Hi there, can anyone guide me to some info on colimation and general maintenance guidelines for a Dob.I havent used mine for ages and not sure how I need to start again. It has been completely covered and kept inside.Being so inexperienced I probably wouldnt notice minor adjustment problems,so would like to know what to look out for etc. Thanks in advanceIf there is a particular colimator that is best for Dobs,I would love to hear about it (and how to use it). Have to admit my ignorance or I will never get back out there.....
Hi Liz,
I was lost on this to until I found this article (http://www.andysshotglass.com/articles.html). This led me to buy a laser collimator and to experiment ie twist the knobs and see what happens. I have only had my scope for a little over a month now and I am very much still learning.
There was a down side to this though Brisbane has had overcast or cloudy nights since I got the device and I have not been able to test my efforts for any great length of time.
Thanks Greg .... Had that video playing while I was trying to fix it, and am using a laser collimator!!! ..... think I have made it all worse!!
jjj has given me a few more tips I will try.
Have one of guys from our club to give me hand in next few days
.. dont want to take over flearosies thread, though may be of help also.
Well ..... I put the laser col in, and got the secondary mirror all aligned so red dot in the middle of col. - easy!!Then tackled the primary mirror - loosened all the thin screws as per video, then went about adjusting the fat screws, checking the mirror after each one, but despite multiple adjustments of these screws, it made no difference, in that the red dot was still about 20mm right of centre.
Checked secondary mirror - now I cant even get the red dot on the collimater at all.
Have given up in disgust at this stage, and will retackle when recovered!!!!
This is what I have experienced so far - I hope this is relevent to the thread.
When I go the scope we (seller and I) tested the the collimation and it was close. I am just learning this stuff as our 114mm reflector didn't seem to need it.
1. The scope is in good physical condition so no external maintenance was required. The scope moves well in all directions. What I did notice is that one of the screws that attach the tension mounts has become loose and needs to be tightened - another reason to remove the primary mirror.
2. The Primary/secondard mirror all seemed to be in good condition ie no cracks or chips etc. What I have noticed is that there is a lot of dust in the tube. I am going to have to take out the primary mirror and follow Mike Salways process on this and give the scope a clean.
3. When I have had attempts at collimation what I have noticed is that the secondary is easy to do but the primary is a challenge. What I did notice is that mirror doesn't move freely when the locking bolts are loosened. So what happened to me is the mirror moved unpridictably and I often had to reset collimation screws and start again.
4. What I have noticed is that the laser collimator gives different results depending on where and how it is placed in to the focuser. The other issue I have learn't is that the laser collimator needs collimation -grr. More learning to do. What I now know is that a laser collimator is just part of the process of aligning the scope.
5. When collimating the scope I noticed that when I move the scope from the vertical to the horizontal the return beam dot moves on the collimator - I don't know why this happens. I suspect that I caused this when I tightened the tension bolts to better hold position when I attach my camera. Loosening them did lessen this effect but it is still there.
This is what I have learned so far - but the biggist thing I am learning is patience - with the weather and everything else including my own skills or lack there of at the moment.
2. The Primary/secondard mirror all seemed to be in good condition ie no cracks or chips etc. What I have noticed is that there is a lot of dust in the tube. I am going to have to take out the primary mirror and follow Mike Salways process on this and give the scope a clean.
3. When I have had attempts at collimation what I have noticed is that the secondary is easy to do but the primary is a challenge. What I did notice is that mirror doesn't move freely when the locking bolts are loosened. So what happened to me is the mirror moved unpridictably and I often had to reset collimation screws and start again.
4. What I have noticed is that the laser collimator gives different results depending on where and how it is placed in to the focuser. The other issue I have learn't is that the laser collimator needs collimation -grr. More learning to do. What I now know is that a laser collimator is just part of the process of aligning the scope.
5. When collimating the scope I noticed that when I move the scope from the vertical to the horizontal the return beam dot moves on the collimator - I don't know why this happens. I suspect that I caused this when I tightened the tension bolts to better hold position when I attach my camera. Loosening them did lessen this effect but it is still there.
Cheers
Greg
Greg,
2. Is the primary mirror itself really that dirty? Only clean it unless it's really needed.
3. When you unscrew the locking bolts, the mirror isn't meant to move "freely". You need to adjust the larger collimation screws, one out then one in each time, to re-angle it and get the right collimation. The locking bolts will just fix it so that it doesn't bounce around on the collimation bolt springs.
4. Hold the laser collimator firmly in the focuser and turn it. If the laser dot moves in a circular path about the primary mirror centre, it's out of collimation.
5. This is probably because of some slack in the focuser. The laser collimator is quite heavy and will exaggerate the slack.
.. dont want to take over flearosies thread, though may be of help also.
Well ..... I put the laser col in, and got the secondary mirror all aligned so red dot in the middle of col. - easy!!Then tackled the primary mirror - loosened all the thin screws as per video, then went about adjusting the fat screws, checking the mirror after each one, but despite multiple adjustments of these screws, it made no difference, in that the red dot was still about 20mm right of centre.
Checked secondary mirror - now I cant even get the red dot on the collimater at all.
Have given up in disgust at this stage, and will retackle when recovered!!!!
Liz,
Find attached a suggested method of restoring collimation if you've lost your red dot. Hope it doesn't confuse you more.
On topic but a bit off thread. My colimination of the 12" secondary seems to be quite difficult, does the secondary mirror go offline easily and need resetting?
If not what would be the cause?
Could temperature changes cause the offset?
I have inadvertantly altered al-be-it very very slightly the spider vein as they seemed a bit loose from new. What kind of accuracy does the spider veins need to be in?
I must admit the collimination of the 12" is better than the SEBEN I have and the 12" does centre on the target as well.
On topic but a bit off thread. My colimination of the 12" secondary seems to be quite difficult, does the secondary mirror go offline easily and need resetting?
If not what would be the cause?
Could temperature changes cause the offset?
I have inadvertantly altered al-be-it very very slightly the spider vein as they seemed a bit loose from new. What kind of accuracy does the spider veins need to be in?
I have a 12" truss DOB so it has to be re-collimated any time I disassemble/reassemble it. Even moving it as a unit can affect collimation.
In a collapsible, the process of collapsing/extending would still result in a slightly different final assembly due to the tolerances involved. I would say that there would still be significant flex in a collapsible if moved as an extended unit. If you are directing the telescope around the sky holding the connecting rods (as I do with the trusses), some slight changes might occur due to flex and tolerances. In moving the scope, usually the secondary mirror is the more stable and the primary mirror collimation is the one that is out.
A slight knock can put the primary or secondary off alignment. Temperature variations could possible affect alignment due to variable expansion/contraction of the steel but this would be hard to quantify.
Even a full tube DOB will have a certain amount of flex when carried.
As long as the secondary is about centre on each spider vane diameter, that's OK.
For general viewing, a little out-of-collimation won't be a worry. After all, you can only collimate to a certain accuracy. But you need good collimation for fine detail (e.g on planets or separating very close binaries). That is why I collimate every time I do viewing.
Gosh, thanks everyone for your responses. Liz, I cant wait to go buy a laser colimator and see what type of a mess I can produce
I was all ready to go outside tonight but a bit breezy here on the plains so will see what tomorrow night presents.My biggest problem is making my OH stay awake so I can get him to carry my scope back inside when I finish. Have verandahs all around the house and I just cant lift it myself. Must get a ramp instead of steps. Gee the scope was obviously the cheap part of the deal!
I had tried again over the last couple of days, but no luck, and was very frustrated and confused, as turning the screws wasnt achieving anything in the end.
Rex from our astronomy club has just popped over, and took him ages to get it right, then had me trying to adjust/loosen/adjust/loosen to achieve the little red dot in the middle, with the secondary then primary/secondary then primary etc. I definitely hadnt been doing it properly!!
I will do it each time now, so I dont lose the knack .... that I am starting to attain.
Thank you all for time and patience, esp Rob ... I will reread all what has been written, and am sure it will be a lot clearer.
It's worth remembering when using laser collimators that they themselves need collimated once in a while to ensure that the laser is aligned correctly along the axis of the barrel.
Sit the collimator in a set of V blocks and put a piece of paper on the wall on the other side of the room marking on it where the laser hits. Rotate the collimator in the V-blocks and check that the dot stays on the mark on the paper. If it doesn't then you need to adjust the collimator collimation screws until it can be rotated without the dot moving off the mark.
Beside making V Blocks which is relatively easy, as I have a shoulder injury I am unable to operate saws or machinery.
I have looked everywhere, any idea where i can get V Blocks. I have tried Bunnings and a couple of specialist tool stores but not much luck.
Malcolm,
You'll probably not use it very often.
Here's my suggestion if you can use a stanley-type knife with a sharp long blade. Watch you don't cut yourself!
Find some old styrofoam packaging and cut 3 rectangular pieces out of it (see diagram). The base needs to be thicker (3cm) for support. The thickness of the uprights can be more than 2cm. Cut the Vs out of the uprights as shown. Attach the elastic bands to the base first then stretch them over the uprights.
My laser collimator is 15cm long but you can adjust the width between the Vs for your own laser. Nothing has to be that exact in the assembly but make sure the V-Block doesn't move when you rotate the laser for collimation check. If the laser points too low, put it in the other way around in the Vs.
As a test, I cut and assembled the whole thing out of styrofoam myself. Took about 15 minutes.
Works fine.
If there is a particular colimator that is best for Dobs,I would love to hear about it (and how to use it). Have to admit my ignorance or I will never get back out there.....
Brisbane has had overcast or cloudy nights since I got the device and I have not been able to test my efforts for any great length of time.
..... think I have made it all worse!!
