Sky Hunting with Small Refractors - March, 2010

[MistrBadgr]

The thing I wanted to do with this post is to talk about multiple stars, just to give the beginner a taste of what goes on with them.  In particular, there are a large number of multiple stars in Orion, which sits pretty much due South of where I live during early evenings in the first part of March.  Due to clouds, rain, and work things, I have not been able to spend as much time with them as I wanted.

I am taking the Double Star Club list of the Astronomy League and will use the ones listed there that are in the hour of 5 RA.  This includes all of the ones on the list that are in Orion, plus a few more.

The author of the list, the club chairman, states that he was able to find them all with a 60mm refractor.  He did have to travel to a dark site in order to see one of them.  Compared to many astronomers, including yours truly, he obviously really knows what he is doing. I hope his eyesight is better than mine, in terms of cataracts, and suspect that they are.  Additionally his refractor was most likely collimated very well, compared to my entry level scopes.  This gives me good excuses if I do not find some of them!

Even though I could not split some of the ones listed below, I have had a lot of fun with the challenge from my back yard. 

Members of the Astronomy League can find each one, log the appropriate information, turn a copy of the log sheets into the club chairman, and receive a pin for doing so.  For each object, one must long the name, date, time, instrument, seeing, power and draw a little picture of what the scene looks like.  There is also a space for comments about what you saw.  This activity can be done with either a manual or goto telescope.  The chairman does make one comment that people using a manual telescope normally have the directions correct, while people using goto telescopes generally do not.

On the picture, two directions must be shown. Like west and North, west and South, North and east, etc.  West is easy with a manual telescope.  West is the direction the stars move toward.  North is a quarter of a turn in a clockwise direction, for a refractor with a normal 90 degree diagonal.

On the list that can be downloaded from astroleague.org, you are provided with the Object Name, RA and declination for location, Magnitudes, the separation in arc seconds, and the position angle (in degrees). (North is zero)

In order to translate the position angle and separation into a spot around the primary star, you need to know one of the directions (I use west), is angle (270 degrees) and rotate in the appropriate direction until you get to the one listed for a secondary star. 

In order to know how far away the star will be from the primary, you have to know what your true field of view is.  The true field of view is the apparent field of view of your eyepiece divided by the power you are using…..or….AFOV times FL of the eyepiece divided by the FL of the telescope.  Once you know how wide the piece of sky you are looking at is, then you can make an estimate about how far apart the two stars should be.

I should do all this for each star….But, I don’t.  I think I may try to muster up a little self discipline, figure out what true field of view would be best before hand, and make a drawing of each item on the list beforehand, allowing for the right/left switch in the eyepiece.  Figuring out the various angles on the fly has not been too hard for me, as soon as I went to using manual mounts and not one of my goto scopes for this activity, as long as I am looking reasonably away from North.  If I am looking at something fairly well to the North, I get all crossed up and start pointing in this direction and that.  I end up almost cross-eyed, with my brain tied in a knot.

Here is the list with some comments about each one.

Beta Orionis:  This is the star Rigel, which is the western foot (my interpretation) of Orion.  Some people probably think of this star as the western knee.  Rigel A is magnitude 0.1 and Rigel B 6.8.  This difference provides a bit of a challenge.  I was able to pick up B from the coma it created.  I would have thought the extra light in the direction of B (to the lower left) was coma from A, if there had not been some spikes coming off B that were perpendicular to a line coming from A.

118 Tauri:  I have not looked for this one yet.  It is located at 05:29:00.3 RA and +25:09 North.  Looking at a star map, this puts it almost exactly between what I think of as the two horn tips of Tarus, Beta and Zeta Tauri.  The primary is magnitude 5.8 and the secondary 6.6.  The separation is 4.8 arcseconds at a postion angle (PA) of 204 degrees.  The magnitudes are almost the same, the separation is more than double what a 60mm refractor is supposed to be able to split (2 arc seconds).  This should be “easy pickings.”

The PA of 204 means its angular postion is a little west of South (180 degrees).  With an apparent field of view of 4.8 arcseconds…the best a 60mm refractor can do is 2 and a 70mm can do about 1.75…the two stars should be close together with maybe one star diameter between them,…... if all goes well.  It will take the highest magnification, most likely that you can do.  If you only have the 25 and 9mm eyepieces that came with your scope, you might want to call around and see if you can borrow a 2X Barlow from someone.

Delta Orionis:  This is Mintaka, the western end of Orion’s belt.  A is magnitude 2.2 and B is 6.3.  They are 52.6 arc seconds apart. The PA is 359 degrees or B is almost straight North of A.  Using a 9 mm eyepiece with an AFOV of 50 degrees and a 700 mm refractor, the field of view will be about 39 arc min wide.  The separation should be a little over two percent of the field. With good weather, etc. this should be fairly straight forward.

Struve 747:  This double is located at RA 05:35:00 and Dec -06:00.  This puts it in the Great Orion Nebula and a little bit South of the Trapezium, which is Theta 1 and 2 Orionis, listed below.  Magnitudes of 4.8 and 5.7 should make this readily visible.  Separation is 35.7 arc seconds.  Therefore, this will look a little tighter than Delta Orionis.  With a PA of 223, B should be NW of A.  I have not looked for this one yet.

Lamda Orionis:  Meissa is the main star in what I think of as Orion’s head.  There are several stars that make up what looks like the star forming a triangle with the two shoulder stars, when you look with your eyes.  Looking through your telescope, Lamda should be the brightest of that bunch. There are several stars in a line with Meissa being the brightest and only one more star above it at a little wider gap than between the ones below.  Magnitudes of A and B are 2.8 and 6.9 respectively.  The separation is 4.4 so this one will be pretty tight for little refractors.  With a PA of 43 degrees, B should be Northeast of A.

Theta 1 Orionis:  This is part of the trapezium. It is a multiple star that contains two of the main four suns of the trapezium and two others that may not be seen with an entry level scope.  (maybe you can, depending on your particular scope and your eyes)  If you do the drawing, just put down what you can see and not worry about any suns that you cannot.  I will let you figure out which part of the trapezium is Theta 1 and which is Theta two.

Iota Orionis:  This double star is the brightest in the group that appears to the bare eye as the bottom star in Orion’s sword with the Great Orion Nebula being the middle one.  With magnitudes of 2.8 and 6.9, you can definitely tell a difference between them.  With a separation of 11.3, you can see good space between them.  With an PA of 141 degrees, B is Southeast of A.

Theta 2 Orionis:  This is the other two of the main four trapezium stars.  It is a double star.  You should be able to find the trapezium

Sigma Orionis:  This one is located below and a little to the right of the left end of Orion’s belt.  This is a triple star and you can see all three.  The magnitudes are 4.0, 7.5, and 6.5.  PA’s of B and C are 84 and 61 degrees respectively.  Looking at it looks like a mother star sort of leading her two babies to the west with the dimmer one closer to mom but a little farther from being directly behind her.  As you can probably tell, this one was fun for me.

Zeta Orionis:  This is Alnitak.  It is the eastern end star of Orion’s belt.  Sigma is below it and to the right.  It is a triple star and really tough for my two refractors.  I was not able to split A and B.  C was to dim for my level of light pollution.  I will try again with a little work on the scopes to make collimation better and under darker skies.  Magnitudes are 1.9, 4.0, and 9.9.  Separations are 2.4” (near the limit of the little scopes), and 58.”  With a PA or 10 degrees, B is basically North of A and shoved up next to it.  C has a PA or 162 degrees which puts it mostly south and a little east of A and comparatively a long way away.  If C was visible to me, I did not identify it as part of the group.

Gamma Leporis:  I have not tried this one yet.  Lepus is the constellation just South of Orion.  In Lepus, Alpha and Beta, the two brightest stars, form a not quite North/South Line.  The top is more to the east and is Alpha.  Gama is the next major star to the east and a little South of Beta.  To me, Lepus with lines drawn on a chart, looks like a running figure of some sort with gamma being the rear knee.  This is a double star with magnitudes of 3.7 and 6.3.  The separation is 96 arc seconds or over a minute and a half.  With a PA of 350, B is almost due North of A.

Theta Aurigae: I have not gotten to this one either.  Aurigae is the constellation that touches Taurus on the North.  In fact, Beta Tauri, the Northern-most horn point of Tarus is the Southern point of the Aurigae constellation itself.  Theta Aurigae is the next major star to the Northwest of Beta Tauri.  Beta Aurigae is the next major star North of Theta with Beta and Theta forming the western-most line segment of the sort-of irregular hexagon that is Aurigae.

The magnitudes of the two components are 2.6 and 7.1.  The separation is 3.6 arc seconds, which will be very close together.  But, within the capacity of a 60 mm refractor which is supposed to be able to separate stars down to 2 arc seconds with both in the 5-6 magnitude range.  On my introductory scopes, Coma may confuse things and make this one tough.  With a PA of 313, B should be Northwest of A.  (remember west is the direction the stars go while you are looking at them through the eyepiece and North is in a clockwise direction.

Well, that should be enough double stars for now.  If this kind of activity interests you and you would like to know more about the Astronomy League, go to www.astroleague.org and check them out.  I am a member and pay dues with my local astronomy club dues.  My club is affiliated with the astronomy league and takes care of the details.  A person can be a member separate from a local astronomy club.  I am not sure what it costs.  I suspect it is in the $5 to $10 per year range.  With that, you will receive a quarterly publication and can participate in the various observing activities like the double star club.  The publication is not like normal astronomy magazines and is much thinner.  It has, however, proven interesting reading for the most part.

Good Luck and Clear Skies!

Bill Steen

[MistrBadgr]

After rereading my post, I found one item that the information could be taken different ways.  The double star list for the Astronomy League Double Star Club contains 125 items.  In order to get the pin, you have to do 100 of them.  If you do all 125, you get special recognition in some way.  For fear of error, I will not go into what that is.  Reading what I wrote after it cooled off a little, I thought someone might think you got a pin for just doing the double stars that I listed.

Thanks,

Bill Steen

[MistrBadgr]

Found another one!!!  Theta 1 Orionis IS the trapezium.  I had seen a diagram in the past that was either wrong or I remembered it wrong.  Theta 2 Orionis is supposed to be in the same viewing field.  I will have to do some research on this.  The Double Star list shows only two stars in Theta 2 Orionis at mag 5.2 and 6.5.  Other things I have read in the last few minutes from a google search indicates there are three stars.  I will check this out over the next few days are post what I find.  Anyone else that knows for sure...please post.

Thanks,

Bill Steen

[MistrBadgr]

Theta 2 Orionis is a triple star system.  It is located in the same field of view as the trapezium, just a little South and east of it.  The three stars form a slightly crooked line that runs basically east and west.  The A star is magnitude 5.2, B is 6.5, and C is 8.1.  A is on the west end, B is 52.5 arc seconds eastward at 92 degrees.  C is at 97 degrees, and 128.7 arc-seconds from A.  That puts C about 2 and a half times as far from A as B is.  The trio should look pretty much like a line of three stars heading west in order with the brightest going first, next brightest next, and the dimmest last.

I have not looked at them yet to see if I can see them and what they actually look like through a telescope.  I should in the next few days.

Sorry for the confusion.

Bill Steen

[Philip Pugh]

When I've finished Messiering around, I'll go back to double stars. They're good targets for simple photography and I have a few pics on my website.

Pride of place goes to Alpha Centauri, as you can't see it from England. I've snapped it from Brazil and New Zealand.

[MistrBadgr]

During this last week, I made a trip to another power plant in Southwestern Oklahoma, about 9 miles SE of down town Lawton, Oklahoma.  The skies are reasonably dark there, just not as dark as near Anadarko.  I did some more work with the double stars listed as well as do some work on my 70mm refractor.

While I was there, I did find all three of the open clusters in Aurigae, M-36, M-37, mad M-38.  Therefore, I know that they can be found with a small telescope.  It just depends on the amount of light pollution and quality of the atmosphere.  Additionally, the Beehive Custer, M-44 and Pleiades M-45 were in good form.

I re-visited Zeta Orionis, the eastern star of three that forms Orion’s Belt.  I could see what I thought was A and C.  C was, naturally very faint.  I simply could not find B.  I was, however, experiencing a lot of refraction spikes that I will discuss in another post, and B might have been hidden.  Later, I looked up Zeta Orionis in The Night Sky Observer’s Guide.  What I found there stated that the separation of A and B was 2.3 (not the 2.4 shown in the Astronomy League’s list) and that it was closing.  The Position Angle showed the same as the list.  But, it had a footnote stating that the position was from the 2000 epoch.  With the stars closing and ten years gone by, these two may have closed enough not to be inside the limits of small refractors. 

The books that list the limits for different sizes of telescopes put the limit on a 60mm refractor at 2.0 arc-seconds and an 80 mm at 1.5.  Since the numbers do not appear to be linear, I am guessing that a 70 mm is supposed to be good for 1.7 if everything is really set up right.  With introductory scopes being mass produced from less than optimum materials.  Most likely even those numbers cannot be attained.  At this point, I am thinking that splitting Zeta Orionis A and B is not realistic for 60 or 70 mm introductory telescopes.  Orionis C can be seen under reasonable conditions with care and patience.  It may wink in and out due to eye movement.

Back to double stars.  I also found Gamma Leporis and Theta Aurigae.  I looked for 118 Tauri and thought I found it.  Later, I began to doubt myself.  The separation just looked too large.  I will re-visit it later.  Theta 2 Orionis ended up being the string of three stars that you can see to the left and down a little in a refractor with a diagonal from the Trapezium.  You cannot really miss them if you can see the Great Orion Nebula.

One of the troubles I am having with double stars is getting a good feel on what different separations should look like.  Looking at the trapezium through my 70mm focal length refractor with a 32mm eyepiece, I can definitely see separation between all four major stars.  I could probably distinguish a gap a little smaller than the one between A and B, which is 8.8 arc-seconds.  I am guessing that my minimum would be 7.0 arc-seconds.  With a 9 mm eyepiece, like the little refractors come with, that number will shrink by a factor of 9 divided by 32 or about 2.5.  Therefore, these small refractors, as they come should not need a shorter focal length eyepiece than 9 in order to see most of the double stars on the list.  Quite possibly, this is the limit of the scopes anyway.  If you decide to do some double-star work, find a few easy targets and make a mental note of how far apart the stars look (center to center and not just the distance between the edges of the disks) with your different eyepieces and maybe make yourself some little drawings inside of circles to represent what that distance looks like in comparison to the total field of view.

Well, enough for now!

Hope you have clearer skies than I do right now!  It has been snowing.

Bill Steen

[Philip Pugh]

Bill, the separation limits that you refer to is known as the "Dawes Limit". In practice, most nights, you are lucky to get within double it. Also, you need high magnification as well, although I have managed to split Castor at 80x magnification. However, don't expect to do this every night.

[MistrBadgr]

Well, I am back in Southwestern Oklahoma, hoping to give you another episode of Star Hunting with small Refractors.  I have my 60mm with me.  I put a new focuser in it, to get away from the 0.965 eyepieces. (Sorry folks!  I got claustrophobic!)  It is all collimated and can get a good airy disk and even show a dim ring around it at high magnification.  Unfortunately, if there were any stars below the cloud deck, the forty mile per hour gusts blew them away.  The ducks and geese were sitting on the pond today with their storm anchors out!

I will keep an eye out for openings/calm periods and hope for the best!

Bill Steen