tag:blogger.com,1999:blog-21962870644652576032024-02-08T13:39:39.979+00:00Hints and TipsJohn: Train-Plane-Birding-Astronomer!http://www.blogger.com/profile/08773289567350616988noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-2196287064465257603.post-12407182264365816792008-07-06T15:55:00.021+01:002009-10-31T12:24:53.008+00:00<!--<a href="http://dasacquireaa.blogspot.com/"><span style="color:#99c9ff;">Acquiring Pictures with a Colour Camera in AstroArt</span><br /><span style="color:#99c9ff;"></span></a><br /><br /><a href="http://dasastroart.blogspot.com/"><span style="color:#99c9ff;">Processing Colour Pictures in AstroArt</span></a> --><br /><br /><a href="http://dascollimation.blogspot.com/"><span style="color:#99c9ff;">Collimation</span></a><span style="color:#99c9ff;"><br /><br /></span><a href="http://dasformulaeofinterest.blogspot.com/"><span style="color:#99c9ff;">Formulae of Interest</span></a><span style="color:#99c9ff;"><br /><br /></span><a href="http://dasdriftalignment.blogspot.com/"><span style="color:#99c9ff;">Drift Alignment</span></a><span style="color:#99c9ff;"> </span><br /><span style="color:#99c9ff;"><br /></span><a href="http://dasdarks.blogspot.com/"><span style="color:#99c9ff;">Dark Frames</span></a><span style="color:#99c9ff;"> </span><br /><span style="color:#99c9ff;"></span><br /><a href="http://dasflatframes.blogspot.com/"><span style="color:#99c9ff;">Flat Frames<br /></span></a><br /><a href="http://dasdarkadaption.blogspot.com/"><span style="color:#99c9ff;">Techniques for seeing Faint Objects</span></a>Anonymousnoreply@blogger.com0tag:blogger.com,1999:blog-2196287064465257603.post-67237362185201221902008-06-18T00:03:00.011+01:002018-07-30T10:27:21.251+01:00Formulae of Interesta. f ratio = focal length / diameter (mirror or objective)<br />
<span class="blsp-spelling-error" id="SPELLING_ERROR_0">eg</span>. A telescope of 600mm focal length with an objective of 80mm <span class="blsp-spelling-error" id="SPELLING_ERROR_1">Dia</span> will give an f ratio of 7.5 f/7.5<br />
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b. Magnification = focal length of the objective / focal length of the eyepiece<br />
<span class="blsp-spelling-error" id="SPELLING_ERROR_2">eg</span>. our 600mm telescope and a 20mm eyepiece give 600/20 = 30 times<br />
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A 2x <span class="blsp-spelling-error" id="SPELLING_ERROR_3">barlow</span> will double the focal length to 1200mm making the magnification 1200/20 = 60 times<br />
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Focal reducers reduce the focal length, of course. The popular .63 FR will reduce the focal length to 378mm so the 20mm eyepiece will give 378/20 = 19 times (18.9 actually)<br />
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c. In photography, the focal length controls the image size<br />
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The formula is Pixel size (microns) x 206 /focal length(mm) = Field of View/pixel (<span class="blsp-spelling-error" id="SPELLING_ERROR_4">arcsec</span>)<br />
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This gives you the amount of sky that each pixel sees.<br />
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<span class="blsp-spelling-error" id="SPELLING_ERROR_5">eg</span>. For our 600 mm telescope and a digital camera with square 7.8 micron pixels, the formula will give us 7.8 x 206/600 = 2.678 <span class="blsp-spelling-error" id="SPELLING_ERROR_6">arcsec</span><br />
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Multiplying this by the number of pixels in the array gives the size of the camera’s Field Of View in <span class="blsp-spelling-error" id="SPELLING_ERROR_7">arcsec</span><br />
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So for a camera with an array of 3024 x 2016 pixels, the result is<br />
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2.678 x 3024 = 2.25°<br />
2.678 x 2016 = 1.5°<br />
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A Field of View of 2.25° x 1.5°<br />
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d. Focusing, a critical job in <span class="blsp-spelling-error" id="SPELLING_ERROR_8">astrophotography</span>, but how close do you have to get to be "in focus"?<br />
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The formula is, In-Focus Zone = Focal Ratio² x 2.2 in microns<br />
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for our example telescope it is 7.5 x 7.5 x 2.2 = 124 microns and with 1000 microns in a millimetre it means .124 of a mil say 1/8<span class="blsp-spelling-error" id="SPELLING_ERROR_9">th</span> of a mil.<br />
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For a Meade <span class="blsp-spelling-error" id="SPELLING_ERROR_10">SCT</span>, it is 10 x 10 x 2.2 = .22 of a mil.<br />
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It's all in the photographic speed of the telescope.<br />
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A fast f/2 system will have to be within (2 x 2 x 2.2 = 8.8 microns) = less than a 10<span class="blsp-spelling-error" id="SPELLING_ERROR_11">th</span> of a mil !<br />
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Not easy with a rack and pinion <span class="blsp-spelling-error" id="SPELLING_ERROR_12">focuser</span>Anonymousnoreply@blogger.com5tag:blogger.com,1999:blog-2196287064465257603.post-14480553142530580762008-06-16T09:44:00.000+01:002018-07-30T10:31:51.831+01:00CollimationCollimation isn’t too well understood, but I don’t suppose it needs to be, provided you know how to “do it”.<br />
Collimation really means ensuring the various pieces of the optical train are in line and square with each other.<br />
When we buy a telescope, we assume, that it’s perfect, and accept the views we get with it as normal for this type of instrument. But it’s worth a check.<br />
<br />
Collimation is really quite a big subject, but can be dealt with fairly simply for our purposes.<br />
First, how do you know your telescope isn’t “perfect”? In simple terms it doesn’t give you clear images, the image is slightly fuzzy, but many other problems can give the same result, from bad seeing, to fingerprints on the eyepiece.<br />
But you don’t need to investigate what’s causing the problem yet, just check for bad collimation to eliminate it as a cause. How? Wind the image of a star out of focus and if the rings or discs you see are concentric, you don’t really have collimation as a major fault. Though you can always improve it.<br />
So, do all telescopes suffer, or does this affliction only occur in certain breeds? All telescopes can get this problem, but some are more likely than others, I suppose it’s in the genes.<br />
Refractors are the least likely to suffer and accordingly few have the means to correct it, only some very high end refractors have the adjustment, 3 screws, built into the objective lens cell, to correct bad collimation, though you don’t expect to have a collimation problem with them anyway.<br />
The only way to correct a normal refractor with bad collimation is to send it back to its Maker.<br />
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Reflectors? Yes, all types.<br />
The degree of affliction is dependant on the design and quality of its execution, but, by and large, all reflectors suffer.<br />
The test as before, use a medium magnification eyepiece, focus on a mag 3 to 6 star, in reasonable seeing, when the star doesn’t jump about. Make sure it’s in the centre of the field of view too. Now unfocus it so that big circles, discs are produced.<br />
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Important points are, make sure the star is in the middle of the field, as other problems will start to interfere if the star is off axis.<br />
And make sure the primary has had time to stabilise its figure. This comes when the temperature of the primary matches the ambient temperature. It can take an hour from getting the telescope out and set up. The “figure” is the shape of the primary’s curve; it changes with temperature. These are only very small amounts but they can be seen in the eyepiece. A closed tube reflector will take much longer than an open tube reflector. But you can do most of the work without waiting, though with less perfection.<br />
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SCT’s, like Meade and Celestron products, have a pierced primary and only have one means of collimating and that is by adjusting the secondary, where 3 screws are provided.<br />
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Newtonians? You must first mark a central spot on the primary, draw a small circle there with a felt tip pen. Sacrilegious? Yes, but be adventurous, after all, the secondary hides this area, so it isn’t a working surface. Make sure the circle is accurately in the middle though.<br />
Then looking through the focuser, preferably using a long extension tube so as to get as far back as possible making the eye line through the focuser to the secondary as central to it’s axis as possible (a tube with a small central hole through it will help too, though much more difficult to use), adjust the secondary side to side etc. till your circle on the primary is right in the middle of the field of view seen through the focuser, and concentric with the rim of the secondary.<br />
That was stage one.<br />
<br />
Stage two is to remove the tube and extension and put a low magnification eyepiece in, and get a star in the centre of the field, and put it out of focus so that the circles are seen. This applies to SCT’s too.<br />
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In Schmidt Cassegrains, the adjustment is in the secondary. In Newtonians the adjustment is in the primary.<br />
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Adjust the screws, a little at a time, trying to remember which screw, which way, till the circles are concentric. After each adjustment, which will take the image to one side, return the image to the centre of the field before making a judgement on that adjustment. That puts quite a lot of work into a few words, particularly if your collimation is far off and you have no drives. Your telescope may be quite long and your arms short, so you can’t see what you’re adjustments do till you’ve got your eye to the eyepiece again. It can be time consuming, but persevere in little steps, don’t be tempted to use a hammer and chisel. A remote eye, in the head of a friend helps, but a webcam is better, and easier on the temper.<br />
When you’ve got the rings concentric both sides of focus, yes, both sides of focus, this means winding the focus through the focus point and beyond, stop for a cup of tea. Then change to a higher magnification eyepiece, and start all over again. Then on to a higher magnification eyepiece and repeat.<br />
If you want it to be as good as possible, add a Barlow and do it again. But you’re approaching the point of diminishing returns. So judge when to stop to suit your needs.<br />
When you get to using more than 2x barlows for this job you’d better start thinking about all the other problems that affect the image you see in the eyepiece.<br />
There are many refinements to consider, but none that makes giant improvements.<br />
That’s all there is to it, so get on with it. You’ll not regret the work.<br />
<br />
Oh, another point. It pays to check collimation every time you use the telescope till you get to know how well it holds collimation. Some hold it well but still need it checked from time to time. Others don’t hold it well at all, but that generally applies to older and cheaper reflectors.<br />
<br />
My preferred way is to use a webcam so I can stay at the working end of the telescope and make adjustments with more care. The chip is so small that the image will always be on the centreline of the telescope or it flies off screen, which forces you to make small adjustments. The picture I see of the rings is much bigger on the screen of the laptop, and with two eyes it’s easier to judge concentricity.<br />
I did this work on our new telescope, the Vixen VMC260L, the other night, standing on a rickety folding stool in the dark with the telescope pointing almost vertically and the secondary at about my eye level, with the laptop on the floor. So with a tiny Allen key in one hand ready to drop onto the primary, and drive controller in the other and my knee against the pier to steady myself. I made the adjustments, but without a Barlow. It’s reasonable now but can be made better, and I’ll do that when it’s on the new mount.<br />
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The cross on the picture at the side is caused by the spider holding the secondary, and when in focus will produce the characteristic spikes. The rings aren't perfectly concentric as you can see, but close enough for the moment.<br />
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Regards BrianAnonymousnoreply@blogger.com0tag:blogger.com,1999:blog-2196287064465257603.post-10301384619415303272008-06-06T14:20:00.000+01:002018-07-30T11:19:27.436+01:00Drift AlignmentTo take long exposure photographs it is important to line up your telescope as accurately as possible, parallel with the line round which the Earth rotates, and the simplest way to do this is Drift Alignment.<br />
It’s simple but can be time consuming, dependant on how accurate you wish to be.<br />
<br />
You’ll need a short focal length eyepiece (giving 200 magnification or more) with crosshairs, you’ll be deluding yourself if you think you can do it without one, and an illuminated reticule would be much easier to use.<br />
You will be working with two stars, one in the east and the other in the south, and both should be within about 5° of the Celestial Equator and bright enough to see clearly at 200x magnification.<br />
You will be allowing the telescope to track the star and be making mechanical adjustments based on what you see it do in Declination, ignoring what it does in RA.<br />
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Your mount’s hold down bolts should be firmly secure, not slack.<br />
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Go to your southern star, within about half an hour of due south.<br />
Orient the lines of the crosshair up and down, parallel to the Dec and RA directions. Set the star up on the crosshair, and watch to see what it does.<br />
You should see movement within a few minutes, in seconds if badly aligned to start with.<br />
Ignore what it does in RA, but note which direction it moves in Dec., up or down from the RA line.<br />
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If your star drifts up, adjust the Azimuth knobs to move the star to the left in the eyepiece field.<br />
If it drifts down, adjust the Azimuth to move the star to the right.<br />
Then retighten the hold down bolts snugly, reset the star on the crosshair again and repeat.<br />
Keep repeating the process till the star stays on the RA illuminated line for at least 10 mins. <br />
Aide memoire? "Left-up in the air". Or "Down-right stupid".<br />
<br />
When happy with that step, reset the telescope to your star in the east, again near the Celestial Equator, some 20 to 30° up.<br />
Set it on the crosshair, again with the Dec line up and down and the RA left and right, and watch for up or down movement.<br />
If it drifts up, adjust the Altitude knobs to move the star down.<br />
If it drifts down, adjust the Alt. to move it up.<br />
Then retighten the hold down bolts, reset the star on the crosshair again and repeat.<br />
Keep repeating the process till the star keeps on the RA illuminated line for 10 mins.<br />
<br />
When happy with that, repeat the process with the star in the south to make sure your adjustment in Altitude didn’t affect the Altaz settings.<br />
BrianAnonymousnoreply@blogger.com0