
2, There is too much play in the thread holding the sec screws in place, the spider hub, so when the scope is moved about, the play in these threads is allowing the secondary to move about under its own weight.
What does not help this second point is that these collimation screws have a sharp round tip that pushes against a soft plastic secondary holder housing. Of if is is metal, it is soft aluminium compared to the steel screws. In either case the holder housing is being gouged by the collimation screws, forming pits. These pits will make the collimation process less stable for the secondary, and allows the secondary to move about under its own weight because these pits allow the collimation screws to also move because of the play in the spider hub.
If it was a loose primary mirror, the movement of the dot would be more of a jolt than a gradual shift.
Is the metal the colli screws are pushing on aluminium? Thats the other problem. Aluminum is still too soft even for smoothed off screws. It will dent under the pressure exerted by the screws.
On another point, you mention you are using a barlow adaptor, I assume it is one of these https://www.collimator.com/accessori...wed-attachment . If so this should actually spread the beam out so you don't get a tight dot on the primary to adjust your secondary. Are you sure it is not the 1mm Aperture stop that you are using?
On another point, you mention you are using a barlow adaptor, I assume it is one of these https://www.collimator.com/accessori...wed-attachment . If so this should actually spread the beam out so you don't get a tight dot on the primary to adjust your secondary. Are you sure it is not the 1mm Aperture stop that you are using?
It is one of those screw on adapters yeah, but not the self-barlowed attachment. It came with the howie glatter (not listed on their accessories page for some reason). Without it on the laser beam is quite broad, and with it on the beam is only around 1mm in size and is a nice tight dot.
I put the scope all back together again, and kept the tube rings right up against the focuser.. guess what - barely any movement of the laser dot now when I rotate left and right. I'm going to order some more tube rings but will do some further tests tonight. All is looking rather promising so far!
Sounds like it is the 1mm stop. Yes it no longer appears as an item on the website. The Self Barlow attachment is essentially the same as the TuBlug except it cannot be viewed from the back of the telescope, so really only useful if used on a short tube.
So I spent today adding in a 1.6mm thick aluminium sheet behind the focuser to stop any tube flex around the focuser. Looks like it's going to work nicely! I gave it a coat of black spray paint so will let that dry and then tomorrow put on some black 3.0 to match the rest of the inside and keep reflections down.
All back together and re-collimated. The laser dot now stays within the circle when I slew counterweights east and doesn't move at all with counterweights facing west.. I'd call that a win!
So it looks like my GSO830 is suffering from tube flexure around the focuser. What do you guys recommend doing next to try and solve this?
