Advice on attaching the secondary mirror?

[SkyHiker]

Isn't that what collimation screws are for?

[turboscrew]

Isn't that what collimation screws are for?

Basically yes, but the middle bolt (height adjustment) doesn't allow that much adjustment.

[SkyHiker]

Isn't that what collimation screws are for?

Basically yes, but the middle bolt (height adjustment) doesn't allow that much adjustment.

Well that deserves fixing then. The center bolt should allow the secondary to move enough. What is the position of your primary collimation screws, are they at their center position or nearly all the way in or out?

[turboscrew]

Isn't that what collimation screws are for?

Basically yes, but the middle bolt (height adjustment) doesn't allow that much adjustment.

Well that deserves fixing then. The center bolt should allow the secondary to move enough. What is the position of your primary collimation screws, are they at their center position or nearly all the way in or out?

Near center. It's just that the collimation is done using the slack of the center bolt. It's just that the center bolt doesn't have enough slack - unless the bolt is pushed up from its hole a bit. I don't have a good picture of it, and I don't want to disturb the silicone now that the mirror is glued.

[turboscrew]

Here's the secondary holder. The tilt adjustment screws are up to give a better view.
The center bolt can move about 2° back and forth in the spider, and the holder can move another 2° back and forth about the center bolt. That gives about +/- 4° as the adjustment range. I think, however, that the true, properly working range is closer to +/- 2°.

[SkyHiker]

Here's the secondary holder. The tilt adjustment screws are up to give a better view.
The center bolt can move about 2° back and forth in the spider, and the holder can move another 2° back and forth about the center bolt. That gives about +/- 4° as the adjustment range. I think, however, that the true, properly working range is closer to +/- 2°.

Sorry what do you mean by 2°(two degrees as I understand it)? The center bolt should be able to move the secondary mirror in the direction of the optical axis, not in an angular measure but in a linear distance measure such as mm. If you expressed the range in mm I would understand what you are saying but I don't get what the 2° means. Any angular rotation should be done by the secondary screws, not the primary.

[turboscrew]

Here's the secondary holder. The tilt adjustment screws are up to give a better view.
The center bolt can move about 2° back and forth in the spider, and the holder can move another 2° back and forth about the center bolt. That gives about +/- 4° as the adjustment range. I think, however, that the true, properly working range is closer to +/- 2°.

Sorry what do you mean by 2°(two degrees as I understand it)? The center bolt should be able to move the secondary mirror in the direction of the optical axis, not in an angular measure but in a linear distance measure such as mm. If you expressed the range in mm I would understand what you are saying but I don't get what the 2° means. Any angular rotation should be done by the secondary screws, not the primary.

The secondary mirror tilt screws can tilt the mirror about +/- two degrees. More than that, the secondary mirror height adjustment bolt won't allow that. Some of the tilt screws are very tight while the others are loose. The holder can't tilt enough.

In the picture there are 3 tilt screws in the upmost position and the center bolt/height adjustment screw/watchacallit in the low position. I'm talking about the maximum tilt angle of the center bolt relative to the spider and the maximum tilt angle of the mirror holder relative to the center bolt.

[turboscrew]

I also wonder how the h.. that can be! I don't seem to be able to bring the secondary above the primary such that they are on the same axis.I measured the secondary mirror, its offset, the holder and set the mirror holder center bolt in the middle of the tube. I can see the focuser side clip, but it's in the shadows unlike the two others. How the heck is that scope supposed to work?

Also, I can bring the primary mirror doughnut in the middle of the Chesire crosshairs, but the reflected image of the crosshairs is somewhere else. Or I can bring the crosshairs and their image on top of each other, but then the primary mirror center doughnut is somewhere else.

I can't figure out what's going on. As if there shouldn't be secondary offset, but I can't really understand how that could be.

[SkyHiker]

I also wonder how the h.. that can be! I don't seem to be able to bring the secondary above the primary such that they are on the same axis.I measured the secondary mirror, its offset, the holder and set the mirror holder center bolt in the middle of the tube. I can see the focuser side clip, but it's in the shadows unlike the two others. How the heck is that scope supposed to work?

That small tolerance does not leave much freedom for positioning the secondary on its holder with your glue, how can that be accurate to 2 degrees?

Also, I can bring the primary mirror doughnut in the middle of the Chesire crosshairs, but the reflected image of the crosshairs is somewhere else. Or I can bring the crosshairs and their image on top of each other, but then the primary mirror center doughnut is somewhere else.

I can't figure out what's going on. As if there shouldn't be secondary offset, but I can't really understand how that could be.

That sounds about right. For the explanation of what you see, check http://www.catseyecollimation.com/pensack.pdf .

[turboscrew]

I also wonder how the h.. that can be! I don't seem to be able to bring the secondary above the primary such that they are on the same axis.I measured the secondary mirror, its offset, the holder and set the mirror holder center bolt in the middle of the tube. I can see the focuser side clip, but it's in the shadows unlike the two others. How the heck is that scope supposed to work?

That small tolerance does not leave much freedom for positioning the secondary on its holder with your glue, how can that be accurate to 2 degrees?

Beats me, but I think I got it done well enough.

Also, I can bring the primary mirror doughnut in the middle of the Chesire crosshairs, but the reflected image of the crosshairs is somewhere else. Or I can bring the crosshairs and their image on top of each other, but then the primary mirror center doughnut is somewhere else.

I can't figure out what's going on. As if there shouldn't be secondary offset, but I can't really understand how that could be.

That sounds about right. For the explanation of what you see, check http://www.catseyecollimation.com/pensack.pdf .

I found out. The primary just was that much off. When I turned all the primary collimation screws to the same length (using vernier caliper), things started to look normal. I had to re-center the secondary mirror middle bolt, and suddenly It looked like quite close to being collimated.

I checked with the laser that the secondary offset was set right, and I left the secondary to that height.
Getting the secondary to look round (collimation cap) was quite easy, and after preliminary primary collimation with laser, it was quite easy to get the chesire crosshairs and their reflection on top of each other, and the hair crossings in the middle of the center doughnut on the primary mirror.

It looks like the twist-lock 2"-1.25" adapter has been fooling me. To get it center eyepieces, laser and chesire, it needs to be turned very tight. Feels even too tight, but otherwise the eyepieces, etc. wobble back and forth.

I think the telescope is in the working order now, and this time with the proper secondary offset.
Good to get it done before the work continues again (in Monday). The next chance to try star collimation may be as soon as February.

[turboscrew]

I think it now looks kinda good?
(My camera wasn't really up to the job.)

Another heavily magnified.

[SkyHiker]

I'm just looking at your offset calculations. The offset of the secondary hardly matters unless you insist that the optical axis and mechanical axis coincide. But that is only important if you use setting circles and the focuser axis is not perpendicular to the Alt axis. Otherwise, if they are not parallel the collimation of the primary compensates for it by making the reflection through the donut parallel to the optical axis defined by your focuser and secondary position. You can move the secondary around to make it fit no matter what the offset is, and to get the correct illumination, by following the standard collimation procedure.

But, for the case where we insist that these axes be parallel, I believe the offset depends on (for visual) where you want the focal point (depends on the position of the primary collimation screws), what FOV the eyepiece has, and (for imaging) the back-focus and sensor size. At F/4 this scope could be used as an imaging Newt though you might need a larger secondary. Vic Menard has a design tool for this at http://www.catseyecollimation.com/designie5.html . Probably too many things to consider. However for imaging it becomes more critical. For instance my Mak-Newt has a small secondary and I can tell the illumination is a bit off. It is hard to collimate this visually if you don't know where to place the eyeball for the image to be evenly illuminated.

[turboscrew]

I'm just looking at your offset calculations. The offset of the secondary hardly matters unless you insist that the optical axis and mechanical axis coincide. But that is only important if you use setting circles and the focuser axis is not perpendicular to the Alt axis. Otherwise, if they are not parallel the collimation of the primary compensates for it by making the reflection through the donut parallel to the optical axis defined by your focuser and secondary position. You can move the secondary around to make it fit no matter what the offset is, and to get the correct illumination, by following the standard collimation procedure.

But, for the case where we insist that these axes be parallel, I believe the offset depends on (for visual) where you want the focal point (depends on the position of the primary collimation screws), what FOV the eyepiece has, and (for imaging) the back-focus and sensor size. At F/4 this scope could be used as an imaging Newt though you might need a larger secondary. Vic Menard has a design tool for this at http://www.catseyecollimation.com/designie5.html . Probably too many things to consider. However for imaging it becomes more critical. For instance my Mak-Newt has a small secondary and I can tell the illumination is a bit off. It is hard to collimate this visually if you don't know where to place the eyeball for the image to be evenly illuminated.

Anyway, it's now done. I kind of like it when I know my gear, and don't have to guess. Especially if there are some problems, it's hard to know where to look next, if you can't be sure how things are set up. I recall calculating that the secondary is a bit big for this tube. I think I actually used the same calculator. I wonder if it's already photo-capable...? Or maybe some kind of compromise between AP and visual?