250px Flextube Collimation

[dcwales2012]

Hi,

Can someone please clarify my secondary is aligned in the focuser. I find it quite difficult with my glasses on just cant get close enough to peep hole as lenses are quite thick, however if I take them off I can't see anything, just a blur.

This picture taken down the focuser tube.

This picture taken down the Cheshire Eyepiece.

I have used both a 35mm collimation cap and the Cheshire EP to reach this point, and both are showing similar results.

Then when I take tube cover off primary it looks like this in the focuser.

Is the secondary out, or the primary?

If I use a laser it just confuses the issue.

If it is correct I will move on to the next stage, and align primary.

Thanks.

[dcwales2012]

I've sorted it.

[Moonshane]

it looked pretty spot on to me in terms of the secondary alignment with the drawtube.

[dcwales2012]

Thanks Moonshane.

Having spent days collimating my scope I've come to the conclusion, collimation caps, Cheshire EP and laser collimators tell lies.

With the cap the secondary appears centred in the drawtube and nice and round, checked it with a Cheshire and the secondary  can be too far down the tube. Use a laser and its not even in the frame. So using just the cap to align both mirrors and what can one see, the focuser  roller on the right, which indicates the secondary is still too far down the tube, and or twisted. So block of secondary from primary with green card and red card, loosen secondary give one full turn  clockwise, retighten check with cap and Cheshire, everything round and centred  in focuser. Check with laser , way out. So plan b, measure and mark centre of tube cover, place cover on bottom half of tube, put laser in focuser  and see where beam falls. Loosen secondary and get beam aligned with tube cover. Recheck with cap and Cheshire, remove tube cover and focuser  tube metal roller still in view to right. Plan c, put focuser tube all the way in and alter secondary so it can be seen by altering secondary up the tube. Use laser again on tube cover and gently tightened three screw to hold in place. Checked with cap making small adjustments to secondary until everything as one. Removed tube cover, adjusted primary mirror, no focuser roller bearing in sight. Checked with Cheshire and still everything spot on and secondary appears centred in tube. Finally, used laser and that hit the donut and the window in the laser.

Not sure how reliable any method is when everything looks ok in the cap and Cheshire, yet is so far out when mirrors appear aligned. Not sure I trust any method at present.

Waiting for gap in clouds for star test, but feel reseaonably confident my collimation is good.

[madsarmy]

The problem with the laser colli is the focuser tube. I had the same problem. Collimate, then take laser out. Put laser back in & it's out again.

The tollerence in the stock focuser are not the best. I ended up getting a moonlite. There are YouTube vidios on how to tighten up the stock focuser.

The way I got over the laser collimation when I was saving up for the moonlite was to look down the main tube to the primary. Adjust the laser dot to the centre doughnut on your primarymirror by adjusting the secodary, then collimate the primary. The next time you check colli make sure laser is in the doughnut from the front of tube again. If not adjust laser until its right. The you can then check the primary mirror.

Now I've got the moonlite I still check secondary to the primary. Collimaton if needed takes 1-2mins tops.

When I got my moonlite I put a metal washer & PTFE washer behind the secondary. I was looking down the main tube when reinstalling the secondary & thought the secondary didn't look centre to the tube. I then measured the spider & it was a cm off centre. The factory setup wasnt to clever.

Once you get used to collimating you won't bat an eyelid about removing mirrors for maintenance.

[Moonshane]

tightening the focuser (assuming it is rack and pinion - the crayfords don't have much if any slop if well adjusted) is a good idea. lasers can be out of alignment though so do check this - there's lots online using a V block.

unless you have a dot on the optical centre of the secondary I don't think it's possible to centralise the secondary to the drawtube with a laser. even then you need to visually check your secondary is round by eye with a sight tube. either way you really need a cheshire / sight tube combo in my opinion.

your secondary was well adjusted to the drawtube in the first pic. assuming your vanes are equidistant along each axis (they don't have to all be the same length as your tube may not be completely round - mine are not) then you have correctly adjusted the first requirement.

I only use a cheshire/sight tube for collimation as this allows for accurate collimation every time. assuming you are using your laser though, after the bit you did above, you then adjust the secondary with the three adjusters so the laser hits the centre of the primary donut.

then adjust the primary so the laser goes through the hole on the cheshire type slanted face. if you have a barlow, put the laser in this and then into the focuser. this splays the laser and projects the donut shadow on the face.centre this on the hole and you are done.

personally, I prefer a cheshire.

[dcwales2012]

Thanks madsarmy and moonshane.

All your suggestions I had actually undertaken. Spider nicely centred, one vane 1mm out so no problem there. I did stand in front of tube and look straight down with laser in focuser, but found it easier to see on the tube cover when aligning secondary roughly. Laser has been collimated and is within a gnats nadger of being spot on.

I have a Crayford focuser and it has virtually no slop, so secondary alignment using cap, Cheshire  and laser should have produced similar results.

My biggest concern is the secondary can be centred in the focuser similar to my photos above, yet it can be still too low. The secondary will appear round and centred, but in affect it can be skewed and twisted to give that impression. Then one collimates the primary believing it ok only to discover the secondary is skewed towards the right hand side of the drawtube.

I have a explorer 150p, which I'm going to seriously put out of alignment, by removing secondary mirror, and getting my son to mess up the primary mirror, then I will reassemble and try to see if this collimation tells me lies.

I'm wondering how many people believe their scopes are collimated correctly, when in fact the secondary is way out.

[Moonshane]

you may know all this but I thought it good to go through my procedure too.

I think your plan is a good one but might not be necessary. once you are happy with the vanes then adjust the secondary adjusters so they are all in the same position in relation to each other. if necessary use a mirror to check the secondary holder is square to the central hub. then get your secondary (with the primary blocked off with a sheet of card) like the first photo.

if your secondary looks like it does in the first photo (and assuming your camera was centrally placed) then the secondary is placed correctly in relation to the drawtube. this is an adjustment that rarely moves at all and hardly ever requires adjustment. this adjustment ensures that any light hitting the secondary is directed up the drawtube centrally making the most of it. assuming your three secondary adjusters are the same length as described above, this can be affected only by spider vane length. the central secondary bolt length and turning of the secondary.

the second adjustment ensures that the whole of the primary is aimed at the secondary (and of course vice versa). this is done with small adjustments on the three secondary adjusters. in my case I centralise the reflected donut on the cross hairs of the 'cheshire'. in your case you ensure the laser dot points at the central donut. again this should not need doing again unless you hit the scope hard. this ensures that all the light from the primary hits the secondary so you are not wasting aperture.

both the above stages have little observable effect upon the images and even if slightly out do not matter immensely for visual use. try to get them right though.

the final adjustment is the primary which ensures the centre of the primary points directly up the drawtube via the secondary. this ensures the sweet spot of the primary is in the centre of field. you know this one I am sure and it has the most effect on visual observing so needs to be accurate.

in reality if your secondary is out and your primary is aligned then this can have an affect but the primary is the key adjustment. thankfully it is the easiest as it's the most frequent to need attention.

personally, I changed from laser to cheshire years ago and have never regretted it once even though my scopes have tubes which are 5 foot long.

[Moonshane]

just to add, I'd use the cheshire all the way through as this can centre the secondary (you may need an extension for this or pull the cheshire out). in relation to the third pic with the cover removed, I'd say the secondary tilt needs adjusting to bring the donut central to the cross hairs. you then need to centralise the black dot of the cheshire in the donut and you are done.

[JamesF]

I did a full collimation on my 10" dob recently and started by checking that a laser collimator always pointed at the same place on the opposite side of the OTA (with the secondary removed, obviously) as the focuser was moved in and out.  If it doesn't then I think attempting to collimate with a laser is always going to give inaccurate results.  I ended up having to shim and adjust the focuser a fair bit to get it working nicely.

My OTA has been dropped fairly hard in the past mind.  That's how I got it cheap

James

[dcwales2012]

Thanks moonshane, and James.

I agree with what you say and believe this is the correct way, however IMO there is reasonable doubt still whether either, collimating cap, Cheshire or laser can give correct alignment of secondary mirror.

[Moonshane]

once you accept that perfect collimation is not possible to get or retain in a standard newtonian you'll be a lot happier. just get it as close as you can and enjoy the scope. close is good; perfect is unachievable.

[JamesF]

Thanks moonshane, and James.

I agree with what you say and believe this is the correct way, however IMO there is reasonable doubt still whether either, collimating cap, Cheshire or laser can give correct alignment of secondary mirror.

I think they can, or certainly get exceptionally close, but you have to keep stepping back to check the accuracy of your tools.  For example

Is the focuser square to the optical axis?

Does the focuser tube run true to the axis of the focuser?

Is your laser collimated?

If you're using Mire de Collimation for checking the positioning of the secondary with respect to the focuser, is the camera collimated?

and so on.  It may be that collimation can be "sufficiently good" without checking all of these things, or that for some scope designs (eg. slower focal ratios) it can be so, but if you want to get it near perfect I think you have to start right from the beginning and check absolutely everything, one step at a time.

James

[JamesF]

once you accept that perfect collimation is not possible to get or retain in a standard newtonian you'll be a lot happier. just get it as close as you can and enjoy the scope. close is good; perfect is unachievable.

I think this is a good point.  Collimation doesn't have to be absolutely perfect.  Just "as good as it needs to be".  A little distortion in the stars towards the edges of the field of view for instance may be an issue if you're imaging with a large sensor, but not if you're viewing using 68 degree eyepieces.

James

James