Star Test

[middy]

Tonight I tried using the Toucam to do a star test. Visually the rings always look concentric either side of the focus. Using the Toucam and Registax to stack frames shows a slight bias towards the upper edge of the ring. Maybe it is time to learn how to collimate. ;)

The other thing that perturbs me about the results of this test is the fact that there is a large black hole in the centre on one side of the focus and 'normal' looking rings on the other side of the focus. Should I be worried about this? I was under the impression that the rings should look the same either side of the focus.

The star used was Sirius and the scope is an 8" Dob.

[huckabuck]

andrew,

the large black "hole" off to one side is your secondary. it appears to me that you have the focus too far in or out. mine works better defocusing outward. for a star test you need to barely defocus. the rings you'll see are hard to define until you see them for the first time and from then on you'll know what your looking for. there should be several that appear(not just the one)

btw your collimation is out!:D

[garymck]

This may or may not indicate a problem.

Do this:

place a round piece of cardboard over the top of the secondary such that it is a 33% obstruction by diameter
focus to a tight point
note the position of the focuser knob
turn it inside focus until the central black spot starts to appear
Note position of focuser knob
turn focuser to outside focus until central black spot starts to appear
Note position of focuser knob

if the distance inside focus from focus point, and outside focus from focus point, are in a ratio of 1:2 or less, then the scope is ok, if more than 1:2 then it is likely the scope has optics that are not OK.

this is a "quick and dirty" test for 1/4 wave front optics.

cheers
Gary

PS the scope should be coole before you test, and not in a place where temperature is fluctuating. Nice scopes can give poor results when being tested at night and the ambient temp is dropping, so make sure it has stabilized.

[casstony]

Andrew, I think your scope was not properly cooled and not properly collimated, so you can't use those images to judge the scope. I find star testing fun, but since I haven't observed with someone who is experienced in this testing I don't place much faith in my own abilities to accurately assess a star test - it's easy to misinterpret the test.

Once you have the scope collimated and cooled, a safer way to assess it is to count how many craterlets you can see on the floor of Plato or look for the e and f stars in the Trapezium - you need stable seeing for these tests.

[ausastronomer]

Andrew,

looking at the images it would appear that the scope is a long way out of collimation, not properly cooled and the seeing isn't very good.

Firstly, collimate the scope properly, both your primary and your secondary are out. Nor is your secondary properly positioned relative to the focuser. Get this right first.

Let your scope cool for a good 3 hours before you attempt to star test it.

At this point you will at least know if the seeing is good enough to continue.

CS-John B

[DougAdams]

I look for Plato's craterlets every month or so but never see them (always see e and f in the Trapezium). Is there a secret to spotting the craterlets? I'm assuming you bag them on day 8.5 or so of the lunar cycle, but no luck. I guess with the moon quite low and it's still turbulent in the early-ish evening doesn't help!

[casstony]

It's been a while since I've observed them but you do need a stable atmosphere. I recall the 4 largest craterlets being very easy to spot in a 10" scope, with another 1 or 2 visible as well when Plato is near the terminator. I also recall seeing the largest craterlets during a full moon, but they appear as white spots rather than shadowed depressions. Perhaps others will chime in with more recent observations.