Collimating an ODK?

[DaveS]

Although I think my ODK 12 is in reasonable collimation I would like to be able to check without having to use a precious clear night in order to do so.

OOUK recommend a Catseye collimator, but looking on the @FLO website they only run up to f/6, whereas the ODK runs f/6.8.

I can see many other options ranging from "pricey" to "how  much?!" but am unsure as to their suitability, partly as I'm not sure about the visibility of the secondary through the corrector, and whether it has a centre spot. The very expensive Hotech job with multiple lasers looks to be a non-starter (Fortunately for my bank balance) as the first step is to remove the secondary.

Does anyone have any suggestions? I can't afford to buy several instruments, one after another, in the hope that one of them will work.

[DaveS]

OK, checked with a hand mirror and there is no centre spot on the secondary, as expected.

[ollypenrice]

Artifical star? The downside is that reflectors may not give an accurate indication of their collimation when pointing at the low elevations implicit in this test.  Is there a roof you could get to? (Half joking...)

Olly

[DaveS]

I have seen artificial stars on the FLO site, and have considered them, but cannot get a long enough throw from my obsy position. Best option would be to set one up in my bedroom or possibly the upstairs landing looking through an open window but the distance is barely 20m.

[Skyline]

Ask @peter shah

[DaveS]

Yep, hope he sees this.

[ollypenrice]

The oft-used artificial star is just a ballbearing illuminated from a little off axis so you don't see the source. Only the bit of the ball nearest the telescope reflects light down it, making it an excellent approximation of a point source. You'll find this system in professional optical shops.

Olly

[DaveS]

Ah, I had forgotten about the ball bearing method, thanks for reminding me.

[andrew s]

Artificial stars are good for collimation but if they are too close back focus can be an issue and in addition will show aberations which don't effect collimation but do effect apparant optical quality. 

Regards Andrew 

[Skyline]

13 hours ago, ollypenrice said:

The oft-used artificial star is just a ballbearing illuminated from a little off axis so you don't see the source. Only the bit of the ball nearest the telescope reflects light down it, making it an excellent approximation of a point source. You'll find this system in professional optical shops.

Olly

Similar to what Es Reid uses.