TheSkySearchers

How exact does the eyepiece-focuser-diagonal alignment to the objective need to be?
Like if I put a laser in place of the eyepiece. how exact would it need to exit the dead-center of the objective lens to give an image without noticable degredations or aberrations?
For an air-spaced doublet refractor scope. 70mm F6

Mine always seems to be slightly off. no matter how I wiggle or fiddle with the connections or trying to shim things. its always slightly high. low. or off to the side. just moving the scope by holding the diagonal causes the laser dot to slightly wander all around the middle 25% of the objective lens. it usually is slightly off to one side and up or down.
and the reflection doesnt go back to the center of the laser collimator tool (Shouldn't the slightly defocused laser dots meet right in the center??
I think i'll have to somehow shim the entire focuser housing/assembly some way.
just depending how i tighten the thumb screws can throw it off.

How important is getting it perfect? or close to perfect?

I still also have an over/under correction issue with the lens air-spacing gap. it seems slightly under corrected. its close but not oquite there. need a thinner spacer ring. I tried little square spacer "tape" things but due to the slight variences in size and thickness the airy disks were always distorted to some sided in some way. so I put it back the way it was. now I get perfect airy disks with rings. but im positive the spacing is slightly off.
but thats a separate issue.

realflow100 wrote: ↑Mon Sep 06, 2021 10:22 am How exact does the eyepiece-focuser-diagonal alignment to the objective need to be?
...
How important is getting it perfect? or close to perfect?

Respectfully, I'll go out on a limb here and say that it doesn't really matter.

I'm a frac-head at heart with around a dozen of them. While it is good to recognize the things you mention in your post, I suggest "That rabbit is not worth chasing."

Clear Skies

with my DSLR it does seem to matter. one corner has more blurryness than the other (looks like focus plane is not perfectly even across all four corners.)
The center is ok but i'm sure it could be better. at least if it wasnt tilted at all. and more straight in-line with the objective somehow?

with an eyepiece i cant notice anything off. looks fine.

I try to keep my scopes collimated and make sure that focusers are properly tensioned. If star test looks good then it is good enough for visual.

For AP one typically remove diagonal and EP, so you just need to make sure that your scope is collimated and the focuser tube is not sagging.

the star test looks ok. airy disk and diffraction rings.
but with just the camera the focus plane is slightly tilted and offset and it bothers me (causes uneven star field.)

realflow100 wrote: ↑Sun Sep 12, 2021 4:38 pm the star test looks ok. airy disk and diffraction rings.
but with just the camera the focus plane is slightly tilted and offset and it bothers me (causes uneven star field.)

I would suspect the focuser then. If focuser tube is loose or you overloading the focuser you will get focuser tube sagging under the weight.

Unfortunately stock focusers on cheap (and often not so cheap) scopes are typically not suitable for AP. Too much play.

realflow100 wrote: ↑Sun Sep 12, 2021 7:21 pm ive shimmed the focuser and tightened the focuser knob screws so its rock solid. but its still showing off! The whole thing is crooked in some way. I dont think its sagging its just plain crooked and not straight
I tried shimming it in all different ways but its off by enough that i cant get it perfect DX like the plastic housing of the focuser is warped or distorted slightly. so annoying.

Unfortunately stock focusers on cheap (and often not so cheap) scopes are typically not suitable for AP. Too much play.

Could it help if you get the camera focused (as well as you can), check the focuser position and try to align with your laser in that very same focuser position? Then it could be good, even if only at that point, however warped the the focuser is, but you mentioned that it's not a problem in visual use.

If the laser dot is within a half inch of center should be ok.
My last scope build the dot was only 1/4" off center, was really pleased with that result.