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 Post subject: Aligning a finderscope
PostPosted: Tue Feb 19, 2019 7:51 pm 
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In the latest edition of Popular Astronomy Robin Scagell describes the two traditional methods of aligning a finder scope with the main scope before it can be used. These are:

1: Point the telescope at a distance object during daylight such as the convenient church steeple that appears in so many of these instructions.
2: Point the main telescope at Polaris (since it does not move much) and then align the finder to Polaris.
To me both of these have severe problems. With the first you have to start during daylight and then wait for darkness – not very good for impatient beginners. You also need to be able to see and locate a distance object that can be identified in the upside down view in the main telescope. Most places I have tried this there is a shortage of distinguishable distance objects. Trees do not usually work as unless you are lucky to have a single tree it is difficult to know which one of a number of trees you are looking at in the main telescope.

In the second method the problem for beginners is finding Polaris through the main telescope even with a wide field eye piece. It is quite easy to end up on the wrong star and then align the finder to Polaris (assuming the beginners can find Polaris).
After lots of frustration with aligning the finder on by 200mm SCT which has a small field of view I thought there must be a better way. Having been inspired by the laser guide stars used for active optics on professional telescopes I realised that if one put a green laser in place of the eyepiece you should be able to see it through the finder and thus align the two easily.

As a result I made the adaptor shown in the attached image. The outside diameter is 1 ¼” the same as an eyepiece and the inner hole is the same diameter as the laser pointer. The locking screw which has since been replaced by a plastic version stops the laser pointer falling out or worse still falling into the telescope! The outer stop is provided by using a parfocal ring as this was easier than turning down a larger piece of bar.

In the illustration you may notice that the laser pointer is angled to the ‘optical axis’. This was not the original intent but drilling the hole in my small lathe required going up in small steps and the first small drill was not rigid enough and wandered off centre. The other larger drills then followed this path. When I first made this I was rather disappointed with the result but then I realised that while an on axis beam would work in a refractor in an SCT or a Newtonian the beam would be reflected straight back to the laser and would not emerge from the front of the telescope. For an SCT or Newtonian the angle needs to be such that it is off centre but hits the secondary mirror. Luckily that is what I accidently achieved!

Using the tool is remarkably easy. You just insert it in the eyepiece holder and switch it on. Looking through the finder scope you see the green beam which fades as it gets further away. It only takes a few seconds to adjust the finger until the point where the beam fades to invisibility is in the cross hairs and the finder and main telescope are aligned.
Of course the normal rules about being aware of pointing the laser at aircraft apply.

A second use is if you are showing other people objects through the telescope, they frequently ask where the object is on the sky. It only takes a second to remove the eyepiece and insert the laser and the green beam emerging from the front of the telescope will point out exactly where it is. The green beam emerging from the telescope looks quite impressive as well.

This tool is not commercially available so you will need to make your own, either from aluminium as I did or perhaps make it from wood as being easier to work or else 3D print it.


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File comment: Tescope Finder laser alignment tool
20120429-153722-small.jpg
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PostPosted: Tue Feb 19, 2019 9:17 pm 
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Take a bow! 8)

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PostPosted: Wed Feb 20, 2019 8:01 am 
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
An interesting but perhaps rather technical way of aligning a finder for a beginner, especially as no one that I know has a metal turning lathe in their shed. I agree that the use of Polaris is problematic as it is very difficult to line up on if the telescope is on an equatorial mount. An equatorial mounting is of course the bane of beginners anyway. If one must align at night, I would suggest the Moon, and perhaps for greater accuracy, a part of it thereof, or a bright planet. As it is generally advised that the beginner (and I would suggest even the expert) should set up a new telescope in the daytime, finding a distant object in the landscape to use to align the finder, it surely not that difficult, and identifying a distant upside down pylon or chimneypot must be easier than deciding which star in the fov is Polaris, or any other star being used. A good idea for the expert maybe particularly for those who like a technical approach but not for the beginner nor those preferring a simpler method. Regards mike the Luddite.


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PostPosted: Wed Feb 20, 2019 8:16 am 
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Hi Mike,

I agree few people would have the skills and facilities to make this out of metal. However one could make it out of wood, 3D print it or even use a bit of ingenuity with an old 35mm film canister (these are 1 1/4" diameter), a piece of 1/2" ID tube and a hot glue gun.

The problem with the moon or other objects some way from the Celestrial pole is that while you are trying to align the finder your telescope mount will not be tracking so the Moon or Planet will move outside the FoV of the main telescope quite quickly. ( Both of my mounts only start tracking after alignment - is this a universal trait ?)

Of course if there was enough interest perhaps someone would start manufacturing them. The angled version works ok in a refractor even though the beam emerges off centre from the objective lens. As a result one angled model should work for all unless it has a very large secondary mirror.

I will try to get some images of the device in use and post them here.

John

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PostPosted: Wed Feb 20, 2019 6:13 pm 
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BAN laser pointers, nothing more inconsiderate to astronomers imaging or viewing to have others shining lasers in the sky

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PostPosted: Thu Feb 21, 2019 2:53 pm 
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Vapour trails from aircraft are a nuisance. Should aircraft be banned?

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PostPosted: Thu Feb 21, 2019 4:06 pm 
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skyhawk,

ESO seem to manage quite well with 4 lasers per telescope on the VLT so who are we to complain ?

The lasers operate all the time they are imaging and they have 10 telescopes on site but still manage not to image the lasers from other telescopes.

John


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File comment: (c) ESO VLT
ESO-VLT-Adaptive Optics.jpg
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PostPosted: Fri Feb 22, 2019 9:45 am 
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brian livesey wrote:
Vapour trails from aircraft are a nuisance. Should aircraft be banned?



Pointless comment

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For info, I am Autistic, Aspergers, ADHD, therefore if I come over as a little "short" on occasions it is not intended, thank you


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PostPosted: Fri Feb 22, 2019 9:46 am 
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JohnM wrote:
skyhawk,

ESO seem to manage quite well with 4 lasers per telescope on the VLT so who are we to complain ?

The lasers operate all the time they are imaging and they have 10 telescopes on site but still manage not to image the lasers from other telescopes.

John



There are NO local streets, residents, pointless comment

https://www.irishastronomy.org/kunena?v ... 1&id=67150

https://laserpointerforums.com/threads/ ... 17.102055/

http://www.skyatnightmagazine.com/forum ... 31369.html

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Celestron 8" Edge HD Evolution, Esprit 120mm triplet, 72mm APO, Sky Tee 2, 6" reflecting scope, William Optics Binoviewer, Quark Daystar Ha Chromosphere on 72mm ED, LVW8mm eyepiece and Celestron 19mm Axiom, matched W.O 10 and 20mm, and a few others, D4s, D810,

For info, I am Autistic, Aspergers, ADHD, therefore if I come over as a little "short" on occasions it is not intended, thank you


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PostPosted: Fri Feb 22, 2019 4:58 pm 
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A large part of the Hazard comes from the fact that a large proportion of the lasers on sale emit partly collimated infra-red radiation see the paper at https://arxiv.org/abs/1008.1452v1. One thing I had not realised that while I knew that the infrared is what heats the eye and destroys the retina this can be reflected at a different angle to the green light and can enter the eye even though you are not looking at the green light. Since infrared is not detected by the eye you are not protected by the blink reflex.

One case mentioned in the paper above is that of windows covered with solar film designed to reflect infrared to keep rooms cool in summer but allow light through into the room. If you point a 'cheap' Green laser at a window with an infrared reflective coating the green light will pass through and the infrared will be reflected and may end up in someones eyes and damage them.

It is not clear to me if the infrared power is measured when a visible light laser is tested or do they filter this out ? It seems probable that the visible light lasers advertised as capable of 'burning' actually heat the material due to the infrared radiation.

That said a properly constructed laser used safely is a valuable tool in all sorts of applications.

John

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