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I am new to telescope .. 

i have Nexstar 5se .. moon seen clearly ..

while watching mars , there is black shadow (circle ) in middle of mars .. same as seen in star .. it goes bigger when increase focus 

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May be shadow of secondary mirror(though it is normally visible only when out of focus). If that's the case, you can't get rid of it physically unfortunately. Maybe magnifying it a bit more would help?

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40 minutes ago, Amit777 said:

I am new to telescope .. 

i have Nexstar 5se .. moon seen clearly ..

while watching mars , there is black shadow (circle ) in middle of mars .. same as seen in star .. it goes bigger when increase focus 

The focuser is not for zooming in. Unless you are using a special zoom eyepiece there is a fixed magnification for each eyepiece telescope combination. The point of best focus is when the object appears at its smallest. If you can see the secondary obstruction you are out of focus and seeing the inside of your telescope. 

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The focus wheel is for making the view clear(ie not blurry). To change magnification, you change eyepieces. The 5se only comes with one eyepiece(25mm plossl), so you may have to buy more.

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The guys are right, what you were seeing was the shadow of the secondary mirror. You can see it on stars and other small sources of light, when they are out of focus.

Mars is tiny compared to the Moon. Here is a video of the Moon passing close by it. The image is from 2007 and Mars was 15.5 arc seconds wide.  Today it is about 12 arc seconds wise.

Right now, the apparent size of the Moon is some 150x bigger than that of Mars.

Stars, btw, are point sources to us, as their distances are enormous.

Edited by Ruud
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As has been said, seeing a black spot or shadow in the centre of the image means that you are not in focus.

As you adjust the focus, the object (eg: Mars or a star) gets smaller and smaller as you get closer to sharp focus. When you are at sharp focus the object will be at it's smallest and there will be no dark shadow in the centre of it. Mars will look like a small pink / orange spot and stars will be points of light.

 

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Thanks for reply ... I thought I damaged my telescope .. hope that is not case ... I’ll try to adjust focus more today 

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1 hour ago, Amit777 said:

Thanks for reply ... I thought I damaged my telescope .. hope that is not case ... I’ll try to adjust focus more today 

Try to focus on the moon and then move to mars without adjusting focus(don’t change eyepieces while doing this). That is what mars should look like. Hope that helps!

Edited by Nerf_Caching
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In fact for any eye piece in the focus tube you are probably closer to being in focus when Mars is at its smallest in the view rather than at its largest, which as the others have said is just Mars out of focus!  When it is that much out of focus you are liable to see the secondary, when you have it as a sharp slightly pinpoint, rather than the sharp pinpoint that you get with a star it is in focus.

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Try this thread, even if you don't read the words scroll down and look at the pictures, which I think are taken with a bigger telescope than yours.  Your picture should be as clear, but perhaps a little smaller :

 

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