Misaligned secondary on an SCT yet in collimation

[DrRobin]

Hi,

If a secondary is in the wrong place on an SCT will it still collimate? I am thinking it probably will, but as the figuring of the misaligned secondary no longer matches the primary/corrector the scopes resolving power will be like the bottom of a jam jar, is that a correct assumption?

Here is why I think star collimation only tells you part of the story.

When a star is on axis, the primary partially focuses the light to what should be the centre of the secondary, which when it is correctly aligned, reflects this light down the tube and to the eyepiece. The light doesn't actually come to a point on the secondary, it covers a small area near the centre and only when the secondary is correctly centred will it accurately reflect this light to the eyepiece.

If the secondary is not correctly centred, it will still be possible to adjust the collimation screws to send a point source correctly down to the eyepiece, by tilting it, but anything with a spread of light, like a planet, won't get focused correctly as some points will be in correct focus and others not.

I assume if you defocus on a star field with secondary in the wrong place then some of the stars will show correct diffraction rings and others won't. When it is in the correct position all of the stars will show concentric patterns across the frame, subject to the flatness and correctors of the scope.

Are these assumptions correct or will it only ever show correct collimation when the secondary is in the correct place?

I am assuming the same argument would apply to the primary mirror and to a much lesser extent, the corrector plate or plates?

Robin

[ronin]

When a star is on axis, the primary partially focuses the light to what should be the centre of the secondary, which when it is correctly aligned, reflects this light down the tube and to the eyepiece. The light doesn't actually come to a point on the secondary, it covers a small area near the centre and only when the secondary is correctly centred will it accurately reflect this light to the eyepiece.

The secondary does not need to be correctly centered. the "cone" from the primary just has to fall on the secondary with none falling off the edge. Therefore the size of the secondary is relevant. If the secondary is off to one side the cone still hits a flat 45 degree nirror and so reflects as required into the focuser+eyepiece.

When you collimate the requirement appears to simply get the view back to the start, to an extent that means that things can be "off" but you conpensate by adjusting angles to get what you want. In effect if the primary is 2 degrees off perpendicular you compensate by adjusting the secondary. So the presumption is "Goody I can see the mirror centre marker in the centre of the collimator."

[DrRobin]

Thanks Ronin,

I agree with what you say for a newt with a flat mirror, but a SCT uses a convex secondary mirror with typically 5x magnification, I am guessing this doesn't work unless it is exactly centred as it won't present the correct profile to the spread of light.

Robin

[ronin]

Apologies, just read the bit that says SCT. So basically all is rubbish/gierish and the ramblings of an idiot.

Not sure how I missed that bit.

OK, stupidity would account for it.

[Mr Spock]

I'm not sure how you would get an SCT secondary in the wrong place as it fits in the middle of the corrector plate.

If you have removed the secondary then you will need to collimate the scope as it will most likely be out. With an SCT that's best done by a star test - adjust the three screws on the secondary.

You can get close before going outside by looking down the front of the tube from about 3mtrs away. You should see concentric rings if all is well, if not, adjust the three screws until it's close.

[Laurie61]

I'm not sure how you would get an SCT secondary in the wrong place as it fits in the middle of the corrector plate.

If you have removed the secondary then you will need to collimate the scope as it will most likely be out. With an SCT that's best done by a star test - adjust the three screws on the secondary.

You can get close before going outside by looking down the front of the tube from about 3mtrs away. You should see concentric rings if all is well, if not, adjust the three screws until it's close.

I guess Robin is talking about mirror flop, something that cant be adjusted for. I think I read somewhere that because the mirror's are spherical this is not quite so bad as it might be ? but I may be getting confused with something else    

[DrRobin]

The secondary holder came loose when I took the secondary out and put a Fastar in. Interesting as looking around a lot of people have had loose secondaries. The secondary holder is slightly smaller than the hole in the corrector, possibly as much as 3mm so it is possible to get it out of alignment.

If you remove the secondary you can sight down the scope and check it is concentric with the main and focus tube, but I understand it needs to be very accurately aligned. I did remove the secondary, put a piece of paper with a bullseye target in place and used a laser in the focus tube, it confirmed that my visual sighting was correct.

The main reason I ask is the secondary was offset, yet it appeared to collimate. The views of Jupiter were however rubbish.

Robin

[DrRobin]

Mirror flop is another potential issue, but I don't think it moves the primary far enough to throw the collimation out, just moves the centre of field of view a little. My scope has dampeners to lock the mirror in place to stop mirror flop, but of course you can't adjust the focus (of the primary) when they are tightened.

Robin

[Laurie61]

I have a c11 xlt, I removed the corrector so I could flock it and there was a little play in the secondary housing. I assumed it needed to be centred and adjusted it, I had moved it slightly when removing the corrector plate. It appears to be as sharp now as previously but I have found that it needs to be spot on with collimation to perform. A 1/32 turn on a bob's knob will make the difference between soft and sharp on my scope.    

[DrRobin]

Agreed. Did you just realign the secondary by eye?

I assume you just collimate on a star? When I do mine the final movement of the collimation knob is just about the smallest movement i can make, not sure if this is 1/32nd of a turn, but it must be a very small amount.

Robin

[Laurie61]

Agreed. Did you just realign the secondary by eye?

I assume you just collimate on a star? When I do mine the final movement of the collimation knob is just about the smallest movement i can make, not sure if this is 1/32nd of a turn, but it must be a very small amount.

Robin

Yes I realigned by eye, I had felt it move slightly and basically put it back where, I thought, it came from When collimating I do use a star and a 9 mm ortho although last time out my Brother collimated it while looking at Jupiter. He said he could see a slight smearing in one direction and adjusted to pull this back over the planet and it popped into sharp view, Not something I would generally recommend but it was only a fraction out.      

[Peter Drew]

My professional experience with SCT's predates the Fastar facility so I've not come across this issue before. I'm surprised that there is as much clearance between the corrector and the secondary holder as suggested. In early Celestron SCT's the outside diameter of the corrector was rough cut and was shimmed with cork pads to centre the secondary which indicates that this is important. Although the secondary optical surface is essentially spherical miscollimation of a sphere is only of no consequence if the tilt "pivot" is at the optical radius. This obviously is not in this instance and is the reason why a very small amount of adjustment of the collimation screws has such a pronounced effect. If the secondary is not central it seems to me that tilting it to compensate could easily result in a less than perfect image. It might just be possible, on close examination, to detect a witness mark on the corrector where the original setting was. 

[billhinge]

I also removed the corrector plate from my C11 when it dewed up on the inside and I decided to flock at the same time

When you remove the corrector you notice the only thing centring  is a set of cardboard shims around the edge - hardly sub mm accuracy and I guess it would shift over time

It is over a year since I last looked as I prefer my 80mm APO for ease of use

I have one of the Hotech 3 laser gizmos, I think someone did a write up recently saying this sort of problem can be addressed with the Hotech.

Edit - found it

see last post

http://stargazerslounge.com/topic/148427-hotech-advanced-ct-laser-collimator/?hl=%2Bhotech#entry2188488

Edited January 30, 2014 by billhinge

[billhinge]

PS 

I have the Hotech unit but I find that it is very sensitive and really the SCT should be on some stable bench to do the alignment ultra accurately and I haven't got around to making such a test bench yet

[DrRobin]

Peter, perhaps it isn't as much as 3mm, but it certainly felt like it.  There was quite a bit of movement in the secondary holder when I did move it.  I didn't undo the corrector plate, it has never been off, so thought it best to leave alone.  The secondary holder has a notch in it to align the secondary when it is replaced after using Fastar.  After my secondary holder became slack it was tightened again, but I noticed just recently that the notch was in the 2 o'clock position and by all accounts it should be in the 3 o'clock position.  The secondary also has a black mark in this position.

I turned it round and then re-centred first by eye (reflection of the secondary and the focus tube) and then replaced the secondary with a paper target and put a laser in place of the eye piece, both confirmed it was now centred.

Bill, I too have a Hotech CT, I think after reading your post before.  I find it difficult to get it spot on, but it did tell me my Crayford was out of line and was useful in re-centering.  I last used the CT before the secondary became loose, but might check it using the procedure you outlined.

I too have been thinking of making an optical bench, a couple of pieces of 4x2 timber, some thick board and a couple of kitchen cupboard levelers should be stable enough to hold the C11 on a saddle plate, or even V-notch and give vertical as well as horizontal adjustment.  The CT has it's own micro adjuster.  Then just put the whole lot on a bench and I will be able to adjust and measure in perfect stability.  It might even work for my Newt?

Now that my C11 has had it's tune up, I just need a bit of clear sky to star test it.

Robin

[billhinge]

Hi Robin

I was thinking similarly, just haven't got around to it

Don't expect any clear weather for months now, I received my Celestron Starsense early this week which means months of cloud

[Jammy]

I know this is a really old thread, but when you say collimated, but mis-aligned; does my photo show what you are thinking?

I'm starting to think my secondary mirror is off centre.