Cheshire versus Cheshire

[erick]

Thought it best to take this discussion out of the laser collimator thread. The story behind this is that I had three collimation tools - a 35mm film cannister with a hole drilled in the bottom, a GSO laser collimator, an Orion combined sight tube/Cheshire. I've been using my film cannister sight tube and my laser and have never felt comfortable that I know how to use the Orion properly, regardless of reading the instructions. Eg. I could not get the crosshairs in focus. One night, I had the chance to compare looking through an Andrews -sourced Cheshire and the Orion and what I saw through the Andrews made sense. So when Darryl (Kokatha man) said he was happy to discard his Andrews Cheshire, I convinced him to send it to me so I could experiment further.

I've had one brief session comparing the Orion and Andrews. When I have a few hours to spare, I'll sit down during the day and carefully work with them until I fully understand. I have printed out all Geoff's (Starkler) guidance on the correct use of a Cheshire and will persist until I have it right.

From the other night's brief session, all I can say is that the view through the Andrews continues to be much less confusing. Both showed the primary mirror centre ring well centred under the peephole, after my usual visual inspection of the secondary mirror position and use of the laser. They have a different size machined countersunk hole that places an image of a dark circle where the peephole is (ie. in the centre of the shiny reflected circle generated from the 45 deg surface. That seems to be affecting how I see it. The Andrews has no cross hairs so I don't have the confusing out of focus crosshairs I see in the Orion (It must be my aging eyes - I've moved my head back from the peephole, but they don't come into focus.)

Both Cheshires will wobble around a bit in the 2"->1.25" adapter I am using, if I push sideways on them. This, of course, moves the images out of alignment. I may do what I have done with the laser I use - adding a layer of contact (yes, school book type "contact") made for a very snug fit in the adapter.

So, more to do yet before I know what is what and achieve confidently, a basic level of reasonable collimation. (Scott (Tannehill) showed me the operation of an autocollimator in the 12" - I have a ways to go before I add that additional level of perfection to my collimation.)

Eric :)

[Starkler]

What is probably confusing you Eric is actually identifying what you seeing. ie what reflection belongs to what.

What can you help here is if you get a piece of paper and fix it to block the view of the primary, so that you only see the secondary in the cheshire. Likewise when trying to align the secondary under the focuser, slipping a piece of white paper against the opposite tube wall will help.

As for out of focus cross-hairs, it shouldn't be too much of an issue given that centering them on the mirror centre spot isn't all that critical. Somewhere inside the inner part of the ring will do. It also helps to bring your eye back away from the cheshire to view both cross-hairs and primary centre ring.

If you are using laser secondary mirror pointing IS critical.

A long cheshire like the Orion is better for two reasons. Firstly the greater length between the peep hole and the cross hairs allows much better accuracy due to reduced parallax error. Secondly it is long enough that the inside edge of the tube can be used as a guide for aligning the secondary under the focuser. You rack your focuser in or out until the secondary edge appears just inside the end of the cheshire tube.

[rmcpb]

I often see comments about the laser or Cheshire wobbling in the 2-1.25 adaptor as a problem. Well it is BUT doesn't this happen to our eyepieces when we are observing and we just don't notice it??

[erick]

Thanks Geoff - more information to help me. I already have a piece of white card and a "bulldog" clip to fasten it to the spider - a bit of bending and the white reflection is all I see in the secondary. I haven't put a piece of paper on the inside wall of the OTA behind the Secondary - I'll do that - a different colour to the white card, I think. Lots to try, thanks.

[erick]

OK, I'll keep and use both these Cheshires (both are called "collimation tools"). I spent a while with them last night and during the day today. I'll use them together with my collimating laser.

In summary, I'll use the Orion primarily for checking secondary mirror position. I'll use my laser collimator as the main tool for secondary and primary mirror tilt, including barlowing it to confirm accurate primary mirror tilt. I'll confirm with either Cheshire tool, but I'll use the "Chinese" Cheshire primarily for checking collimation in the dark.

A couple of comments on the two tools, Orion and "Chinese". The Orion is clearly a better machined product and has the additional length and the crosshairs. The "Chinese" is mostly plastic. It has grooves on its end to which crosshairs could presumably be glued, if one wanted, but given its short length, the accuracy would be questionable, and they are exposed so I suspect they would soon be bent or otherwise damaged.

The Orion is a snug fit in my 2"-->1.25" adapter. The "Chinese" needed two ridges arising from the mould which formed the plastic body scraped off. It was too loose in the adapter, but one layer of contact fixed that.

I now understand my confusion. The second and third attached pictures (apologies - small point and shoot with poor control over focus or exposure) show that the "hole" in the Orion that forms the dark spot in the shiny reflection is considerably larger than that in the "Chinese". The "Chinese" produced a small spot which sits neatly inside the ring centre spot on my 8" and the triangle centre spot on my 12". The dark spot from the Orion overlaps the ring and triangle considerably and makes it more difficult to accurately overlap them - it's probably just me, but the small "Chinese" spot was very easy to align, especially in the dark using a redlight torch for illumination. Additionally, the presence of the crosshairs and their reflection in the view did not assist me in centering the dark spot on the primary mirror centre.

I did lots of experimentation with all three tools - rotating them in the adapter, rotating the adapter in the focusser, racking the focuser in and out. All three show good consistency - not german engineering accuracy, but nothing went dramatically wrong with the reflections or return laser beam when I did all these things. For me, a visual newbie, I reckon that is all I need at present.

Yes, and I checked the laser collimation result with each Cheshire in turn. Again close enough for my purposes. If the Secondary hasn't been knocked out of position (eg rotated on the centre screw while bouncing to darksky site), I trust my laser to give me a good result. I'll check it with either Cheshire and tweak, but the "Chinese" after dark is my preference.

(Yes, and I have a v-block made up on which I check my laser's internal collimation over a distance of several metres every so often - certainly after battery change.)

Any collimation of a newtonian reflector is obviously better than no collimation, and I don't believe one needs to spend too much money or expend too much effort to get a good result for visual astronomy for a relative newbie like me.

My 2 cents worth! :)

[Kokatha man]

Glad it's of use Eric: the addition of the flower prints are very "becoming."

Cheers, Darryl.

[Starkler]

This matching of centre spot dimensions to cheshire hole size is an important consideration that doesn't seem to get discussed much.

I have a Synta cheshire and also a cats eye telecat combination tool.
What i have found is the the cats eye triangular spot with 1/4" centre hole is an excellent fit to the hole of the synta tool,and allows extremely small errors to be seen.

My 15" currently has a ring binder spot which doesnt fit either tool very well. My collimation is close, but never great simply because it can be a couple of mm out before a misalignment is visible in the cheshire.

I must get around to upgrading that centre spot.