New twist on an auto collimating eyepiece?

[5ash (Philip)]

I have an 8" f4 SW image optimised Newtonian and use a Cheshire eyepiece and film canister with a peephole to collimate it. I haven't been happy with the final result as even when satisfactorily collimated a peep through the canister peephole leaves me unsure wether the secondary holder vanes intersect with the image of the peephole in the secondary reflection. To overcome this I took the lenses out of a cheap 4mm eyepiece and replaced them with a small washer with equally small hole. Thus replicating my film canister peephole. Within the eyepiece body the hole originally containing the lenses was surrounded by a plastic ring with 4 supporting vanes of plastic rather like a secondary holder. These I painted with multiple layers of luminous paint which when exposed to a bright light glows brightly. Now with this in the eyepiece holder it is easy to See if the vanes of the secondary coincide with the peephole. The 4 vanes surrounding the peephole can be clearly seen in line or out of line with secondary mirror vanes. Makes me happier.

Philip

[5ash (Philip)]

Have been playing with this with the cap on the telescope and you can see both the glowing cross and central glowing circle plus the centre spot ring on the primary mirror , which means you can use this to collimate in the dark after charging the luminous paint with light.
Philip

[Don Pensack]

Just a note:
In the collimated newtonian, the secondary vanes will not cross the peephole reflection unless the secondary is mounted with full offset (away from the focuser and down the tube).
In the conventional model newtonian, where the secondary is centered under the spider vanes and centered in the tube from wall-to-wall, the process of collimation will build an offset into the system. Therefore, the spider vanes will not line up with the peep hole when the scope is collimated. You should not even use the spider vanes as an indicator for collimation at all.

[5ash (Philip)]

Hi Don , I haven't been collimating with respect to the secondary holder vanes , but was more interested in observing the intersection of the Cheshire peephole with the centre spot ring of the primary. With the cross hairs on my Cheshire and my less than perfect eyesight I find the crosshairs interfere with my view. The peephole eyepiece with luminous circle and 4 vanes enables me to see both more clearly and not as shadows. Finer collimation is easier as the centre of the " centre spot ring" and the dark hole of the peephole can be easily seen , even with the cap placed on the front of the telescope. Fine collimation can be carried out in the dark by virtue of the luminous paint on the sight tube.
Regards Phillip

[Don Pensack]

Phillip,
Let's talk about the crosshairs in a sight tube (or combination tool). If you look through the peep hole, you'll see two sets of crosshairs:
--one near your eye, seen directly, and, often, somewhat out of focus for we with older eyes.
--one distant, seen in reflection, and seeming to cross only the reflection of the tool as we see its bottom in the reflection of the secondary.

The one you have to align with the center mark on the primary in order to collimate the focuser axis with the center of the mirror (i.e. by adjusting the tilt of the secondary) is the near-to-your-eye crosshair set, NOT the distant one. [the distant one is made more visible by painting the bottom of the crosshairs white, by the way]

If you cannot focus on the near crosshairs, try backing away from the peephole. To hold your head steady, put a thumb on your face and rest your little finger on the top of the tool. This will allow you to focus on the crosshairs. You don't need to see the whole mirror--only the center mark on the primary and the center crossing of the crosshairs.

The goal is to line up the drawtube of the focuser with a line reflecting off the secondary and hitting the center of the primary. The peep hole, the center of the crosshairs and the primary center mark are 3 points on a line.

Now if the crosshairs are not there, but the front of the tool were painted white, it's conceivable you could line up the peep hole, the center mark on the primary, and the bright ring created by the front of the collimation tool (again, by ONLY adjusting the secondary), but the front of the tool would only be seen in reflection, and that would entail primary collimation accuracy to see it lined up concentric with the peep hole. The crosshairs are there for a reason--so you can line up the focuser axis before adjusting the more critical primary mirror tilt.

When adjusting primary tilt, you obviously adjust the primary to bring the center mark (which you see directly in reflection off the secondary) with the center of the tool, whether its a dark area, as in a cheshire, or simply a large peep hole. Getting this accurate is critical.

But if you wish to adjust the focuser axis/secondary tilt, then you will have to define the center axis of the focuser drawtube. This can be done with a laser, or it can be done with crosshairs in a sight tube.

I've always wondered why sight tube designers didn't use a gun-sight reticle, i.e. a crosshairs with a small circle in the center. You don't have to focus exactly on the crosshairs, then. Other possibilities are double crossed wires, like #, or points that almost meet in the middle, like ><.
Because you certainly aren't the only person who has trouble focusing on the near-to-the-eye crosshairs and the distant centermark on the primary at the same time.

Pulling the eye back helps. Having the center mark be a reflective white ring instead of a dark mark helps. Painting the inside of the wires white can help when there is less light. Note that it is always easier to use a sight tube when the scope is pointed at a bright afternoon sky.

What about the distant reflection of the crosshairs as seen in the reflection of the tool in the center of the secondary's reflection from the primary? They line up behind the near crosshairs when the primary is collimated.

Remember that because collimation builds offset into the secondary, the crosshairs of the spider vanes will NOT intersect the center of the optical axis if the secondary is centered between the walls of the tube/UTA.

Don

[5ash (Philip)]

Thanks for all the info Don , it adds to my understanding of collimation . Vey grateful for your assistance .
Regards philip