Problem with collimating secondary (new Bobs knobs)

[Grunthos]

I received some of Bobs Knobs the other day and fitted them (one at a time) to the secondary assembly on my new 250PX.

Went out tonight and, using my usual laser collimator, couldn't seem to centre the red dot in the middle of the primary.

No matter which combination of screw tightning/loosening I tried the bubble did not want to move into the centre circle. I suppose I could have screwed one or two of the knobs really hard but didn't think that was a good idea.

Am I doing something wrong?

Should I perhaps slightly loosen the centre screw to slightly move the secondary mirror (whilst scope parked in horizontal position of course) then try the collimation screws?

:icon_redface:Have I perhaps (when using the original screws) slightly gouged grooves in the secondary stalk as in this thread?

Hopefully it's just something basic. I do like the concept of collimating without an allen key...

[Merlin66]

How far is the laser dot from the centre of the mirror?

Was in in collimation before you changed to the knobs?

[spaceboy]

Just start collimation all over again Astro Babys Guide to Collimation

[Chris H]

It sounds like the secondary is too far down the tube and the Bob's knobs are screwed in fully so they are all on maximum adjustment.

[Grunthos]

Thanks for your swift responses.

The collimation was challenging (but not impossible) before installing Bobs Knobs. I remember thinking when I first tried to colllimate (pre BK) that there wasn't much to play with.

Now (post BK) the red dot is maybe 5-10mm from the middle of the primary.

Looking at "Collimation Step 2" on the Astro Baby site - maybe I should maybe try and adjust the centre screw? which adjusts the secondary mirror towards/away from the primary as well as holding it in place?

I'm starting to assume I got a slightly duff dobo which wasn't optimally aligned and I need to start and check everything from scratch?

(I'll put the hammer and chisel back in the tool box for now - joke! )

[arbacia]

Is the secondary mirror correctly oriented towards the drawtube of focuser?

Collimation screws make a balancing movement of the secondary but, by regulation of the central screw, the secondary can be moved also along the tube and -previosuly to tighten the central screw- and also can rotate respect to the tube.

Use a sight-tube inserted as an eyepiece to center the secondary as seen from the hole of a sight-tube and move the secondary until it looks perfectly circular in shape and centered.

Once be centered the laser beam should be able to reach the spot of the primary mirror.

Patricio

[Grunthos]

I'm also going to have a little play with the primary mirror before diving in and moving the secondary about (+ where do you find a cheshire collimator and 35mm fim canister on Boxing Day?).

Am starting the become more and more tempted into buying Vic Menard (although I will "keep it simple" at this early stage...).

I suppose I am just going through the usual collimation blues that some new reflector owners seem to contract .

[Gina]

Glad I've got a frac now

[arbacia]

Grunthos, tilt the primary mirror once the secondary is correctly aligned.

Collimation should run from the eye towards outside. First collimate secondary, then primary.

[Grunthos]

Well, I seem to have sorted it out (don't exactly know how or why ).

Had another play around with the secondary adjustments (using Bob Knobs)....loosened them off tried a few combinations and eventuallygot the red dot in the middle of the primary. The secondary adjustment screws still seem to be tightened up a lot but maybe that's just me.

Primary mirror adjustment was a piece of cake. Tightening up the locking screws seems to throw the scope out of collimation so I just tighten them up a little one at a time (about 3 or 4 times) and that seems to work.

So...a lot of palavar. No doubt it will get easier, faster, more straightforward as I grow more experienced and confident.

Thanks for all your posts folks.

Seems like I've still got a bit of reading and YouTubing to do!

[Jason D]

The proper method to align the secondary mirror using a laser collimator is to first loosen the 3 secondary set screws by a tiny amount -- just enough to rotate the secondary mirror freely. Then rotate the secondary mirror to get the laser beam as close as possible to the primary center as shown in the 1st attachment. Then start making adjustments using the 3 set screws to get the laser beam to the center. Make washers from a plastic container such as a milk plastic container to make fine adjustments easier as shown in the 2nd attachment. If you do not want to remove your secondary mirror to install the washer, you can cut the washer as shown in the 3rd attachment then slide it in.

Jason

[arbacia]

Nice method, Jason

[Grunthos]

So I don't touch the centre screw unless I want to remove the secondary mirror?

[Jason D]

So I don't touch the centre screw unless I want to remove the secondary mirror?

Touching the central bolt has nothing to do with directing the laser beam to the primary mirror center. Use the central bolt only to center the secondary mirror under the focuser in case the secondary mirror is little too high or low in the scope's tube.

Jason

[Jason D]

You might run into the following scenario: You adjust using one of the 3 secondary mirror set screws and the laser beam seems to be moving in the right direction towards the primary mirror center. All of a sudden, the set screw becomes tight. The other set screws do not seem to help in moving the laser beam in the right direction. If only the “tight” set screw could rotate little more, the laser beam will hit the primary mirror center. Now what?

If you ever run into the above scenario, here is what you will need to do: Loosen ONLY the tight set screw until the secondary mirror is free to rotate. Rotate the secondary mirror until the laser beam is closest to the center (overshooting the center by a tiny amount is even better). Now use the 3 set screws to get the job done. You might have to repeat.

Jason

[spaceboy]

This is why I don't like laser collimator's. Can't beat the old fashion Cheshire in my book. Grunthos my advice to you is to play with the collimation as often as you can while the clouds are upon us. Make sure the OTA is level so nothing falls on to the primary and if need be put a pillow under the secondary just in case that was to drop. I have seen so many beginners fear collimation in the worry they are going to mess up the collimation and the views. In truth you should check your collimation every time you use the scope. I dread to think how many beginners felt the collimation remains unchanged on new scopes even though they have traveled half the globe getting to their door. I think laser collimator's offer a false sense of security to beginners and most will end up with false collimation either due to the laser calibration being out, the collimator is not square in the focuser or the secondary is already miles out. If you have a large newt then it is an unavoidable fact that collimation has to be done regularly so as they say practice makes perfect

[Tim]

The other thing is, you can get what looks like good collimation with a simple point laser and yet the secondary can be rotated slightly out of square with the focuser.

And sometimes when you tighten up the 3 screws to put the laser on the donut, it reveals that the mirrors position is actually rotated out of place, although when you first started it looked ok. Jason points out the method to fix this above. Always worth going back to an empty focuser or sight tube through the process to make sure the secondary still looks round.

My 10" newt has a very large (100mm) secondary and is extremely difficult to get right. Even the properly adjusted Catseye sighttube doesn't help as it obscures the edges of the mirror.

[todd8137]

Just start collimation all over again Astro Babys Guide to Collimation

as nice as astrobaby is,this method is so long winded that most people give it up

[Grunthos]

as nice as astrobaby is,this method is so long winded that most people give it up

Surprisingly - not really (at least for me:))

I've just spent a pleasant few hours reading up and watching YouTube videos on collimation. Makes much more sense to me now!

TBH Astrobaby is good concise summary of what needs to be done.

Executive summary: I hadn't really RTM!

Have now.

Should, with practise and patience, be able to sort it all out (and have a bit of rewarding fun in the process).

Thanks again to SGL memebrs for your inputs.

[Moonshane]

it'sone of those things that you think you'll never get and once you get it, you wonder how it took so long. really, don't worry about it as eventually it will all click and then seem a lot easier. do though check it each time and as you suggested you'll only have to make minor adjustments to primary in future.

some tips:

to check whether the secondary is round can be confusing - with nothing in the focuser, move back about 3 inches and the secondary silvered side will be more obvious.

ignore the relationship between cheshire cross hairs and the spider vanes

make sure your centre spot is actually in the centre.

make sure you have enough time when you first start and if you get really frustrated, walk away and have another go the following day.

if the views looks good with good detail and contrast, sharpness and quite tight stars then don't worry so much an observe!

[Paulb3098]

I've got the same set up and had the same problem, this is how I collimate my PX:

1. Centring the secondary, IMHO this should be a one off exercise done indoors in the daylight.

2. Once you have set your scope up, slightly loosen all the “bobs knobs” so there is some small play in the secondary mirror (the centre screw will make sure the mirror doesn't drop or lose it's center position)

3. Hold the stem of the secondary (carefully not touching the mirror) and line up the laser with the centre circle

4. Tighten up the bobs knobs one by one

5. Check the primary

6. Job done.

It may not be the right way but it works for me

[Grunthos]

OK, think I'm getting the hang of this now.

Had to wait until I could make my own collimation cap from an old 35mm film canister (none supplied with my new dobo). Finally found one in the attic...

Proceded with check of secondary as per guidance given by Astrobaby, Astronomy Shed vids and SGL members (esp. Jason D).

Didn't take me long to get everything aligned.

I think, for a first attempt, it is "good enough".

(Next job is to flock the back and stem of the secondary with blackboard paint!)

[Moonshane]

the secondary certainly looks good enough for observing. the primary will of course need to be adjusted and this is the more vital adjustment of the two. it looks as though this is still out looking at the (out of focus) spider vane reflections?

[Grunthos]

looking at the (out of focus) spider vane reflections?

Not spider vane reflections.

I just used A4 paper inside a red and separate blue plastic folder (like these) which were in my brief case.

The A4 paper (I just grabbed some folded receipts off the study table) had folds in them which is what you are seeing.

Now waiting on a cheap and cheerful 2x barlow to arrive to sort out the primary mirror (might check the collimation of my laser collimator whilst I'm waiting. Where's all that old pvc plumbing pipework gone?...)

[Moonshane]

aha silly me!