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Help me my telescope shows only black sights


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i have a National Geographic 90/1250 and i regularly look through it at night to see planet and stars. but for some reason if i look through it while looking at a star or planet, then i just see a black circle with light around it. Kinda like a black hole surrounded by light.

and if there is a light source, nearby my telescope then i can see the planet/star if i pull out the lens i'm looking through, out of my telescope and i look into the hole where my lens is supossed to be then i can see it. am i doing something wrong? i really want to look at space but even if i look at the moon i only see light nothing else? help....

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I am not sure, but could it be that you are way out of focus?  The best thing is to point at the Moon which is bright, then it will be easy to get focus, to see the craters.  Then keep a note of where the point of focus it and try to get it at that point every time.  Once you have focussed on the Moon your planets will also be in focus. 

Carole 

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Two possible basic thoughts come to mind here,  "eye relief" and "central obstruction".

Eyepieces have eye relief distances varying from almost touching to as much as 20mm.  This means that your eyeball needs to be a precise distance away from the lens and fully central to it for an image to form in your eye.  If you're too near or too far or slightly off centre, you see blackness or partial blackness.  Most eyepieces have extending rubber eye cups which help, but some don't, and spectacle wearers have to fold them down anyway.

As has been suggested, try practising with the telescope during daylight, where it is much easier to judge the correct distance and align the eye centrally.  At night, trying to locate the dot of a star can be very difficult if one has to contend with getting used to the eye relief distance for the first time as well.

The second thought is that telescope ( National Geographic 90/1250 ) is a Maksutov-Cassegrain , so it has a secondary mirror in the centre of the light path. This means that with certain levels of brightness and certain magnifications, you will see a central black obstruction, which is the smaller secondary mirror inside the middle of the front glass.  It's particularly noticeable during daytime viewing, when your eye iris opening reduces below a certain point.

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14 hours ago, JaydenvdH said:

but for some reason if i look through it while looking at a star or planet, then i just see a black circle with light around it. Kinda like a black hole surrounded by light.

If this telescope is a Maksutov or other type with a central obstruction, then the first explanation that comes to mind is that the image is out of focus.  This is exactly what you see with an out of focus Mak or SCT or newtonian. So focus it: easiest done in daytime on a distant object, or on a bright star at night.

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13 hours ago, Astro-Geek said:

Two possible basic thoughts come to mind here,  "eye relief" and "central obstruction".

Eyepieces have eye relief distances varying from almost touching to as much as 20mm.  This means that your eyeball needs to be a precise distance away from the lens and fully central to it for an image to form in your eye.  If you're too near or too far or slightly off centre, you see blackness or partial blackness.  Most eyepieces have extending rubber eye cups which help, but some don't, and spectacle wearers have to fold them down anyway.

As has been suggested, try practising with the telescope during daylight, where it is much easier to judge the correct distance and align the eye centrally.  At night, trying to locate the dot of a star can be very difficult if one has to contend with getting used to the eye relief distance for the first time as well.

The second thought is that telescope ( National Geographic 90/1250 ) is a Maksutov-Cassegrain , so it has a secondary mirror in the centre of the light path. This means that with certain levels of brightness and certain magnifications, you will see a central black obstruction, which is the smaller secondary mirror inside the middle of the front glass.  It's particularly noticeable during daytime viewing, when your eye iris opening reduces below a certain point.

O ok. Im just a beginner and this the first time dealing with a real advanced telescope. I've never had any experience before so now i get that i need to learn alot about it. But if i would focus it during daylight. Where should i aim/focus it on?

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Have had several new members rock up to our club, with scopes they have purchased from National Geographic, Australian Geographic, down under, and when used laser to checked collimation, found way out

Some do not even have small dot on primary mirror of dob

Had one, where secondary mirror was mounted incorrectly, and was not able to adjust

If still having issues, rock up to your local club, and members there only too happy to assist you

John

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46 minutes ago, JaydenvdH said:

O ok. Im just a beginner and this the first time dealing with a real advanced telescope. I've never had any experience before so now i get that i need to learn alot about it. But if i would focus it during daylight. Where should i aim/focus it on?

When I Google " National Geographic 90/1250 ", it comes up as a Maksutov, and that design holds its collimation much better than Newtonians, which need tweaking almost every time they're moved. ("Collimation" is the accurate lining up and angling of the two mirrors inside reflecting telescopes, and needs to be as perfect as possible to get sharp clear images).

To practice focusing in daylight, it's best to pick something that's as far away as possible, but with fine detail, such as leaves or branches on a tree, or brickwork etc..

You should be able to focus a nice sharp and clear image, just like a normal terrestrial telescope, except that it may be upside down and reversed left to right, (which is normal for an astronomical telescope).

In daylight, you may also see a central black out of focus dot, which is the secondary mirror, partially blocking the view. Again, don't worry, this is quite normal, because during daylight, your eye's iris will be smaller than at night, when it is darker and it opens up, and the dot will no longer be visible.

Move your eye back and forth from the eyepieces to get used to each eyepiece's "eye relief", which is the distance it needs to be from your eyeball to focus the image on your retina. It will vary with different eyepieces from a couple of millimeters to as much as 20 millimeters. When your eye is in exactly the right place, you will see a full circular image which then needs to be focused.  When you're new to the game, it can be very frustrating getting used to finding  exactly the right spot in relation to the eyepiece when looking at stars, because they're just a white dot on a black background.  Whereas in daylight, it's a full bright circular view.

 

ng90 1250.jpg

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Hi @JaydenvdH and welcome to SGL. :hello2:

What eyepiece(s) are or were you using at the time?

Mak's and SCT's do have a lot of winding in or out of the focusser when new to acheive focus. Eye placement at the eyepiece is critical too. I found that out when with my Meade ETX105 before the 're-mod' and using a TeleVue 6mm 'Radian' e/p. Once you have found the 'sweet-spot' it should be easier to focus other e/p's, unless they are parfocal.

EDIT 26.02.2019 Another thing too, don't be tempted to buy a 'Barlow' lens or 'Powermate' clone... they can make things worse as you have a narrow field of view. I never use one in mine, unless I am using my TeleVue 'Ranger'. That said you can buy a thing called a Magni-Max*. They screw into the eyepicee nose and give about 1.6x - 1.8x extra magnification depending on the manfacturer.

 

* other brand names are avaiable.

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12 hours ago, JaydenvdH said:

first time dealing with a real advanced telescope

If you ever buy another one can I respectfully suggest that you ask for recommendations on SGL before parting with hard cash for something sold by National Geographic? Some of their telescopes have a very poor reputation and are not value for money.

That said I guess it should be possible to get some sort of passable use out of that model, and it does sound like you are out of focus.  As noted above during the daytime and pointed the opposite way from the sun see if you can focus on a distant object like a spire, chimney or tree on a horizon.  Unless you can make that work well during the day time I would say your chances of succeeding at night are less than evens.

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On 24/02/2019 at 13:46, JaydenvdH said:

i have a National Geographic 90/1250 and i regularly look through it at night to see planet and stars. but for some reason if i look through it while looking at a star or planet, then i just see a black circle with light around it. Kinda like a black hole surrounded by light.

and if there is a light source, nearby my telescope then i can see the planet/star if i pull out the lens i'm looking through, out of my telescope and i look into the hole where my lens is supossed to be then i can see it. am i doing something wrong? i really want to look at space but even if i look at the moon i only see light nothing else? help....

 

2 hours ago, michael8554 said:

Be aware that on many Maks the focus knob may take many many turns to get close to focus, don't be afraid to go from one end of the range to the other in search of focus, but once it gets hard to turn, time to reverse direction.

Michael 

Yes, most likely way out of focus.  Turn the knob in the direction that the circle of light surrounding the black hole gets smaller.  It may be hard to tell at first.  As Michael says above, if you hit the focus limiter, stop, don't force it, and reverse direction because you've turning it the wrong direction.  Hopefully you'll get to a sharp focus before you hit the other limiter. ?  It may take a dozen or more revolutions of the focuser knob to reach focus from the limiter position.  If a star or planet never gets to a sharp focus between focus stops, something is seriously wrong with the scope (such as a broken focus mechanism) and should be returned for replacement.  Maks are pretty tough, though; so this is unlikely.

If it reaches sharp focus, but the image looks distorted into an egg shape when slightly out of focus while centered in the eyepiece, then collimation may need adjustment.  If that's the case, check back here for further instructions on how to collimate a Mak.

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Oh, and if it focuses like mine does check there is not a locking nut on the focus mechanism which has been turned either by accident or design.  If this has been tightened no amount of twiddling of the focus knobs will drive the mechanism in and out.  So stand back twiddle the focus knob and check that the mechanism appears to drive in and out.  If it doesn't find a screw in that area and see if it untightening it makes a difference

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On 26/02/2019 at 01:08, Philip R said:

Hi @JaydenvdH and welcome to SGL. :hello2:

What eyepiece(s) are or were you using at the time?

Mak's and SCT's do have a lot of winding in or out of the focusser when new to acheive focus. Eye placement at the eyepiece is critical too. I found that out when with my Meade ETX105 before the 're-mod' and using a TeleVue 6mm 'Radian' e/p. Once you have found the 'sweet-spot' it should be easier to focus other e/p's, unless they are parfocal.

EDIT 26.02.2019 Another thing too, don't be tempted to buy a 'Barlow' lens or 'Powermate' clone... they can make things worse as you have a narrow field of view. I never use one in mine, unless I am using my TeleVue 'Ranger'. That said you can buy a thing called a Magni-Max*. They screw into the eyepicee nose and give about 1.6x - 1.8x extra magnification depending on the manfacturer.

 

* other brand names are avaiable.

i was using 2 eyepieces K25mm and K12mm

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