Collimation for an 11" SCT.

[WicklowSkies76]

Please have a look at the video link below.

https://1drv.ms/v/s!AuFbGvbjMa5pnIUoKJDLW_Mnc6sYWQ?e=W2dfJp

I bought the scope used and it has never been collimated so I am guessing it needs collimating by the video attached? It's a C11 XLT and the defocussed star was captured using a Neximage 5 camera.

One thing I noticed when trying to expose the collimation screws (there's a 'Fastar Compatible' twister on the front) is that the whole front section (not the glass outside case) rotates when twisted. So it looks like the entire housing (either side of the glass) is rotating when twisted so I can't really unscrew anything.

[dweller25]

The fastar unit can only be tightened up by taking the corrector off but you need to be VERY careful.

Hopefully you have a serial number on the fastar which needs to be parallel with the dovetail. 

If you do that make sure you mark the correctors orientation as it must go back in exactly the same position and make sure any shims holding the correctors edge go back in exactly the same way too.

I’m assuming something like a tree was in the way when you took that video as the flat edge should not be there ?

Edited September 18, 2019 by dweller25

[haitch]

1 minute ago, dweller25 said:

The fastar unit can only be tightened up by taking the corrector off.

If you do that make sure you mark it’s orientation as it must go back in exactly the same position.

I’m assuming something like a tree was in the way when you took that video as the flat edge should not be there ?

You can undo the outer ring with the grip on that holds the secondary mirror, take the secondary out then put your finger through the hole to grip the other side to allow you to tighten the secondary holder. Jusat be careful not to drop the secondary - I had a £139 bill and a 12 month wait for a new secondary to arrive from the US! 

The rings look quite concentric apart from the flat section - are you using a flexible dew shield? They sometimes flop across the aperture which could cause the flat edge.

[haitch]

[Star101]

When replacing the removable mirror, be careful to align that notch with the recess. If you miss align them, you may manage to slip the mirror back into location and jam it. So you cannot adjust anything.( Voice of experience here   - Had to wait until the air temperature changed to allow me to pull the misaligned mirror back out and place it back correctly.)

[dweller25]

43 minutes ago, haitch said:

You can undo the outer ring with the grip on that holds the secondary mirror, take the secondary out then put your finger through the hole to grip the other side to allow you to tighten the secondary holder. Jusat be careful not to drop the secondary - I had a £139 bill and a 12 month wait for a new secondary to arrive from the US! 

The rings look quite concentric apart from the flat section - are you using a flexible dew shield? They sometimes flop across the aperture which could cause the flat edge.

Brilliant solution - My only experience is with a C6 which has a much smaller secondary so my hand would not fit through.

[WicklowSkies76]

16 minutes ago, Star101 said:

When replacing the removable mirror, be careful to align that notch with the recess. If you miss align them, you may manage to slip the mirror back into location and jam it. So you cannot adjust anything.( Voice of experience here   - Had to wait until the air temperature changed to allow me to pull the misaligned mirror back out and place it back correctly.)

When I try to loosen the outer ring, the whole secondary holder rotates. So I can't get enough purchase on the outer ring to twist it off. I think it might be jammed! Do I need to let it cool down or heat up to free it up ?

[Star101]

The outer ring screws off leaving the mirror with the three screws in place. You have to try and pull the mirror straight out, it does not twist.

[WicklowSkies76]

Right - but what I mean is when I try to screw off the outer ring, the entire secondary holder twists. Make sense?

[WicklowSkies76]

1 hour ago, dweller25 said:

The fastar unit can only be tightened up by taking the corrector off but you need to be VERY careful.

Hopefully you have a serial number on the fastar which needs to be parallel with the dovetail. 

If you do that make sure you mark the correctors orientation as it must go back in exactly the same position and make sure any shims holding the correctors edge go back in exactly the same way too.

I’m assuming something like a tree was in the way when you took that video as the flat edge should not be there ?

It was the flat edge that worried me... perhaps that's not a sign of bad collimation?

[Star101]

I see. So the whole inner part, mirror,  rotates within the 11" glass!!

[WicklowSkies76]

1 minute ago, Star101 said:

I see. So the whole inner part, mirror,  rotates within the 11" glass!!

Precisely! The glass remains fixed but the whoe inner and outer part either side of the glass rotate. So I can't get any purchase on the outer ring with the grip, to twist it off.

 

If My collimation is OK this won't be too much of an issue. I thought the flat part in the vid was a sign of poor collimation? Seems not. Could be the dewshield possibly.

[Star101]

Bad collimation is where the stars look like the old silk fan to keep cool. Slight bad collimation is where a out of focus star looks cone shaped.

The video you posted looks like the dew shield is in the way, not a collimation issue.

If you do need to collimate the C11, and all other possibilities to remove the mirror have failed, Including pulling at an angle while twisting the ring, then you will have to remove the glass aperture plate. Easy to do but you need to make markings ( tape or chalk type ) on the glass and SCT before removing the glass to then get access to the back to release the mirror.

 

[WicklowSkies76]

3 minutes ago, Star101 said:

Bad collimation is where the stars look like the old silk fan to keep cool. Slight bad collimation is where a out of focus star looks cone shaped.

The video you posted looks like the dew shield is in the way, not a collimation issue.

If you do need to collimate the C11, and all other possibilities to remove the mirror have failed, Including pulling at an angle while twisting the ring, then you will have to remove the glass aperture plate. Easy to do but you need to make markings ( tape or chalk type ) on the glass and SCT before removing the glass to then get access to the back to release the mirror.

 

Thank you so much for the help. I will try the pulling at an angle first (albeit gently). I did take it off once, sounds like I didn't screw it back in properly or did so too tight!

[Cosmic Geoff]

I have collimated one SCT and checked the collimation on another.  My non-expert opinion is that the video does not show any need for collimation and you should leave well alone. Just check that you have not made things worse by twisting the Fastar around.

[haitch]

Hold the centre part still whilst you turn the outer ring - have you got Bob’s knobs like mine? They might give a bit more purchase on it.

[haitch]

If you have to remove the corrector plate don’t worry it’s not as  major an operation as it seems - just remember to make sure it is marked for alignment before you take it out m.

[haitch]

1 hour ago, Cosmic Geoff said:

I have collimated one SCT and checked the collimation on another.  My non-expert opinion is that the video does not show any need for collimation and you should leave well alone. Just check that you have not made things worse by twisting the Fastar around.

I’d like to know that the secondary holder is sufficiently tight though. Collimating SCTs is fairly easy with a star test.

Edited September 18, 2019 by haitch

[x6gas]

As has been said, you can remove the whole corrector / secondary assembly - it's not hard, you just have to be careful.  If you are doing this, you might like to consider flocking the inside of the scope at the same time.  As I understand it (and IIRC) the corrector glass has usually been marked to help orientation but this is not always the case.  If not use a marker pen to mark the glass and the scope OTA.  Needless to say, use the correct size cross-blade screwdriver as the screws can mar quite easily...

I am led to believe that the orientation of the secondary mirror is not crucial; that would seem to be borne out with my experience at least; I had the same issue as you in that my secondary was spinning so I had no idea of the original orientation.  Star tests and analysis with CCD inspector didn't show any appreciable difference with different orientations of the secondary (and I wasted a fair few clear nights confirming this), but that could, of course, just be my scope...

 

[johninderby]

Orientation used to be important in earler Celestron SCTs when mirrors were hand finished and were a matched set but now with machine finished mirrors doesn’t seem ro matter any more.

[haitch]

2 hours ago, x6gas said:

 

I am led to believe that the orientation of the secondary mirror is not crucial; that would seem to be borne out with my experience at least; I had the same issue as you in that my secondary was spinning so I had no idea of the original orientation.  Star tests and analysis with CCD inspector didn't show any appreciable difference with different orientations of the secondary (and I wasted a fair few clear nights confirming this), but that could, of course, just be my scope...

Given that Celestron are willing to sell replacement secondaries I would very much doubt there is any matching involved.

[haitch]

2 hours ago, johninderby said:

Orientation used to be important in earler Celestron SCTs when mirrors were hand finished and were a matched set but now with machine finished mirrors doesn’t seem ro matter any more.

I think there is still some matching involved with primaries and corrector plates just not secondaries

[haitch]

If you do flock the tube be careful what you use - I had a friend with a 9.25 who had no end of problems with shedding fibres 

[johninderby]

Buy the proper non shedding flocking material as sold by FLO. Regular flocking as found in a local shop does indeed shed.

Also when flocking an SCT or Mak get some acetate 600micron sheet to make a slip in liner and apply the flocking to the acetate sheet. Much neater job than trying to stick it to the inside of the tube. You could also use Protostar Flockboard but the cost of importing into the UK is prohibitive.

 

 

Edited September 18, 2019 by johninderby