Collimation issues

[ZOG]

I thought my collimation was pretty good on my Flextube 300p.

Using the standard and the barlowed laser method it looked spot on, that is, it did until I received my cheshire from FLO yesterday.

Well I had to check straight away just how accurate the barlowed laser was compared to the cheshire.

What a shock I got. Following Mels excellent tutorial I centred the secondary under the focuser, not much offset apparent, I would have expected more in an f5 scope but anyhow, not to bother as I read somewhere the other day that if you had the secondary centred, then the offset would be taken care of.

The first problem arose when I tried to see all three mirror clips in the secondary, it took a lot of tweaking but eventually I got there (I used a home made colli cap for this). Dropped the cheshire into the self centreing adaptor and took a look The cheshire cross hairs were nowhere near the centre spot but above and left of it. No donut shadow either but when I turned one of the collimation screws it moved away from the centre spot so I readjusted and moved it back inline. No amount of tweaking of the primary would bring those cross hairs central.

After kicking a few things around the room and muttering words heard only in the land of the irate Geordie I decided to give up with the cheshire and reset the whole thing again with the barlowed laser method, got it dead centre and dropped the cheshire in again to check. MILES OUT. Then I noticed something, the advantage of the flextube is that you can see where the laser hits not just on the primary but on the secondary as well, and this laser didnt seem to be hitting the right place. Instead of hitting dead centre of the secondary, it's hitting about a third of the way down from the top of the mirror.

This being an f5 scope I'm not sure where the beam is supposed to hit the secondary but I'm pretty sure it shouldn't be where it is. This of course poses a problem, the laser, both straight through and barlowed is telling me that my collimation is correct, even diffraction rings on a star test are even, but, I'm now fairly certain that my secondary is too far down the tube and the laser is telling me lies.

Could someone with a flextube f5 scope please stick a laser in the focuser and let me know exactly where the beam hits the secondary ?

Sorry for being so long winded but I wanted to get all the information in here and I'm afraid that the next thing I kick may just be something valuable.

Thanks

Bill

[arad85]

If everything is collimated perfectly, the return laser light should hit in exactly the same place as the sent laser light. That is, you only have a single spot...

[rawhead]

I thought my collimation was pretty good on my Flextube 300p.l

You have my sympathies. However good it looks using whatever method I always have a nagging doubt that the collimation is out.

When I used to play the guitar I was always fiddling with the tuning. Never seemed to be quite perfect.

I wonder how many of us using Newts have them truly collimated correctly.

Hope you get it sorted

Cheers

Jon

[dark knight]

When you get get the answers you need, please let me know too. I'm having major nagging doubts about our 12" Lightbridge (also F5)...:icon_scratch:

[mdstuart]

So frustrating isnt it.

The key seems to be to get the secondry right first showing the all the edge clips of the main mirror in a nice circle centred under the eyepeice holder as viewed through a hole in a 35mm film canister.

Then start on the main mirror using the chesire to get everything to line up. I like to keep a note on a piece of paper of how many turns I make each way on the main mirror so I can always return to where I started before trying something else.

It is possible just be careful not to overtighten anything or allow the secondry to fall off or put your hands on any surface and a lot of patience..

Then finally line up Vega defocus in each quadrant of your view and see if you get nice circles on high power both sides of focus..

Then leave it alone and only check every so often unless the stars look odd....

Mark

[Whippy]

To be honest, I think tools such as lasers and cheshires can only take you so far. I work to the theory and doing a star test is more reliable because what you see in the eyepiece is what you see with your eye, not how it looks through a cheshire or coli cap.

Have a look at this: A Star Test Primer .

Tony..

[Merlin66]

The secondary "off-set" in large fast (f ratio<5) causes the physical centre of the secondary mirror to be displaced DOWN towards the main mirror and AWAY from the focuser.

The laser beam should therefore NOT hit the secondary in the middle but some distance above...

There was a previous thread where I showed this result.

[dark knight]

for newcomers to the hobby, collimation has to be one of the hardest things to get ones head around, yes we understand that the optics have to line up, it's how it is achieved that causes the problem,,,for those who have been doing it for a long time please remember patience is a virtue and we all started out at the beginning once.

[Merlin66]

http://stargazerslounge.com/diy-astronomer/77359-100-secondary-off-set-newtonians.html

This was the previous thread on the subject of secondary off-set and collimation.

[Doc]

Maybe your laser needs collimating. Thats the problem with lasers. I would tend to believe the cheshire over the laser.

[Sparrow]

To be honest, I think tools such as lasers and cheshires can only take you so far. I work to the theory and doing a star test is more reliable because what you see in the eyepiece is what you see with your eye, not how it looks through a cheshire or coli cap.

Have a look at this: A Star Test Primer .

Tony..

I'd agree with this except for the fact that it's much easier to collimate in daylight than at night - particularly with cold hands. If I'm looking through the eyepiece I can't actually reach the primary mirror screws (8" Newt). Adjustment involves a small allen key and a screw so it's not easy in the dark. I wouldn't be without my Cheshire collimator and home made hole centred cap, but I do still use a bright star as a final check.

I must admit I do miss the easy collimation on my previous scope (Schmidt) which I fitted Bob's knobs to - only took a couple of minutes so I could check and adjust it every time I used the scope if I wanted to.

[Whippy]

No arguing with that. I used to use a laser during the day to get it close and star test it at night. FWIW, you can fit knobs (Bob's or otherwise) to pretty much any mirror primary or secondary.

Tony..

[ZOG]

Maybe your laser needs collimating. Thats the problem with lasers. I would tend to believe the cheshire over the laser

Mick, thats the problem, when I centre the secondary in the focuser and tweak it to get all 3 mirror clips in view, the cheshire cross hairs are nowhere near the primary centre spot and no amount of tweaking of the primary will get them where they should be, hence my idea that the secondary is displaced too far down the tube, at a guess I'd say that the laser spot (well collimated BTW) is hitting the secondary 15 to 20 mm above centre.

Ideally if someone with the same scope could tell me where the laser hits their secondary, I should be able to take it from there.

I measured the length of the vanes from the centre screw and they are 185mm all round so no offset there.

Merlin, I see your equation to calculate the offset, but I'm not sure how to measure the "T" distance.

Bill

[ZOG]

Aha, Whippy, I was looking for Bobs knobs for the secondary but they don't list them for SW scopes. Would the Celestron starhopper screws be the right ones ?

[Whippy]

Aha, Whippy, I was looking for Bobs knobs for the secondary but they don't list them for SW scopes. Would the Celestron starhopper screws be the right ones ?

Probably, not definately. Maybe Bob himself could answer that for you?

Tony..

[Doc]

Mick, thats the problem, when I centre the secondary in the focuser and tweak it to get all 3 mirror clips in view, the cheshire cross hairs are nowhere near the primary centre spot and no amount of tweaking of the primary will get them where they should be, hence my idea that the secondary is displaced too far down the tube, at a guess I'd say that the laser spot (well collimated BTW) is hitting the secondary 15 to 20 mm above centre.

Ideally if someone with the same scope could tell me where the laser hits their secondary, I should be able to take it from there.

I measured the length of the vanes from the centre screw and they are 185mm all round so no offset there.

Merlin, I see your equation to calculate the offset, but I'm not sure how to measure the "T" distance.

Bill

For a fast scope I would say 15-20mm above centre is about right.

Have you tried tightening all three primary collimation knobs down and then back them off 1 turn.

Then only use 2 out of the 3 to adjust the collimation.

[Merlin66]

In the previous thread I mentioned the alignment is displaced (Off-set) by an amount = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives a secondary mirror off-set of 5mm, this is equal to a point 1.7 x 5mm = 8.5mm along the surface of the secondary mirror towards the front ( ie sky end) from the physical centre of the secondary.

For your 300 f4.5(?) the physical offset in the secondary would be approx 6.3mm not "20mm or so".......

[Doc]

In the previous thread I mentioned the alignment is displaced (Off-set) by an amount = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives a secondary mirror off-set of 5mm, this is equal to a point 1.7 x 5mm = 8.5mm along the surface of the secondary mirror towards the front ( ie sky end) from the physical centre of the secondary.

For your 300 f4.5(?) the physical offset in the secondary would be approx 6.3mm not "20mm or so".......

You got me thinking now as I've checked mine and roughly found the centre and my laser dot is approx 15mm above centre. My scopes a 16" F4.5

My collimation is pretty good, I can split double double pretty easy. And very good star tests.

[Merlin66]

The physical off set on a 400mm f4.5 would be, assuming 350mm from the centre of the tube to the focus:

= 1.7 (350/4*(4.5*4.5)) which is approx 7.4mm. If the secondary is over-size then this off-set is not so critical. You can lose light/ efficiency one way or another.

[Doc]

The physical off set on a 400mm f4.5 would be, assuming 350mm from the centre of the tube to the focus:

= 1.7 (350/4*(4.5*4.5)) which is approx 7.4mm. If the secondary is over-size then this off-set is not so critical. You can lose light/ efficiency one way or another.

As for as I know all LB's come with oversize secondary mirrors.

[ZOG]

Hmmm, I just done some measurements,

The length of the mirror is 100mm so physical centre would be at 50mm

The laser strikes the mirror at 35mm so an offset of 15mm the same as Micks.

so why the heck do the barlowed laser which is supposed to be almost as accurate as the cheshire, and the cheshire itself, disagree so much ??

My head is being slowly battered by this and I feel a kick coming on

[dark knight]

lol Zog,,,I'm just about to do a cross between The Who and a Rolling Stones concert, throwing everything around the garden at the moment, won't make the collimation better, but it's good to get that aggression out of the way first,, so glad this OTA belongs to the wife lol

[mdstuart]

Wish I could come down and help.

Get the secondry right first...So using a hole in a 35mm film canister you should see a perfect circle of light which is the reflection of the main mirror. This circle should be centered in the view with the same gap all around the edge in the view down the eyepeice tube. The view should be lined up with the main mirror in that you can see all the three clips around the edge of the circle reflection.

In this state the secondry mounting should be more or less pointing down the tube ie no slanted to any side etc...

Get this right first and be sure before moving on to the main mirror...

Once everything is right if you put the cover on the tube and look down the eyepeice tube you should see your eye reflected from the centre of the mirror with the little circle sticker where your eye is...

Have you looked at Astro Babies description and diagrams as that is where most of us learned to do it...

Mark

[dark knight]

Kind thought there Mark, it's just so frustrating knowing that once shown, everything drops into place and you wonder how you never got it in the first place. It's the need for some reassurance when it just doesn't seem to be going right,,you know "I see this through the colli cap, here you have a look and tell me what you see". Thankfully Dave (Milliways) only lives 5 ml up the road and has kindly offered to pop round for a few beers and a demo...thank you, thank you, there is a god after all....

[albedo0.39]

Hey Zog - where do you live ? If even remotely not far from me, I would be happy to pay a house visit and I'll bet you in 10 mins we'll have it sorted with a cheshire (preferred) or a Laser.

Happy to help ?

Steve