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Sky-Watcher 200P Dobsonian problem.


seven_legs
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Hi all,

I brought this scope around last October and only took it out twice. First time was just after lockdown in April at Belper site. That time i did not use it as it was cloudy.

Second time was for the Perseids meet up a few days ago.

Anyway I thought I would look at the thin crescent moon and was surprised to see a yellow tinge around the moon and around the edge of the 31mm Tele Vue nagler 82º eyepieces. 

With Jupiter there was a horrible red tinge to the bottom of the disc and had a general false colour around it. I thought perhaps it was because it was low down in the sky.

The stars looked ok but I thought M13 looked a bit dim.

I did not take any other eyepieces with me. 

Today I Noticed that the primary mirror had like a mist, film layer on the surface with a little cleaner area on the edge. I left it about a hour just to make sure it was not reacting to different air temperature.

I pointed the scope to the sky and again the yellow tinge around the eyepiece. I tried another 1.25 25mm eyepiece and that was fine.

Next I borrowed another 2" Explore Scientific 100° eyepiece from Rob and the same yellow tinge around the eyepiece was there.

I am tempted to clean the mirror but I'm not sure that will do anything.

 

Any ideas would be welcome.

 

 

Edited by seven_legs
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Sounds like you are suffering from C A. or possibly coma if it is occurring on the larger eps only.

How is the collimation looking.

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I think that I would be looking at collimation first, the two colours you keep seeing are next to each other on the spectrum, so using different eyepieces will show different parts of whats bouncing back up the tube. 

See if you can get a picture of the primary mirror,  I suspect that it might be a bit dirty, but I don't think that's what's causing the fringing, it depends what's on it. 

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12 hours ago, Graham said:

Sounds like you are suffering from C A. or possibly coma if it is occurring on the larger eps only.

How is the collimation looking.

3 hours ago, BAZ said:

I think that I would be looking at collimation first, the two colours you keep seeing are next to each other on the spectrum, so using different eyepieces will show different parts of whats bouncing back up the tube. 

See if you can get a picture of the primary mirror,  I suspect that it might be a bit dirty, but I don't think that's what's causing the fringing, it depends what's on it. 

 

 

The is collimation is fine at the moment. 

As i mentioned before i have used this Eyepiece on various scopes in never had a problem. 

I've tried taking a photo but mirror is to far down the tube. 

Might just take the mirror out to have a close look. 

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Put some tape on the mirror cell and the tube as reference mark's so you are sure it goes back where it came from, you don't want to be introducing other errors that could muddy the water.

Pm me your mobile and I'll talk you through cleaning the mirror, it's not difficult, you just need to be organised.

Edited by BAZ
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14 hours ago, BAZ said:

Put some tape on the mirror cell and the tube as reference mark's so you are sure it goes back where it came from, you don't want to be introducing other errors that could muddy the water.

Pm me your mobile and I'll talk you through cleaning the mirror, it's not difficult, you just need to be organised.

thanks Baz,

I am fine with cleaning mirrors as i've done it with my previous scopes, the 14" and the 16" scope. This one should be nice and easy lol.

But thanks for the tip and offer, really appreciate it thanks

will take a close up photo of the mirror when its out the scope.

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