Circles

[Langy]

A few weeks ago after I had ordered my tripod I watched loads of YouTube videos and read lots of articles before my scope had even been delivered, wanting to learn as much as I could so that I'd have some ideas about how to use it when I finally had it setup.

Now I can remember one video I watched was about setting up and using circles on a small telescope. All started well and after half way through the topic went on about how bad circles were on a small telescope in that they were not accurate enough. Fair enough I thought and then the video ended. So it was a video on setting up circles, told me how bad they were and don't use them because they are of no use, why not call the video "Why not to use circles on a small telescope!"

So I know that they are not accurate, but they must still have some use. So to me if they could be used to quickly point you in the right direction of an object then they have helped no end, a bit like using the finder scope. It's a bit like using a satnav, if you enter the postcode you don't expect it to take you right to the door of where you are looking for as a postcode can still cover a reasonable sized area.

If someone thinks that I can use circles this way then perhaps you can just give me some instruction on how to set this up as looking up the coordinates seems rather confusing for the first time.

Below is a picture of the mount on my telescope.

Now if I am roughly polar aligned (telescope setup in my postage stamp back garden) the folowing instructions are in the manual.

1. Locate a bright star near the celestial equator. The farther you are

from the celestial pole the better your reading on the R.A. setting

circle will be. The star you choose to align the setting circle with

should be a bright one whose coordinates are known and easy to look up.

2. Center the star in the finderscope.

3. Look through the main telescope and see if the star is in the field.

If not, find it and center it.

4. Look up the coordinates of the star.

5. Rotate the circle until the proper coordinate lines up with the R.A. indicator. The R.A. setting circle should rotate

freely.

NOTE: Because the R.A. setting circle does NOT move as the telescope moves in R.A., the setting circle must be

aligned each time you want to use it to find an object. However, you do not need to use a star each time.

Instead, you can use the coordinates of the object you are currently observing.

Now my back garden is quite low down so seeing the horizon is not physically possible, I'm also new and star names are only coming along very slowly.

From what I can read into this I swing the scope around to something as far away from Polaris as possible. In my case with my available FOV in the garden and start knowledge Betelgeuse or Sirius (later in the evening when it's high enough) would be two of my chosen candidates.

Then I lookup the coordinates of the selected object and change the RA setting circle (the one in hours bottom left of picture using top scale for northern hemisphere) to the correct coordinates for the object which should then have set everything up.

Now firstly when I either look at Stellarium or SkyMap Free on my Windows 8 Phone I have all sorts of coordinates as listed below.

Times Based on now when writing this at 15:53 UK time 30 Jan 2014

Stellarium

RA/DE (j2000) 5h55m10.3s/+7°24'25.6"

RA/DE (of date) 5h55m56s/+7°24'31"

Hour angle/DE: 18h25m22s+7°24'31" (geometric)

Hour angle/DE: 18h25m35s+7°28'57" (apparent)

Plus a few others which I don't think are relevant.

SkyMap Free

RA/Dec 88.79°/7.41°

So first question is which one and which part do I use?

Now according to the note you have to set it every time you use it, I would have thought that if you are setting up in the same place it would be correct each time, or is this because the coordinates alter as we rotate around our solar system? If so then it would be a case of slightly adjusting them each viewing session.

I've not tried to move the RA dial yet as until now I thought it was fixed and you had to release something first, but thought it time to investigate.

Hopefully I won't get told to forget Circles after all this typing and looking up numbers!

[ardbeg74]

Hehe, i guess that video was the astronomy and nature TV one. TBH after watching that i never went anywhere near the setting circles

As to the coordinates in stellarium, just pick one, they are all close enough cos the scales arnt accurate to that level of precision.

There is a reasonable explanation of how to use the setting circles here: - bout half way down the page

http://www.skyatnightmagazine.com/feature/how-guide/how-tomaster-setting-circles

I might give it a go myself and see, but i don't expect it to be any better than a chart and finder scope. And the latter doesn't require calculating h m s offsets in my head

Cheers

Mark

[ardbeg74]

Sorry Langy, just noticed that you have 2 sets of different coordinates.  Use this one : RA/DE (of date) 5h55m56s/+7°24'31". Though there isnt much in it between that and the 2000 figures.

Hour angle im not sure about, looks its alt-az related as it changes over time in stellarium. If you go int configuration / information you can turn all those off and just have the ra/de of date showing. Less clutter on the screen

Cheers

Mark

[Paul M]

Small setting circles on small mounts really are only useful for getting you in the ball park. Probably ok for getting you pointed towards the right direction for a planet or something that you can see naked eye once you get there.

But for deep sky stuff you're probably better off learning to star hop with a good atlas, because that is what you'll end up doing. Tiny setting circles are barely useful for degrees let alone minutes and seconds.

Star hopping is fun and a great way to learn the sky too. Small setting circles are nothing but frustration!

I'm not entirely sure why manufacturer fit them on mounts that are attractive to beginners. Partly to make the mount look "technical" I think.

[LouisJB]

those settings circles on eq1 or eq2 are a bit unreliable, I think they rely on grease or something to stay when slewing and move when using the fine-tuning adjusters.

I sort of gave up, although the theory is simple enough. Dec is fixed, RA depends on time. Calibrate by centring on a known object and set the RA circle to indicate that. 

Now slew to the new target until the RA and DEC read what stellerium/star atlas say it should be - bingo it should be there, although in practice the circle might have slipped or move by the time you've done this, so it's going to be a bit hit and miss at best.

[Langy]

Thanks for the replies. I have the wonderful RDF on the scope which I can get within a few degrees of an object I'm looking for, so is rather hit and miss.

When I git home tonight I tried to turn the RA dial and it is extremely stiff, so I don't think that it will be slipping about too much.

If the RA setting is time based then I guess you get to an object first and set it, then used the dials to find your next object. If you vies for too long then you need to recheck the coordinates of the object you are viewing and then adjust the RA again before heading to the next object.

I guess the SkyMap coordinates are just the degrees version of the normal Dec / RA. Which means that I could use that and convert them easily into hours based on the degrees of the clock. I usually always have my phone with me so it would be easier to look for coordinates on there.

RA/Dec 88.79°/7.41

[Mr Q]

   Setting circles ARE very useful in locating objects providing:

   1. The scope's mount is properly aligned to the celestial north pole.

   2. The wheels are big enough for accuracy (ease of readings)

   Unfortunately, most "setting circles" are just not big enough for such accuracy. But even then, they may be of some use to get you to the general area the object lies in.

[Langy]

That's what I was planning to do Mr Q. The RA is very very stiff on my mount so may have to work on freeing it out a bit before I attempt to use them.

[Mr Q]

   Are the circles mounted in rubber? If so, try some silicone lubricant spray to free them up without damaging the rubber or markings on the wheels.

[Langy]

I don't know, but the plastic covering to protect them was still in place when I got the telescope. That plastic goes right under the main frame to the center.

[acey]

If the RA setting is time based then I guess you get to an object first and set it, then used the dials to find your next object. If you vies for too long then you need to recheck the coordinates of the object you are viewing and then adjust the RA again before heading to the next object.

RA and Dec are not time based - both are effectively fixed co-ordinates on the celestial sphere. But the celestial sphere itself effectively rotates around the Earth every 23 hours and 56 minutes. Any star moves across the sky in an arc of a circle, and the circle is fixed - the star always rises to the same maximum height above the horizon, always reaching that point when it lies due south, i.e. is on the merdian. But this occurs at a different time each day, because the star "goes around the Earth" 4 minutes faster (on average) than the Sun "goes round the Earth", and it's the Sun, not the stars, that defines the length of our day.

This means there is a "star time" (sidereal time) in addition to the familiar solar time. When a star is on the meridian its RA is equal to the sidereal time. At any other time there is a non-zero angle between the star and the meridian - its "hour angle" - which changes constantly. This is what's "time based". Sirius has RA 06 hours 45 minutes, so whenever Sirius is due south, the sidereal time is 06 hours 45 minutes. If the sidereal time is anything else, then Sirius will not be due south. So when you use setting circles, you need to calibrate the RA dial by pointing the telescope at a star (e.g. Sirius) and turning the dial to the RA for that star. And you need to keep re-doing this calibration, because the hour angle (not the RA) keeps changing.

It's a fun exercise and a good way to learn about celestial co-ordinates. But for actually finding stuff on the sky, I prefer to use a star map (or planisphere), get my scope pointing at a naked-eye star I can identify, then star-hop to my target.

The little setting circles on commercial scopes are basically toys. The far larger setting circles on classical observatory instruments enabled astronomers to measure positions with arcminute accuracy, which is how they were able to make star maps.

[acey]

RA/DE (j2000) 5h55m10.3s/+7°24'25.6"

RA/DE (of date) 5h55m56s/+7°24'31"

Hour angle/DE: 18h25m22s+7°24'31" (geometric)

Hour angle/DE: 18h25m35s+7°28'57" (apparent)

Plus a few others which I don't think are relevant.

SkyMap Free

RA/Dec 88.79°/7.41°

A follow-up to explain the above data...

I said that RA and Dec are "fixed" co-ordinates on the celestial sphere. This is not strictly true, because the Earth's axis wobbles over time ("precession") meaning that RA and Dec co-ords also change slowly over time. So you should really specify them according to some particular time in Earth's history. Because the change is small, it's usually enough to do this to the nearest half-century, so we generally use co-ordinates for 2000, and will continue to do so until about 2025, when 2050 co-ords will be the more accurate. This is why the first set of co-ords specify "j2000" (j is for "Julian date" - never mind), and the second say "of date". Note that the difference is tiny.

The hour angle is specified as "geometric" or "apparent". This is because atmospheric refraction shifts the apparent position of a star by a small amount (like water in a glass making a drinking straw look bent). Again the effect is very small.

Finally, RA is usually given in hours and minutes, but it refers to points on a circle (it's like longitude on Earth's surface) so it can equally be expressed as an angle. This is how it's given in the SkyMap co-ords - though these differ markedly from the Stellarium ones, suggesting an error somewhere.

[Langy]

Thanks acey, that clears a lot of things up.

I take it that the of date is the supposingly accurate measurement of the date / time when the coordinates are being viewed.

Is there a reason why there is the difference between the 5h55 and 18h25 or is this that error you finally mentioned?

[acey]

Yes, "date" here means the date that you put into Stellarium. The hour angle (18h25) is different from the RA (5h55) because the RA is fixed but the hour angle changes, and depends on the particular date and time (and location) that have been put in. You would aim your telescope at the target and set the RA circle to 5h55 to calibrate it. The error I referred to was actually an error on my part, since I glanced too quickly at the SkyMap co-ords and read the Dec as 7deg41, which would disagree with the Stellarium figures 7deg24 or 7deg28. But SkyMap says 7.41 deg, i.e. just under 7 and a half, so the figures look consistent.

Some confusion can arise since RA is measured in hours and minutes. Remember that it actually refers to an angle (like longitude). We use that way of thinking in everyday speech if we say an object is at, say "3 o'clock", meaning it is on the right of a circular field of view. Sidereal time is also measured in hours and minutes, and when an object is on the meridian its RA is equal to the sidereal time (and its hour angle is zero). After 1 hour of sidereal time, the object will be one hour of RA past the meridian, it will still have the same RA co-ord, but its hour angle will be 1.

Sidereal time can be measured with a sidereal clock. Think of it as a clockface with numbers up to 24. It does a full revolution in 23hours and 56 minutes of our normal clock time (civil time), i.e. one hour of sidereal time is a little bit shorter than an hour of civil time. Professional observatories all had sidereal clocks. A sidereal wristwatch isn't practical, because whenever you change longitude you need to recalibrate your clock (just as we change our watches when we go across timezones). If you want to know the sidereal time I assume you can get it from Stellarium (I never use Stellarium so I don't know). There's an online sidereal clock here:

http://www.jgiesen.de/astro/astroJS/siderealClock/

When I want to know the sidereal time I just use a planisphere: set it to the date and time, look at the South point, and read it off. Not hugely accurate, but good enough for picking well placed targets ahead of an observing session.