Crayford focuser drawtube tilt adjustment

[lux eterna]

Hello,

Does anybody know if it is possible to adjust the drawtube tilt on a Crayford ? On each side of the focuser,just above the focus knobs, there is a group of three screws which can be seen here. I guess it is the usual design with one screw pushing and the other two pulling (or the other way around). But can these be used for tilt adjustment ? Or what are they used for ? I am asking mostly out of curiosity, but I do have a slightly tilted focus plane with my dslr so maybe I should get a 1.25" laser collimator and test the drawtube tilt by rotating the focuser.

Regards

Ragnar

 

[LightBucket]

Hi,

i think you have to loosen the centre screw and either tighten or loosen the other two, they are adjustable to make the focuser smoother or tighter, but not sure about tilting, as they should be factory set, but be careful, as if they are to tight you can break the bearings.... that's how mine worked anyway...and it's very similar.

whay do you need to tilt the drawtube...? Is it not inline..?

[Merlin66]

The answer is usually to address the squareness of the focuser on the OTA. WIth a refractor you can use a laser pointer to verify the focuser is square to the optical axis.

 

[LightBucket]

Just now, Merlin66 said:

The answer is usually to address the squareness of the focuser on the OTA. WIth a refractor you can use a laser pointer to verify the focuser is square to the optical axis.

 

Can you explain how to do that with a laser, I am interested...

[lux eterna]

Thanks for the suggestions.

@LightBucketMerlin66 is right - I want to make sure that the focuser drawtube is perpendicular to the optical axis. If it is slightly tilted I can not get all parts of the image in focus simultaneously.

My intention to do that is to put a pre-collimated 1.25" laser into the eyepiece holder and then rotate the focuser housing to see if the laser dot is (not) moving.  My OTA is a Meade LX200 8"  so I would have to remove the corrector plate first, but that is no big deal.

Ragnar

[LightBucket]

3 minutes ago, lux eterna said:

Thanks for the suggestions.

@LightBucketMerlin66 is right - I want to make sure that the focuser drawtube is perpendicular to the optical axis. If it is slightly tilted I can not get all parts of the image in focus simultaneously.

My intention to do that is to put a pre-collimated 1.25" laser into the eyepiece holder and then rotate the focuser housing to see if the laser dot is (not) moving.  My OTA is a Meade LX200 8"  so I would have to remove the corrector plate first, but that is no big deal.

Ragnar

That's all well and good, but when you put the corrector and secondary mirror back you will have to re collimate, and then that needs to be perfect or you have wasted your time....

good luck

[m.tweedy]

one of these would address all issues 

https://www.firstlightoptics.com/hotech-collimation-tools/hotech-advanced-ct-laser-collimator.html

[Merlin66]

Light bucket,

on a refractor just make a paper disk to fit the aperture and mark the center.

fit a collimator laser to the eyepiece holder and adjust the focuser until the laser spot falls exactly on the center point......job done.

collimating the objective is another issue........

[Merlin66]

Ragnar, are you sure the problem in the focuser?

the SCT thread usually gives a very square outcome. Could it be the camera mounting?

[lux eterna]

No, I am not sure there is any issue with the focuser. I just want to test how good (or bad) it is. I do not suspect fhe focuser housing for being tilted, but I guess the drawtube could be. 

 

[Peter Drew]

If you use a holographic laser pointer it will project a bulleye target, this would show the concentricity or otherwise of the Crayford focuser tube. 

[LightBucket]

2 hours ago, m.tweedy said:

one of these would address all issues 

https://www.firstlightoptics.com/hotech-collimation-tools/hotech-advanced-ct-laser-collimator.html

Not at that price, I could buy a new scope for that...

[m.tweedy]

If 

5 minutes ago, LightBucket said:

Not at that price, I could buy a new scope for that...

if you are in the North East we (Sunderland Astro) are doing a collimation  workshop on Sunday (and yes we have one of the Hotec SCT collimaters).

 

Sorry if i have hijacked the thread from the OP. Delete if necessary.

[lux eterna]

I just fixed it.

First I fit a laser pointer inside a tube that fit the 1.25" EP adapter. The laser output end was centered carefully inside the tube with a piece of foam, but the laser rear end had to be tilted pretty much to compensate for the non-collimated laser diode. Anyway, I managed to align the laser rear end so the red dot on the floor moved only 1 mm when I rotated the complete 2" - 1.25" EP adapter / tube and laser combo inside the draw tube.

Then I rotated the focuser housing (and everything in it), which was lying with the nicely flat machined flange on my vice, with the draw tube protruding between the jaws. A steel ruler, attached with two magnets, was supporting one side of the drawtube so it would not move sideways, and the vice jaws was barely touching the drawtube. The red dot was some 15 mm off center when I rotated the focuser. Distance from laser to floor was maybe 90 cm.

After some experimenting I got it right. The two small screws in each group of three (see first post) are pushing while the middle is pulling the bearing assembly. I also had to adjust the tension screw under the focusing axis a little.

Ragnar

 

 

[m.tweedy]

excellent solution

 

[lux eterna]

I might as well continue this old thread. The focuser was much better aligned after the above alignment, but another focuser problem was still there :

Egg shaped stars on longer exposures. I was hunting the source of this for a long time until I found that the focuser draw tube was to blame for much of it. Tightening the adjustment screws harder did not help much, so I ended up with this solution. Not so pretty, and not compatible with a focus motor, but it really works - it has a big impact on star roundness. I think I have learned the lesson of gravity and a slowly rotating object (my scope & camera). The force of gravity will constantly change direction, and so will any flexing parts do.

The upper wingnut is focus lock while the lower is just a locking nut for extra stabilization to the stainless steel bracket. Each time I refocus I have to release the bottom allen screw. One of the three thumbscrews that hold the camera is replaced with a crosshead screw (under the small hole) so a screwdriver is needed when attaching the camera.

Ragnar