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Meade 10"f/6.3 and Nexstar 8SE view comparison?

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#1 tim57064

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Posted 12 July 2013 - 10:43 AM

I have been comparing the views between my Meade 10" f/6.3 and my Celestron Nexstar 8se. I have come to the realization that I prefer the view thru the 8se. The views in the 8se seem to be clearer with DSO's like Nebula,Galaxies and clusters.The Planets are also clearer. I have not checked collimation on the 10" and do not know why I haven't yet the stars seemed to be okay.There is nothing wrong with the mirrors or the corrector plate.
Has anyone else compared the views between these 2 scopes before? Could the 10" just need collimating? I am selling it to a friend and want to make sure it is the best it can be before doing so. Another thing to add here is that I used the same eyepieces in both scopes,brand and size were identical. I used my Meade QX Wide angle 20mm ep's and my Burgess WA 8mm fov60 degree.

#2 Eddgie

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Posted 12 July 2013 - 11:38 AM

Collimation will have little effect on the performance of a telescope on targets like Galaxies or Nebula (extended targets).

Most likely the difference is due to the magnification differnces. If your sky is light polluted, most likely the larger exit pupil you get at f/6.3 (using the same exit pupils) may be resulting in a bit more washed out sky appearance.

Try using them with as close to the same exit pupil as possible.

#3 tim57064

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Posted 12 July 2013 - 12:59 PM

Thanks Eddgie. When you say "Try using them with as close to the same exit pupil as possible",what are you referring to?I have forgotten so much over the years as I was out of the hobby, for so many years.
I remember that using the same eyepieces,focal length and so on, in different size scopes,you will have different magnifications,Correct? So lets say I am using the 20mm QX70 in both my 8SE and my meade 10" f/6.3. What would the different magnifications be? As for the skies that night,yes I am in a light polluted zone,yet not sure what it is. I believe just inside orange near red zone. What is the formula for figuring your eyepiece magnification? I had it written down somewhere yet cannot find. Is the magnification of the eyepiece affected by the degree of the field of view of the eyepiece? Just trying to remember. Thanks, Tim

#4 Eddgie

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Posted 12 July 2013 - 01:40 PM

The exit pupil is calculated by dividing the magnification into the aperture.

If you use the 20mm eyepiece in the 8" SCT, you get about 101x (assuming that you are using 1.25" diagonals because it will be higher than this if you are using 2" diagonals). This will give you right at a 2mm exit pupil.

In the 254mm scope working at f/6.3 (again, assuming a 1.25" diagonal) the focal length is only 1600mm so the 20mm eyepiece gives 80x.

Not only is the magnification lower, but the exit pupil is now 3.175x That is a pretty huge difference.

The target is going to be smaller in the 10" f/6.3 scope, and also brighter, but because the sky brightness is also much higher, the view can appear washed out.

To make a fair comparison, you would need to use the 10" scope at 127x (for a 2mm exit pupil) or the 8" scope at 64x.

I suspect though that the difference you are seeing is because of the difference in sky brightness due to the exit pupil size. The brightness of the sky I think is the proportional to the area of the exit pupil A bigger exit pupil means a brighter target, but a brighter background as well.

#5 tim57064

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Posted 14 July 2013 - 08:47 AM

The exit pupil is calculated by dividing the magnification into the aperture.

If you use the 20mm eyepiece in the 8" SCT, you get about 101x (assuming that you are using 1.25" diagonals because it will be higher than this if you are using 2" diagonals). This will give you right at a 2mm exit pupil.

In the 254mm scope working at f/6.3 (again, assuming a 1.25" diagonal) the focal length is only 1600mm so the 20mm eyepiece gives 80x.

Not only is the magnification lower, but the exit pupil is now 3.175x That is a pretty huge difference.

The target is going to be smaller in the 10" f/6.3 scope, and also brighter, but because the sky brightness is also much higher, the view can appear washed out.

To make a fair comparison, you would need to use the 10" scope at 127x (for a 2mm exit pupil) or the 8" scope at 64x.

I suspect though that the difference you are seeing is because of the difference in sky brightness due to the exit pupil size. The brightness of the sky I think is the proportional to the area of the exit pupil A bigger exit pupil means a brighter target, but a brighter background as well.


Eddgie, Thank You so very much for your help on this subject. Your explanation was very clear and concise for me.Now,I just need to remember it by writing it down in my astro notebook. Thanks again and Clear Skies to you. Tim

#6 orion61

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Posted 14 July 2013 - 10:12 AM

Yhat F6.3 came off an LX6. That was pretty early in the production process. I have seen some stunning 6.3 optics, I have also looked through some that were not so good.
You need to do a test on a Star, say Polaris with your highest power eyepiece and double check the collimation.
There a several factors that may influence the final viewa.
A fast scope needs to be spot on with it's alignment, The larger scope is more succeptable to seeing condition.
Also using the same power, per inch of apeture is a good test.
We need to figure which of these are affecting your views..

#7 tim57064

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Posted 14 July 2013 - 10:54 AM

Larry, thanks for your input. I have sent you a PM. Tim






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