200p Dob eyepiece advice - BST / Maxvision

[Reeny]

I want to double check I am doing the right thing before I hit the button and spend £120 on two new eyepieces.

I was out last night in the mist, dew, and low cloud.

The Revelation plossls were great for the moon - full mag at 200x and 250x was OK

But the stars wouldn't focus in.

The collumation appears to be OK, but they had diffraction spikes from the mirror cross bars, and showed as an oval disc.

I couldn't get the stars to a pin-point unless I was using low magnification.

Question-1

Is the bad focus due to the moisture in the air?

Question-2

Or are the 52degree Plossls struggling, needing me to upgrade to the EP's below.

Question-3

If I get myself a 24mm BST 60degree / and 16mm Maxvision 68degree - will they be OK with a 2.5x  Barlow lens

Magnifications will be 48x / 75x / 120x / 188x

I need to justify the expenditure in my head.

Thanks in advance for your comments.

[ronin]

Your scope appears to be f/6, I would have thought that the revelation plossl's would have been fine, and that any coma would have been minimal.

The spikes are as you say a consequence of the spiders, the oval stars really sound like something concerning mirror alignment - what collimator are you using? I often see pictures but wonder how many times the actual optics behind them is understood. Just getting things sort of "in the middle and central" is not all of it.

Another question is how well/flush is the focuser to the OTA, have heard where the focuser is not seated correct and an angled focuser would cause oval stars.

Wouldn't like to say get the eyepieces until you get point like stars, since the problem is very likely not the eyepieces you have and would remain with any upgrade.

Can you borrow another barlow, was "low magnification" without the barlow and high magnification with it?

If so then look at the barlow, the lens in it may not be seated correctly. Could be a bad barlow as well if removing it from the system removes the ovals.

[Dude_with_the_tube]

i would suggest to wait and try the Ep`s again when seeing conditions are better.Yesterday was very poor seeing i can confirm that too.I have a perfectly colimated 10" dob and i was unable to get a good seeing out of anything!

your scope is F6 so should not be demanding on any type of EP`s.Make sure your scope is colimated,secondary aligned to your focuser and give another bash when the seeing conditions are better.

[Reeny]

Will do - I will hang on until I test it out properly.

I don't have a collimation tool - I use the film roll canister.

The ring on the mirror was very slightly of f by one diameter = 5-10mm maybe.

I tweaked the collimation a little - but the view to the trees 1/4 mile away still wasn't that good using the Rev's.

Focus is possible. But the sweet spot is very very tight, and drifts off easily.

The scope is brand new yesterday.

So the gear is still getting used to me and my expectations.

The fact that the moon was crystal clear is very confusing.

I was hoping the problem was condensation in the air, in the eyepieces, or on the mirror.

More testing is required.

Thanks for the input.

[Reeny]

Thinking back to last night, the stars looked like they were distorted due to coma.

But it was happening during focus adjustments, not from being near the edge.

The problem occured right across the view.

If I can get the focus right, the oval appearance might dissappear as well.

Roll on clear, crisp, dry skies.

[Reeny]

The oval disc was east west in the viewfinder before focus, 4 pointed star in focus (the spider), changing to a North South oval after focus.

All is well at the moment.

I collumated the scope and tweaked the secondary mirror back into line to get rid of the astigmation described here.

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

In the meantime, the maxvisoons have have arrived today.

They are bigger and heavier than expected - especially the 24mm.

 

Can someone tell me what the cover extensions are for please.

I assume they are adjustable to shroud from stray light, closing the gap between your eye and the optic. 

The photo shows the rubber covers extended.

Cheers

[YKSE]

My guess is that the cover extension is for better eye placement, I wear glasses so I always have the cover all the way down. if you search in the forum for 24mm maxvision, there's a loooooong thread about the EP, lots of info there.

Yes, the 24mm is among heavier 1.25" with its 330gram something. When I got my first 32mm plössl, I thought that it was big. Now I'm paying more attention to weight of each EP I read about.

[Reeny]

Just had a peek through the new lenses on the old scope.

There was a little bit of eye and eyelash reflection which disappears with the covers extended.

Hopefully a clear sky tonight will allow me to test them out fully.