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Author Topic: Halloween-ghostly - the Pleiades test  (Read 1873 times)

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Offline Astrogeordie

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Halloween-ghostly - the Pleiades test
« on: 18:23:03, 31 October, 2014 »
Here a suggestion to test coma correctors, flatteners, reducers and what else is near the detector.

Background: I had an old Baader coma corrector with T2 thread. Later I got a TS coma corrector for M48 which illuminated the field of my Skywatcher 200/1000mm Newton better but showed some reddish reflections when trying to get subtile details out of the image of the Pleiades. I decided to sell both correctors to buy one with large diametre but no ghosting.

I chose the Skywatcher 0.9x coma corrector. With now f/4.5 I had a go at the Pleiades - see attachment 1.

You see some reflections opposite of bright stars as referenced to the image centre. and some interesting blue arcs that are surely not signs of gravitational lensing.

My NEQ6 pro did not track well on this occasion in Kielder Forest. I later found out why: The grass gave way despite of the fact I pressed each leg down with my full body weight. So the image drifted randomly during 21 3min exposures, as the load on the three legs varied and the grass acted as a spring. As the stars moved, the reflections moved in a different way with respect to the stellar reference frame. Hence, after using the Kappa-Sigma stacking in DeepSkyStacker, I got the image shown in attachment 2 - free of ghosting apart from a slight brightening in the middle. However, once my mount tracks better I would keep the ghosts.

I sent the coma corrector back and now I have the Baader Mark III, M48 and hopefully little or no ghosting.

Conclusion1: The Skywatcher 0.9x coma corrector is not suitable when you have bright stars in the field. The TS coma corrector is quite good but produces some dim ghosting which I never saw before imaging the Pleiades. The old Baader seems to be okay, and I will do the test with the new Baader as soon as possible.

Conclusion2: The Pleiades test is very effective as you can check the ghosting as well as the field performance of correctors, all in one shot.

Conclusion3: Sometimes it pays out to have poor tracking.  ;)



Acknowledgement: To First Light Optics as they took the coma corrector back with no quibbles.

Clear skies,

Juergen

My scopes: 30 year old Tasco 60/700 on 10K mount and a few others ...

If you have no friends, just buy a lathe and things will change.

Offline dciobota

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Re: Halloween-ghostly - the Pleiades test
« Reply #1 on: 17:31:05, 01 November, 2014 »
 Lol, interesting test Juergen, never thought of a wobbly mounting to be that useful.  One other way I've identified ghosting is to pic a target with an offset bright source (like the Flame, which has Alnitak to one side) and take a pic on both sides of the meridian.  Unless goto is dead on, there will be offset between the two frames, and the ghosting will move about.

 One other thing I think may affect ghosting is the distance to the chip.  I think the closer the filter to the sensor the less ghosting?  Or is it the other way around?
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Offline Astrogeordie

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Re: Halloween-ghostly - the Pleiades test
« Reply #2 on: 18:44:45, 02 November, 2014 »
Hi Daniel,

as closer you are with a filter to the chip, as more likely it is that a double reflection shows up as a ghost. Ghost images are always a problem if there are two glass air surfaces with long radii or flat ones. However, in principle a reflection of one glass air surface can just be reflected on any other surface in a given optical system and the ghost can show up in the focal plane if it is focused enough to do so. Usually in optical analysis both cases single reflection and double reflection are looked after, with the double reflection being of bother in refractive systems.
Clear skies,

Juergen

My scopes: 30 year old Tasco 60/700 on 10K mount and a few others ...

If you have no friends, just buy a lathe and things will change.

Offline dciobota

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Re: Halloween-ghostly - the Pleiades test
« Reply #3 on: 18:14:19, 03 November, 2014 »
 Ahh that makes sense, thanks Juergen.
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