Sky-Watcher Classic 200P Dobsonian problem

[seven_legs]

Hi all,

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

Second time was 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 my friend 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.

[Roy Challen]

Wide angle eyepieces (80 degree+) can occasionally exhibit what is called the 'ring of fire'. I've forgotten the proper name for this. I'd be surprised if Televue EPs showed this though.

Planets can also display false colour due to atmospheric dispersion. This happens when you look at an object through a lot of atmosphere, ie when it is low down. There are devices that can correct for this.

Unless your mirror is really dirty, I'd leave it alone. The easiest thing to do would be to wait until the object you want to view is higher up. Also you could try an eyepiece such as a plossl (50 degree) or a wide angle eyepiece such a BST Starguider.

Hope that helps.

 

Edited August 14, 2021 by Roy Challen

[niallk]

Hi @seven_legs - do you know if the scope is collimated?

If you have a cheap 25mm plossl (perhaps the scope came with one?), it would be interesting to know if you see the same effect if the scope's optics and focusser are well aligned & collimated.

[seven_legs]

22 minutes ago, niallk said:

Hi @seven_legs - do you know if the scope is collimated?

If you have a cheap 25mm plossl (perhaps the scope came with one?), it would be interesting to know if you see the same effect if the scope's optics and focusser are well aligned & collimated.

 yeah the cheap eyepiece that came with the scope does not have the yellow tinge around the edge.

the 2" eyepiece is tried and tested on various scopes it is a great eyepiece.

checked collimation and thats fine.

thanks for the reply

[Tiny Clanger]

1 hour ago, Roy Challen said:

Wide angle eyepieces (80 degree+) can occasionally exhibit what is called the 'ring of fire'. I've forgotten the proper name for this. I'd be surprised if Televue EPs showed this though.

Planets can also display false colour due to atmospheric dispersion. This happens when you look at an object through a lot of atmosphere, ie when it is low down. There are devices that can correct for this.

Unless your mirror is really dirty, I'd leave it alone. The easiest thing to do would be to wait until the object you want to view is higher up. Also you could try an eyepiece such as a plossl (50 degree) or a wide angle eyepiece such a BST Starguider.

Hope that helps.

 

Well I learn something every day 🙂

 

 

[seven_legs]

30 minutes ago, Tiny Clanger said:

Well I learn something every day 🙂

 

 

thanks for the reply,

but I am not using a Barlow lens.

I have used this eyepiece on many scope off various sizes up to 16" and observed planets and such at different heights in the sky and it has been fine.

I saw Mars at opposition 2018 through a 16" dob and was so low down in the sky I had to go on my knee's to look through this eyepiece and it was great.

I feel something not right with this scope. there must be other people with the same scope and eyepieces not having problems.

 

[John]

2 hours ago, Roy Challen said:

Wide angle eyepieces (80 degree+) can occasionally exhibit what is called the 'ring of fire'. I've forgotten the proper name for this. I'd be surprised if Televue EPs showed this though....

 

 

The Nagler 31 does show a glow around the field stop and for the 1st few degrees of it's field:

It's not noticeable when the eyepiece is used for deep sky type observing. I don't use my Nagler 31 for lunar / daytime observing.

 

Edited August 14, 2021 by John

[seven_legs]

Here's a not very good picture of mirror.