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 Post subject: Reflector Telescope
PostPosted: Tue Feb 16, 2016 5:40 pm 
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I was going to get a 8" or 10" dobsonian but in the mean time have acquired a 150PDS and whilst its designed for photography I'm using it for observing using an eyepiece extention to get focus and have noticed that the centre of my vision is darker and it looks like I'm seeing the secondary mirror.

I have tried the societies small 100mm reflector and I'm getting the same issue. I have tried a few different eyepieces and it does improve but does not totally go away. I have also tried observing with and without my glasses and even tried using my contact lenses.

Strange, any ideas

Regards Bill


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 Post subject: Re: Reflector Telescope
PostPosted: Tue Feb 16, 2016 10:10 pm 
take lens cap off...... :shock:


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 Post subject: Re: Reflector Telescope
PostPosted: Wed Feb 17, 2016 5:09 pm 
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Bill,
What you are describing is when the exit pupil from your telescope exceeds how wide your eye pupil is open. It can be caused by old age (sorry) or if you are testing the telescope during daytime or if you are using a very low power. What happens is you are not seeing the full aperture and the secondary shadow becomes almost as large as the field of view as your eye can see.


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 Post subject: Re: Reflector Telescope
PostPosted: Thu Feb 18, 2016 8:55 am 
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Thanks David

Strange but now explained it makes sense, I was testing them out on the Moon and observing whilst there was a full Moon, and yes I was using a 32mm and 40mm eyepiece as I like the wide FOV.

I will try a shorter length eyepiece next time I get the chance, hopefully I can work around the problem.

Regards

Bill


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 Post subject: Re: Reflector Telescope
PostPosted: Thu Feb 18, 2016 1:35 pm 
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As a rule of thumb for the "older" observer, I use 5 x the focal ratio of the telescope = the longest eyepiece focal length you can use. So for a f/5 scope then 5x5= a 25 mm eyepiece. Gerald's binos at f/7 can use up to 35mm eyepieces and he has 32mm ones - perfect for us oldies. You don't get the effect with refractors but you are actually stopping the aperture down if you have too low a power.


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 Post subject: Re: Reflector Telescope
PostPosted: Thu Feb 18, 2016 2:39 pm 
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Thanks David never gave it a thought as I have used the 40 and 32 on my ED80
I will do the maths to see which eyepieces will work with a 8" Dob

Will have to wait for a clear night to test out which of my eyepieces will work with the 150pds

Once again thanks for the guidance

Bill


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 Post subject: Re: Reflector Telescope
PostPosted: Thu Feb 18, 2016 6:43 pm 
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Well a break in the cloud, got the 150pds out

I'm using a field flatterer and my 28mm produces a dark patch, the next one down in my collection is a 15mm, good views of the moon no darkening in the centre.

I will see if I can get my hands on a 2" 20mm or nearest to see if my old eyes can cope :shock:

Regards

Bill


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