How to use a cheshire collimating eyepiece

[jonathan]

I had a look down my Cheshire but didn't see a major problem, it maaaaaybe could have been tweaked, but the three screws on the front of my SCT were very tight and turning them slightly didn't seem to make any difference to the position of the circles.  I'm wondering if it's different for an 8" SCT, maybe more turn is required?  I've tried to find a decent video on using a plain Cheshire with an SCT but they all seem to use a camera, laser collimator, or artificial star.

[AstroMuni]

2 hours ago, jonathan said:

I don't have many bright walls inside the house and there isn't enough distance for an artificial star (we have small, dimly lit rooms... old house thing).

You could shine a torch at the opening. It just needs enough light so that its visible when you look through the hole.

[Pixies]

23 minutes ago, jonathan said:

I had a look down my Cheshire but didn't see a major problem, it maaaaaybe could have been tweaked, but the three screws on the front of my SCT were very tight and turning them slightly didn't seem to make any difference to the position of the circles.  I'm wondering if it's different for an 8" SCT, maybe more turn is required?  I've tried to find a decent video on using a plain Cheshire with an SCT but they all seem to use a camera, laser collimator, or artificial star.

Just out of interest. How does one collimate an SCT with a Cheshire? What do you see when you look down the tube?

[jonathan]

1 hour ago, Pixies said:

Just out of interest. How does one collimate an SCT with a Cheshire? What do you see when you look down the tube?

I see some light and dark circles and an out-of-focus crosshair.  I think the concept of getting all of the circles aligned down the middle is the same, but I just haven't seen anyone do it with a Cheshire.  I'd do it using a bright star that's out of focus but I'm still waiting for a good clear night with no thin cloud.

Edited August 26, 2020 by jonathan

[RT65CB-SWL]

1 hour ago, Pixies said:

Just out of interest. How does one collimate an SCT with a Cheshire? What do you see when you look down the tube?

I have never used a 'Cheshire' collimator when collimating my C6/SCT-xlt and ETX105 and use a collimation cap; (same as the one shown in the video); and/or artificial star. You should see concentric rings. If they are slightly oval, then it will need collimating. You can use a laser collimator, but that too will need to be checked for collimation.

[Starwatcher2001]

I always used a cheshire to collimate my dob, and when I bought an SCT thought I should be using it on that too.

After a lot of messing around on the SCT, I found the cheshire was just confusing things. You don't adjust the primary mirror in an SCT, just the secondary. I also spent a happy few days with a touch, tinfoil and a pin making artifical stars to try collimating during the daytime, but I've found no real substitute for just using real stars.

I made some notes for myself, which may be helpful. I'm not putting these up as the only or correct way to do this, it simply works for me and helps me work out where the screws are on my scope, and which one to twiddle!  If a screw is too tight to tighten more, then slightly slacken the other two. 1/12 of a turn is enough.

If the Fresnel rings are all over the place the atmosphere probably isn't steady enough to collimate, enjoy the scope and tweak another night. If the rings look weird, particularly on one side, it's probably tube currents and needs longer to cool.

NOTE: This is using a star diagonal. The directions will be wrong without one.

Edited August 27, 2020 by Starwatcher2001
Note on star diagonal