Collimating an ST80?

[Stu]

I noticed the other evening when observing doubles with the ST80 at high power that the collimation is a little out, nothing disastrous but it is definitely affecting the performance of the scope.

I was expecting to be able to loosen the front ring and give the tube a gentle tap to settle the elements but it seems not. Has anyone done this? Does removing the whole objective by unscrewing the four screws give me access to anything?

Thanks in advance 👍

[Jim L]

The screws that hold the lens cell to the tube, like the screws that hold the focuser to the tube, all allow for a small amount of movement. That freedom of movement of the lens cell and the focuser can, on the one hand, cause mis-collimation, and on the other hand, help you to collimate the scope if it is out. I loosen one set or the other enough that I can move the lens cell or focuser, but against some resistance, all while checking collimation. When it’s right I tighten the screws while doing my best to maintain correct collimation.

As to wiggling the crown and flint objective, that’s a hit or miss proposition because the two elements are held relatively loosely within the cell. You can tighten up the fit of the elements within the cell using small pieces of thin adhesive tape at several spots along the plastic cell annulus. Doing so improved the objective alignment for my own ST80. Good luck!

[happy-kat]

I took mine apart I followed atronomyshed video on the ST80 it also covered collimating it

Edited November 15, 2023 by happy-kat

[Franklin]

51 minutes ago, Stu said:

Has anyone done this?

Some designs have the retaining ring on the inside, behind the rear element, so it is still possible to do the "slap the tube" thing but the problem then is checking to see if you've got it right. You have to keep putting it all back together and cheshire testing. Quite a faff, I'd just live with slight off-collimation and keep the scope for low-power wide field stuff.

[happy-kat]

This one

https://youtube.com/watch?v=Ylb7xnc_03U&si=AEfpjBegxXAtiyOz

[Stu]

50 minutes ago, happy-kat said:

This one

https://youtube.com/watch?v=Ylb7xnc_03U&si=AEfpjBegxXAtiyOz

Thanks very much, really useful. I’ll have to recheck when I’m home tomorrow, but I could see an obvious retaining ring on the front of the scope to loosen but might just have missed it. I’ve got a Cheshire eyepiece so will give that a go, it’s only a small tweak that it needs but it should improve the performance nicely I think.

[Stu]

1 hour ago, Jim L said:

The screws that hold the lens cell to the tube, like the screws that hold the focuser to the tube, all allow for a small amount of movement. That freedom of movement of the lens cell and the focuser can, on the one hand, cause mis-collimation, and on the other hand, help you to collimate the scope if it is out. I loosen one set or the other enough that I can move the lens cell or focuser, but against some resistance, all while checking collimation. When it’s right I tighten the screws while doing my best to maintain correct collimation.

As to wiggling the crown and flint objective, that’s a hit or miss proposition because the two elements are held relatively loosely within the cell. You can tighten up the fit of the elements within the cell using small pieces of thin adhesive tape at several spots along the plastic cell annulus. Doing so improved the objective alignment for my own ST80. Good luck!

Thanks Jim, appreciated. I’ll give it a go.

I know I shouldn’t bother too much, but I want to do a comparison with the LT80 I’ve got, and don’t want it to be unfair because the ST80 is misaligned.

[John]

Last time I had an ST80 the lens retaining ring was at the front of the objective cell. It had a plastic rim around it and to get any grip on that, the dew shield had to be removed. I think that slid off the objective cell as I recall. The retaining ring could have been overlooked though because, unlike many others, it did not have any location slots or holes in it to take a tool. It just has a smooth top plastic rim. I've had a look at a Skywatcher Evostar 90 objective cell that I happen to have knocking around this evening and that uses a similar approach. 

This is not my ST80 but it is how it looked with the dew shield removed. The lens retaining ring is marked with red arrows:

 

 

[Stu]

5 minutes ago, John said:

Last time I had an ST80 the lens retaining ring was at the front of the objective cell. It had a plastic rim around it and to get any grip on that, the dew shield had to be removed. I think that slid off the objective cell as I recall. The retaining ring could have been overlooked though because, unlike many others, it did not have any location slots or holes in it to take a tool. It just has a smooth top plastic rim. I've had a look at a Skywatcher Evostar 90 objective cell that I happen to have knocking around this evening and that uses a similar approach. 

This is not my ST80 but it is how it looked with the dew shield removed. The lens retaining ring is marked with red arrows:

 

 

Thanks John. Clearly I need better glasses 🤣🤣. Will try again tomorrow. I suspect tweaking with the focuser alignment will be a more likely remedy to the problem but we will see.

[happy-kat]

I think this is with the lenses removed, ST80 Orion livery, and before I then blackened the inside and on the shiny bits.

 

Edited November 16, 2023 by happy-kat

[happy-kat]

[Stu]

Thanks all for the input.

I’ve had a play around using an artificial star and a 2.4mm Vixen HR planetary eyepiece. I found the most effective improvements were achieved by loosening the front ring (once I found it 🤪) and tapping the tube gently to settle the elements into place, and secondly tightening up the focuser using the two small grub screws. It is now pretty stiff, but at least it is square and doesn’t wiggle. 

Whilst not absolutely bang on, I’m pretty satisfied with it now and am looking forward to giving it a try out. I took a few images of the in and out of focus artificial star, they seem ok. Sorry for the grubby eyepiece 🤪.

[Jim L]

The rings look nice and concentric, Stu, though if I squint hard enough I believe I can detect just a touch of what the experts call chromatic aberration.

[John]

That looks pretty good to me @Stu 👍

 

[Stu]

16 hours ago, Jim L said:

The rings look nice and concentric, Stu, though if I squint hard enough I believe I can detect just a touch of what the experts call chromatic aberration.

Gosh, you must be really sensitive to CA Jim 😱🤪🤣🤣🤣🤣

[Stu]

On 15/11/2023 at 20:27, happy-kat said:

I took mine apart I followed atronomyshed video on the ST80 it also covered collimating it

Watched the video, some good stuff in there. One thing I didn’t get at all was the use of a Cheshire. All I could see was a bright circle and cross hairs, nothing of use. What was I doing wrong??

[John]

9 minutes ago, Stu said:

Watched the video, some good stuff in there. One thing I didn’t get at all was the use of a Cheshire. All I could see was a bright circle and cross hairs, nothing of use. What was I doing wrong??

Apologies if you know this Stu. What we are looking for is the reflection of the illuminated face of the cheshire. It shows on the rear faces of the lens elements as two small disks of light with a dark central hole. Ideally they will be one on top of the other. If the objective is tilted or the focuser is askew, they show as two disks not overlapped or even side by side.

Apologies again if you know all this and have seen such illustrations before.

Edited November 18, 2023 by John

[Stu]

10 minutes ago, John said:

Apologies if you know this Stu. What we are looking for is the reflection of the illuminated face of the cheshire. It shows on the rear faces of the lens elements as two small disks of light with a dark central hole. Ideally they will be one on top of the other. If the objective is tilted or the focuser is askew, they show as two disks not overlapped or even side by side.

Apologies again if you know all this and have seen such illustrations before.

Thanks John. No, I don’t really know much of that! I tend to use a laser for collimation on Newts. I can’t actually relate what I was seeing with your images though so I need to have another go.

[John]

2 minutes ago, Stu said:

Thanks John. No, I don’t really know much of that! I tend to use a laser for collimation on Newts. I can’t actually relate what I was seeing with your images though so I need to have another go.

I find that the the 45 degree face of the cheshire needs to be really well illuminated to see the reflections clearly.

[Jim L]

To build upon John’s excellent advice, I use a small penlight held up to the side of the Cheshire eyepiece. It’s also helpful to remove the crosshairs at the end of the Cheshire as they’re nothing more than a distraction when collimating a refractor. Sometimes they unscrew, sometimes you’ve got to cut them.

Next, you’ll need more hands than you’ve got to hold the scope, hold the penlight, rotate the Cheshire, and adjust the focuser or lens cell alignment, so mounting the scope in a stand or low tripod makes life easier.

 

If you aim the scope at a dark matt surface the reflected circles within the Cheshire eyepiece will be easier to see clearly. Bonus points if you buy yourself a close focus monocular to make collimating even more precise but you tell your wife you bought it for her to look at hummingbirds in the garden and she has no idea it’s really a tool for your telescopes.

This SvBony monocular won’t focus close enough for the short focal length ST80, but it works for most of my other refractors. My wife’s never seen this configuration, and I plan on keeping it that way.

All set to go with #8 and #12 Wratten filters in the tripod basket.