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Old 27-11-2017, 04:55 AM
buill
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seeing secondary mirror in view

hi new here got my little girl a reflector telescope as she is big into space and am trying to figure it out its this telescope but with a different brand name on it
https://www.aliexpress.com/item/High...342890122.html my problem is when i look at the moon with it its fine but if i focus on a star i can see the secondary mirror in the center of the image blocking my view of the star. Tried googling but cant find info on it anywhere if anyone could help me out it would be great im probably doing something stupid as i had no idea there was a bit that went into telescopes thanks.
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Old 27-11-2017, 05:48 AM
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Allan_L (Allan)
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Hi Buill
Sounds like what you are seeing (when looking at a star) is the "defocussed" view.
Stars are so far away that all you will get even under magnification, is a tiny pinpoint of light, when focussed properly. So turn your focus wheel until the image shrinks down to a small sharp point of light, and the image of the secondary mirror will not be a problem.

Regards and Welcome to IceInSpace
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Old 27-11-2017, 06:17 AM
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OzEclipse (Joe Cali)
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There are two circumstances under which you will see the secondary mirror

1. When the eyepiece focal length is longer than about 7x the f ratio.

2. When the eyepiece is not focussed.


1. When the eyepiece focal length is longer than about 7x the f ratio.

If you have the exact scope shown in the link, it is 76mm diameter and 700mm focal length. It's f ratio is 700/76=f9.2
Consequently the longest eyepiece you can use before seeing the secodary is around 60mm. It is very unlikely you'd have an eyepiece of this focal length. The longest eyepiece shown as supplied is only 20mm.


2. When the eyepiece is not focussed.

Is it possible you are using the high magnification 4mm eyepiece and the thing they call the magnifying lens to look at stars? If so, and the image is not reaching focus it is normal to see the silhouette of the secondary mirror spider against the out of focus image.

A very good quality 76mm scope will have a maximum usable magnification of about 150 power. This instrument is not likely to fall into the "very good" category so you probably don't want to use the 4mm eyepiece.

The eyepieces supplied give these magnifications
20mm........38x
12.5mm.....61x
4mm.........190x

The barlows or what they call magnifying lenses will push the magnification way too high and shift the focus. Perhaps that is why the image may be out of focus. Suggest you don't use the barlows at all with the 4mm. The 1.5x or 2x with the 12mm eyepiece will probably be the highest useful magnification.

This is all generic advice. If you can give more specific info about how you were using the scope, eyepieces etc when you saw the secondary we can possibly nail down the problem. The scope will magnify 525x but won't ever produce a clear image at any magnification over 150x possibly less.

The pictures shown were not taken with that telescope. You will be able to see the Moon in good detail, see the rings of Saturn, maybe a couple of belts on Jupiter. You won't see galaxies like that shown in the photo. Concentrate on bright nebulae, globular and open clusters rather than galaxies which are almost all quite faint.

Here are some bright targets to try first (on a moonless night)

Nebulae

Orion Nebula
Eta Carina
Tarantula Nebula

Globular Clusters
NGC 104 (aka 47 Tucana)
Omega Centauri (not in a good position for observing now - Autumn is better)

Open Clusters

Jewel Box
NGC 3532

Joe
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Old 27-11-2017, 08:12 AM
buill
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thanks for replies i was using the 4mm to view the star and did try focusing but the secondary was always there ill try once i get a cloud free night to just use the 20mm eye piece ,one other question would i get a better image if i totally remove the end of the scope rather than just the cap in the middle? this would allow for more light to enter the scope? just wondering and thanks so much for replies my little one loves space can name all the planets dwarf planet and dwarf planet candidets tell you the obital times temps ect and i will get her a better scope if her interest continues but need to get the most of this one for her for now
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Old 27-11-2017, 08:16 AM
croweater (Richard)
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Yes mate, remove the end cap-cover. Cheers Richard
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Old 27-11-2017, 09:15 AM
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Scorpius (Dave)
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I agree with what has been said above. This is a low budget simple scope, so do not expect too much from it - Certainly a 4mm plossl is is not really suitable.
The center cap is basically there to reduce light from bright objects like viewing a full moon. Normally you should remove the entire cap for general viewing. Years ago I started with a similar scope off EBay, I recall it has very low grade plossls and they were only 3/4" diameter.

Having said that You should be able to view with a 20mm eyepiece the planets of Jupiter and Saturn albeit very small and some of the larger stars like Sirius. Vega and Arcturus.

For a similar price there are a number of 2nd hand 8" dobsonian scopes available which will give far greater viewing satisfaction.

Clear skies
Dave Perth
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Old 03-12-2017, 02:42 AM
buill
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finally got a clear night last night and tryied it out was able to focus the mirror out with the 20mm,12mm eyepieces but the 4mm i cant see anything but darkness through it even on the moon it blurry it might be broken as i did get it second hand but as one post here stated it might not work with this telescope.
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Old 03-12-2017, 12:35 PM
raymo
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The small cap's actual purpose is to reduce the aperture[and internal heat build up] when doing solar observing with a solar filter fitted to the eyepiece.
This practice is now a no-no due to the risk of eye damage should the
filter shatter. Unfortunately, these filters are still available on ebay.
There is no point in just removing the small cap when viewing the moon
because this reduces the aperture to about 50mm which in turn reduces the image's brightness. The scope's resolution is reduced by more than 50%, which is the last thing you want when looking at all the interesting features
on its surface. Incidentally, if you want to view the sun you can attach
some Mylar sheet to the aperture exposed when the small cap is removed.
raymo
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