Collimation and vignetting

[hoops86]

Hey Guys,

Was wondering if you could give me some advice… So I recently bought a used meade lx90 and only been able to use it once. But randomly yesterday I was messing about with it during the day and noticed something odd. When looking through the eyepiece I could clearly see what I believe to be vignetting? the top of the view (a small section of the view at the top was all black). The eyepiece is a 25mm BST its been used loads of times before on my old scope and its been perfect. Never seen this before. I even twisted the eyepiece round to see if the black bit moved round with it but it stayed at the top.

Now im relatively new to all things astronomy and im yet to venture down the road of collimation… I done a rough star test on the only night ive had the scope out and it look ok to my inexperienced eyes! Now my question is…. Are the two related? Would a out of collimation scope give me these kind of results? If not any idea why its like this and how would i fix it?

Ohh and i have no idea if it makes any difference or not but the object i was focusing on was only maybe 50 yards away.

Sorry for the stupid question i just have no idea what im doing

Cheers

Chris

[rwilkey]

Hi Chris, I don't think it is viginetting or a collimation problem.  The source is purely with how you use the eyepiece.  When this happens you should use the eyepiece with the eye-shield fully wound up, this should prevent the problem you are experiencing, which I believe is 'blacking out', a common trait with long eye-relief ep's.

[nicks90]

i get the same issue with my revelation astro 32mm ep. its all down to eye placement. too near or far, to high or low - i get black out areas.

get the eye in the right place, lovely view!

[hoops86]

Cheers for the replys guys!

Im not questioning either of you as im a complete novice at this stuff. However im not convinced this is the case. As i know all about blackouts! I have a few eyepieces that do this. However the bst's have NEVER ever caused me this issue. I didnt want to reply before i checked it out again and i can confirm this is 100% not a blackout issue.

I tested it with 3 different eyepieces all different makes to confirm its not the eyepiece. i used a 12mm bst, 40mm meade 4000 and a 7mm x-cel lx all showed the same results! Nothing changed when moving my eye placement about.

I then removed the diagonal and used the eyepiece directly into the back of the scope. Problem dissappeared! This leads me to think the problem is the diagonal! That might not be 100% straight? I will try and pick up a new diagonal and see if it helps.

Btw....before i noticed this i had an issue with my video camera when focusing.... the out of focus star only showed me a "half donut" and when i went past the point of focus the "half donut" switched sides from top to bottom. I asked the guys in the VA section but none of them could explain why it was like this. If the diagonal is the cause of my problem... do you think that could be the reason why i seen these odd results?

Thanks for the help its much appreciated!

[nicks90]

In a word.....yes

If the issue goes away with the diagonal removed, its got to be that.