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What is this effect?


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I was taking advantage of the brief windows of very clear skies last night and spent most of the time mucking about with my NexImage camera and Saturn. I then spent an hour or so generally looking here and there and finished up looking at Mars for a while.

As I was looking at Mars a I noticed a very small star close to its disk seemed to disappear behind the planet and very soon (a few seconds later) re-appeared. I continued to watch the star and it was definitely moving away from the planet. Initially I thought I was actually detecting the movement of the planet against the background stars but then I realised it was not possible - it was moving far too quickly. It was definitely moving relative to the stars though. Then I moved the scope by a very small amount and the star, which was below and right of the planet moved above it by a significant distance. I moved the scope up and down and the star moved above and below the planet. The greater the movement of the scope the more exaggerated the relative movement of the planet and the star.

I was using my Nexstar 8SE with a 20mm Plossl eyepiece at the time, so not a huge magnification.

Can anyone tell me what causes this effect? Is it simply parallax?

I feel quite noobish now.

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An internal reflection?
definately sounds like a reflection artefact

Could be actually. I took a look at the scope in the daylight yesterday and there are several small pieces of white polystyrene on the inside surface of the corrector lens. These have most likely come from the packing pieces I used in the home-made porting box that I use because I put it away in a hurry without the back cap on a week ago when I was surprised by a sudden rain shower.

I think maybe they are causing some strange reflection or diffraction effect.

Its a bit annoying. ;)

I have no idea how to clean them out. :)

Edited by jaquesnoir
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It will be difficult to remove any debris without removing the front corrector plate. Not something I would advise unless you exercise great care, as the plate is orientated in a way that allows the mirror to perform at it's optimum. There will probably be small spacers between the plate and the front ring flange, also there could be marks indicating the plates exact position. The secondary mirror is mounted in the centre too, and probably best not to disturb that.

I would tolerate the small reflections rather than risk making things worse.

There will be dealers who would do it, but for a price, and unless you could deliver it to them, transport costs could be expensive.


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Cheers mate,

Thanks for the advice. I'm a bit peeved about it but it is tolerable. I did wonder if I could get the specks with a small neoprene suction tube and squeezy bulb inserted via the rear orifice (urgh! sounds medical :)).

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