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Thread: Enhanced Planisphere and R.A. slide rule

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    Default Enhanced Planisphere and R.A. slide rule



    I modified my planetarium with a few lines and circles and turned it into a more useful device. The most useful feature (and the reason I started the project) is that it lets me predict when pointing at an object near zenith may cause the diagonal to hit the base of my LS8 and a safe time to view it. I've added a meridian line with declination markers, a scale and zero point in red on March 21 on the perimeter to calculate the R.A. of a star, and added hour numbers for 13 - 23. There's circles for zenith, a danger zone for viewing near zenith at my latitude, circumpolar stars, and the celestial equator.

    plan1.jpg

    The 'markings' are detents in the clear plastic overlay pushed in with a dull pencil and a 'dead' pen. The graphite cleans off easily but the ink from my 'Lazarus' pen took more effort. Beware! I did the circles with a pencil, the lines with the pen. The pen was too sharp and tended to catch on the surface and produced spotty and pitted circles but it drew sharp lines and circular detents on the R.A. scale.

    Drawing the circles requires an even pressure on the surface, keeping the pencil centered over the selected star while turning the plastic overlay and keeping the bottom stationary. I used a mixture of heavy books to get the right wedge to lock in the bottom. To get a feel for the technique try using a finger with increasing pressure. Practice with the smallest circle using a dull pencil and very light pressure. The graphite deposited will help reduce the friction. If it gets too dark clean it off with a dry wipe.

    I picked a star with the same dec as my latitude and used it to draw my zenith circle. Then I drew a circle 5* south of it to mark the bottom of my danger zone. Next I drew the circumpolar circle which turned out to be 4* north of the zenith circle and used that as the top of my danger zone. Last up was the celestial equator and done! Finally I cleaned off the graphite and admired the work.

    I drew the meridian line from the pole rivet to the southern horizon pip at 12 o'clock and marked off the dec lines by using stars with correct dec. I'll list the stars I used at the end to save you the trouble and atlas work required to find them. I finished my danger zone trapezoid by drawing two lines from the pole, one from 11 am and the other from 1 pm and marking them from the top of my circumpolar circle to the 5* south circle. Danger zone complete! In theory this will give me a 1 hour heads up warnng and a time when the object is 1 hour past zenith and safe to view.

    plan2.jpg

    For the R.A. calculator I needed a way to mark off the minutes. I noticed that the hour markers are about 15 day markers apart. So each day marker is about 4 minutes of R.A. The scale is close but not exact so there will be some small error in the result. 5 of the day markers are about 20 minutes of R.A. so I pressed in pips at day markers March 26, and April 5. There's a solid line on March 31 and I put a useless pip there too. These marked off 20, 40 and 60 minutes. Then I drew the lines at 10, 30 and 50 to complete the scale. I drew a red line at March 21 for the 0 hour marker and colored the top of it to make it easier to find.

    plan3.jpg

    The R.A. accuracy is a good ball park figure. I get within 2 min. of R.A. on some stars but most of the results are within 5 minutes. The declination marks on the meridian line are a 'slide rule' eyeball method and are less accurate. It's better at the middle latitudes than it is below the equator. It depends on how accurately the chart was projected and printed and the precision of your lines and circles.

    To find the R.A. of a star - put it on the meridian line and read the result from the red marker at March 21 down the scale to the nearest hour marker. As an example use the star Vega in Lyra. Bisect it with the meridian line and then notice that it falls between 6am and 7am which is between 18 and 19 R.A. From the red line down to the 6am marker there's about 42 indicated on the scale. It's a little south of the 40* line so the estimation of it's position by the planetarium is 39* 18h42m. The actual position via Wiki is 38*47' 18h37m.

    To find the area of the sky using R.A. and Dec - assume you want to find the irregular galaxy pair NGC 4656 and 4631 on the chart. The location is at 32.1* 12h43m. Put the 12 hour marker on the red line and use the scale to move it back 43 min. Look on the meridian line and find the correct approximate dec and the location is found to be in the constellation Canes Venatici. If a line between Denebola and the bright star in Canes is drawn, it looks to be close to that line. It narrows down the location for a more dedicated search. It also shows when it's at meridian and at the best viewing time, in this case on April 1 at midnight, April 16 at 11 P.M. ect.

    Now with an object's R.A. and Dec. coordinates I can easily find the area of the sky and the constellation where it's located. Also, when reading an observer's report, I can whip it out and follow the footsteps of their journey. With the extra features it's turned into a better tool for exploring the sky and it helps me prevent a diagonal crash. Old Dog, New Tricks?

    70 Kappa in Draco : next to last star in tail
    60 Ruchbah in Cass : star at point of shallow 'V'
    50 Mirlak in Perseus : also called Mirphak : star where the 'legs' join
    40 Sadr in Cygnus : star at the middle of 'cross'
    30 Zeta in Cygnus : star in south wing, second star from center star of 'cross' (Sadr)
    20 Gamma in Leo : star in 'question mark', second star north from Regulus
    10 Altair in Aquila : put mark between Altair and close north star (gamma) on the circumference of Altair
    0 Mintaka in Orion : west star in belt
    -10 Saiph in Orion : east 'foot' of Orion, across from Rigel
    -20 Graffias in Scorp : top bright star of north 'claw'
    -30 Zeta in Canis Major : bright star south of Murzim on west edge of constellation

    Just out of curiosity I checked the vernal and autumnal equinox days using the zenith line at the celestial equator and found the dates when it crosses the ecliptic. I got March 17 and Oct 2. I also checked it in a dark area and the markings show up well under a red light.
    Last edited by bladekeeper; 09-18-2016 at 09:45 PM. Reason: requested title edit...
    bladekeeper, Jnielsen and Gabby76 like this.

  2. #2
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    Default Re: Enhanced Planetarium and R.A. slide rule

    Very cool well done! Could be very usefull for predicting meridian flip requirements

    I am modding a planisphere to show my tree obstructions so I can tell when a target will be available and when it will end.

    Basically for now just observing the sky and when a certain star is 'just barely' above a tree top I stick a little sticker below it. Eventuallly I will connect the dots.

    Anyhow don't want to sidetrack your thread just some additional ideas.

    Oh .. its called a Planisphere
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    Default Re: Enhanced Planetarium and R.A. slide rule

    OOPS! Guess I should have researched / checked my title.

    Would a moderator please update the title?

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    Default Re: Enhanced Planisphere and R.A. slide rule

    Meh who cares everyone knows what you meant

    Oh and liked the meridian line, that's a great idea!
    Last edited by UlteriorModem; 09-18-2016 at 09:53 PM.
    Scopes: Orion 130mm APO Triplet, Orion 8" f4 astrograph, 10" RC Mount: Celestron CGX-L Guiding: Orion 60mm f4 scope, Starlight Loadstar, PHD2 software - Cams: Planetary: ASI120mc, DSO:Atik 383L+, SGPsoftware Processing : Regisax, Deep Sky Stacker, Star Tools, Pixinsight Assistant: One very large cat
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