Help please! Collimating an RC with a Howie Glatter

[inFINNity Deck]

Hi Peter,

were those images taken with a real or an artificial star?

If I look at the last image and flip the lower left and upper right I can see there is some room, although marginal, for improvement (ignore the yellow target, only look at the outside and inside of the doughnut):

 

As can be seen when the outer rim is nicely round, the inner rim does not match.

Te ronchigrams do show several paterns:

- a sand-wave like pattern
- straight diffuse lines parallel to the spider in direction approximately 20°

The first could be the result of seeing when done on a real star. If done using an artificial star it could indicate issues with the mirror.
The second is very strange, have no idea how to explain it.

Normally ronchigrams are made closer to focus, so with fewer lines (also taken with a RC10 using an artificial star):

The ronchigram is not very sensitive, so errors in the inter-mirror distance of several millimetres are quickly made. The focal length of the RC10 varies by about 10-11mm per 1mm change in inter-mirror distance. In your case this could mean that the inter-mirror distance is off by slightly more than 2mm (your focal length is off by 25-26mm).

HTH,

Nicolàs

[davies07]

Hi Peter,

Sorry for the slow reply. 

I agree with Nicholas, there is something strange about your Ronchi patterns. I've consulted with Es Reid and he agrees that something is not right. He wonders if air currents might be responsible. To test this wave a piece of card in front of the scope to see if the patterns change. 

Alternately, Es wonders if the patterns are due to 'zones' in the primary mirror, these could be due to a manufacturing fault or stress. I wonder if the mirror clamp(s) are too tight. My eight-inch scope has the mirror secured with a single screw-in ring. I once had this screwed in rather too tightly and it produced very strange Ronchi patterns. To check this, and if your 10-inch is the same arrangement as my eight-inch, reach into the tube and unscrew the shade tube. Then check the tightness of the central securing ring. You should be able to unscrew it easily with thumb and finger. If it is too tight then unscrew it and let the mirror relax for a while before gently re-screwing it, but not tightly.

The first time I unscrewed the centre screw I discovered that the O-ring,  that sits between the centre structure and the mirror, was perished and I had to replace it.       

Good luck,

David

[neil phillips]

On 20/04/2020 at 18:44, lukebl said:

No, unfortunately I can't see the secondary donut when looking down the tube for some reason. It's too dark down there!

On the point regarding a subsequent star test, surely that would simply tell you whether or not you'd collimated it properly, but wouldn't help you regarding the cause? If it proved that the collimation was still out, you wouldn't be able to tell whether it was the focuser, secondary or the primary which was at fault and you're back to square one.

Seeing the doughnut can be seen using a cheshire and torch on the cheshire side reflector. On my CC though