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thepominlaw

Newly Purchased Skywatcher 100p

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Hi all, 

Finally have something to begin skywatching again after many years, saw a Skywatcher 100p on amazon warehouse deals for £36 in "acceptable" condition as they are usually three times that thought i would take a chance. Well it arrived today and someone somewhere has made a mistake i think as it was still sealed and new. Nothing inside looked like it had been touched. 

Anyhow not used one before (reflector) so i thought i would give it a go, using the 25mm eyepiece is it normal to see the shadow of the other mirror in the centre of view? 

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Hi.

Do you mean you see the shadow when your eye is far away from the eyepiece, like in these cases (my Schmidt-Cass and my dob):

20180921_154145.thumb.jpg.1855838c7b09d760423f0c39c635d2b1.jpg

20180921_154444.thumb.jpg.fee931379c43a1fa75f401bdd7583cc3.jpg

Or do you see it when your eye is close, in the observing position? The first case is quite normal, the second probably means your 25mm focal is a bit too long. For an f/4 scope the basic low-power eyepiece is on average 5 times the f/ratio, thus it should be no more than 20mm.

A longer focal generates lower power and a larger exit pupil, the disk of light in the center. When it's wider than 5mm, for some people it can create that shadow effect. Is it there only in the daytime, or also in nighttime observing?

The size of the exit pupil is simply the scope's diameter divided by the magnification. Your 25mm yields 16x, thus a 100mm / 16 = 6.25mm pupil, a little too much for most eyes.

Edited by Ben the Ignorant

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This will indeed be down to the fast (f4) optics which will yield a largish 6.25mm exit pupil with a 25mm eyepiece.

Edited by Alfian

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Yeah, the telescope is f/4, so with a 25mm eyepiece you get a 25/4 = 6.25 mm exit pupil.

If your own pupil is much smaller that this, like during the day or when you  look at the Moon, you may begin to notice the shadow of the secondary.

Have you tried a stronger eyepiece yet? With an 5mm eyepiece the exit pupil will be 5/4 = 1.25 mm. Even during the day that should be smaller than your won pupil, so that all the light in the exit pupil can enter your eye.

(If the exit pupil gets larger than the observer's pupil, light at the edge of the exit pupil is lost, and the shadow of the secondary will begin to fill the observer's pupil.)

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I have the Zhumell Z100, aka the Orion "SkyScanner" 100mm, both sold here in the U.S.  The same as your own, and all three made by Synta...

kit2.jpg.44b475d5c79019fa88ef59a32b256ea1.jpg

Yes, the secondary shadow...

1.jpg.0a0af43e43a76f03a56f1cc4ede6bd3b.jpg

But that's during the day.  Newtonians overall were not designed for use during the day, for land targets, although they can be used for observing the Sun, and with a safe solar-filter fitted over the front opening.

However, at night it comes into its own.  The Moon, at 13x, and through a 30mm Plossl...

011217-30mm.jpg.6969c36ea0d1a6db5268c077557263a8.jpg

011217-30mm2.jpg.3d643e404968c756024e59397f9c8660.jpg

Enjoy.

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Ah, this also explains one of the reasons why I've found it difficult to take photos of the moon with my phone held close to the 25mm eyepiece... because the camera lens of a phone is so small.

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Finally got out tonight still getting the circular shadow though first photo taken with 20mm celestron it shows the shadow, it’s also the same with my 15mm celestron. Shadow doesn’t appear in second photo taken with 10mm.

also realised iPhone no good for taking photos little disappointing 

01476C8A-EBF5-4E9C-BF46-018BF1E3DF08.jpeg

E68DC479-3E75-4948-874B-69C0EB1AEC47.jpeg

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Smartphones are good for taking photos through the eyepiece but you need an adapter to put the phone's eye at a very precise and very small spot. These were made with my 127mm scope, my Samsung phone, and Celestron's NexYZ adapter.

20180709_043006.thumb.jpg.396f2f378c2703b46c233c7b03fde2dc.jpg

20180802_020317.thumb.jpg.4fa0eda930af4ef346f79968c731d595.jpg

20180802_025236.thumb.jpg.c5e481eb63c6fd649a609611ea76c25c.jpg

Single, raw images with no processing.

The mirror ghost can be annoying but if it disappears with higher power eyepieces, that's less of a problem since you'll be using those for photography.

Correction: the first pic was through an 80mm semi-apo.

Edited by Ben the Ignorant

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5 hours ago, thepominlaw said:

Finally got out tonight still getting the circular shadow though first photo taken with 20mm celestron it shows the shadow, it’s also the same with my 15mm celestron. Shadow doesn’t appear in second photo taken with 10mm.

also realised iPhone no good for taking photos little disappointing 

01476C8A-EBF5-4E9C-BF46-018BF1E3DF08.jpeg

E68DC479-3E75-4948-874B-69C0EB1AEC47.jpeg

Within your first photo, I don't think that your camera was focussed properly, which is why the shadow appeared.  Practice makes perfect, it is said.

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I have a Panasonic mirrorless Camera would it be worth buying a t2 Mount for it and using the scope as the lens?

 

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