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Strange Artifact Question

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#1 daveingu

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Posted 31 January 2013 - 11:15 PM

I was out tonite enjoying the Clear break (-10C), has since started snow squalling. Thought I would give Sirius a shot. I noticed a strange spike at around 7 o'clock when I was at around 250x that I have never noticed before. I got to thinking about what was causing it. I have a helical focuser so changing focus didn't change it, so can't be the eyepiece. I think maybe it could be coming from the edge of my secondary which I haven't gotten around to darkening. Has anyone else seen this before?
Dave

#2 BillC

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Posted 01 February 2013 - 12:24 AM

One thing that might trump that is, at -10C your TRANSPARENCY was probably great, but your SEEING was probably terrible.

If you're dedicated enough for -10C, you deserve a medal . . . and one those neat little coats with the sleeves that fasten in the back! -10C . . . BBBBBBRRRRRRRRRRRRR!

Cheers, :jump:

BillC

#3 dave brock

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Posted 01 February 2013 - 01:18 AM

I have a helical focuser so changing focus didn't change it, so can't be the eyepiece.



Not sure what you mean by this.

I think maybe it could be coming from the edge of my secondary which I haven't gotten around to darkening.


I don't think anything not being blackened can cause a spike as such. They are usually caused by something protruding into the lightpath like the spider vanes for eg. Try looking for a screw head or bolt or oversize mirror clip maybe.

Dave

#4 GlennLeDrew

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Posted 01 February 2013 - 01:58 AM

A better descriptiprion would be helpful:
Length, brightness and orientation are just three qualifiers which immediately come to mind.

I'm provisionally wondering if ice crystals in the air might be the cause. Only a possibility if oriented exactly vertically with respect to the horizon...

#5 mconnelley

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Posted 01 February 2013 - 04:01 AM

Hello:

I have seen things similar to what you describe. In my case, it was caused by a heat plume from the center of my Cassegrain primary mirror. I saw a short spike or flare to only one side, that was changing slowly like slow seeing. If you go way out of focus (until you see a big donut), you can see the source of the heat plume.

Cheers
Mike

#6 daveingu

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Posted 01 February 2013 - 08:27 AM

Hi, I will try to describe the issue a little better. What I was seeing was a fairly short and wide spike oriented at about 7 o"clock. There was no corresponding spike on the opposite side (1 o'clock), I didn't make note of what happens as I go in and out of focus but will have to investigate further. The focuser is a helical type, so the eyepiece rotates as you focus.
The scope is a 7" dob with a Seronik style curved spider and I was using an Orion 7.5-22.5 zoom in a generic shorty style barlow.
Dave

#7 Mirzam

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Posted 01 February 2013 - 08:37 AM

See if your other eyepieces show the same thing.

JimC

#8 RossSackett

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Posted 01 February 2013 - 08:54 AM

...and see if Jupiter shows the same thing. Sirius is pretty bright and will put subtle diffraction artifacts front and center.

#9 RossSackett

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Posted 01 February 2013 - 08:59 AM

It's puzzling the spike is only on one side--if it's in the scope I'd expect it to be radially symmetrical. Did the spike change orientation when Sirius was in different places in the field of view?

I'm assuming the spike had an "optical" look to it, rather than atmospheric (eg., Sirius on the edge of a contrail).

#10 Jon Isaacs

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Posted 01 February 2013 - 09:04 AM

Hello:

I have seen things similar to what you describe. In my case, it was caused by a heat plume from the center of my Cassegrain primary mirror. I saw a short spike or flare to only one side, that was changing slowly like slow seeing. If you go way out of focus (until you see a big donut), you can see the source of the heat plume.

Cheers
Mike


That was my thought. At -10C its unlikely that the scope will be thermally equilibrated. From Canada, Sirius is quite low on the horizon, hot air will be rising vertically across the field of view.

Jon

#11 daveingu

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Posted 01 February 2013 - 09:21 AM

Well, it did stay in the same location as Sirius moved through the fov. I was looking over my neighbours house , so a heat plume is possible. I will have to revist with some different eyepiece and maybe try Jupiter.
I was trying to see B , so I will be out when it is clear again, by the way, I am fairly sure I did see it for a few brief moments.
thanks
Dave

#12 GlennLeDrew

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Posted 01 February 2013 - 10:36 AM

The heat plume described would be occurring inside the telescope. Being 'sheltered' from any outside breeze, however slight, would result in it remianing rather stable in appearance. With further cooling to equilibrium, such a plume should diminish and eventually disappear.

But for objects low in the sky, there will remain an artifact caused by the prismatic dispersion of the varying density of the atmosphere. At sufficiently high powers it will be seen as a vertically oriented color fringing, with blue above and orange below.

Heat plumes over neighbors' homes will cause fhe air to vigorously 'boil', which makes the image smear out more or less uniformly, and not in a specific direction.






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