Observing session 20/01/2006

[lunator]

Last night the clouds finally moved on.

The sky was crystal clear at 5pm and had the vivid blue colour that signifies a good night sky later.

I completed the neceessary domestic duties e.g. putting the children to bed and loading the dishwasher. By 8pm I was ready to go.

Tonight I had a very specific observing plan.

I have made a £5 micrometer and put an occulting bar across my 10mm plossl. I am still waiting for my crosshair eyepiece to arrive. (I will show you how I made this in another thread, it is pure blue peter )

I was curious to find out if you could get accurate measurements from such a device.

I had a specific target list

SHJ 40 AC

HJ 365AC

V (32) Auriga

I chose these as they had been measured recently and the primary are bright and very obvious.

I also wanted to observe some of my old favourites

Eta Cas

Zeta Per

Beta Ori

Zeta Ori

and split theta Auriga which despite what it says in Norton's Star Atlas (test for 100mm) can be a test for a 200mm in light polluted skies.

I started with Eta Cass as it was the first double I split and when it is near zenith I find the colour contrast is even greater. It was an easy split in the 10mm plossl and the colour was bright yellow and a deep red.

I moved down and looked at Zeta and beta Ori. Zeta was more difficult than usual to split.

I started my project I had not loggeed the PA of any of thes stars.

SHJ 40 AB (52 Tau) was last measured in 1999 PA 256 Sep 49.2.

I used the bar I had inserted to work out which way was West and turned the Dial to 270 degrees. I then moved the bar so it occulted both stars simultanously and moved the pointer to the position the pointer it stopped at 258 degrees. I then moved the Bar to a north/south orientation to time the difference in RA between the primary and secondary. This gave a mean time differnce of 3.5 seconds.

Using the formula P= 15.0411xtxcosd/sin theta, where P= seperation. t=time in seconds, d= stars declination and theta= position angle.

My results were PA 258

p= 15.0411x3.5xcos27.2103 /sine 258 = 48"

I was quite pleased with these results as a 1st effort. The PA is a couple of degrees off, but the Sep is within 1.5".

I followed the same exercise for HJ 365AC and V (32) Auriga. My results as a comparison to the last data are as follows (WDS data in brackets)

HJ 365 AC PA 193 Sep 54.3 (PA 194 Sep 58.7)

V (32) Auriga PA 208 Sep54.7 (PA 206 Sep 55.9)

From this a concluded that it is possible to get accurate measurements. My accuracy will improve as I get used to the process of aligning the outside dial/pointer with the internal bar. I will also have to improve the Polar alignment of the scope to ensure that the dial is more accurately calibrated.

All in all I was pleased with the results.

I also split theta Aur. and looking at star magnitudes I calculated that the faintest stars I could see were Mag 12+ which from my site is London is a rare sight.

Cheers

Ian

[daz]

Sounds like an ingenious device Ian, and one that works well!

Just goes to show that very often, we don't need to spend fortunes to do astronomy!!

Will the project appear in the Workbench?

[GazOC]

I tried a few doubles last night but for the life of me I could'nt split Beta Orionis even at x250, I did leave it a bit late on in the night when Orion was pretty low down. I had a couple of "maybe" moments but nothing I can defineatly claim as a split.

[lunator]

Gaz

Rigel can be tricky if it is too low down.

The big prblem is that the primary gets smeared out and covers the secondary.

If you are using a Newt the secondary is in the 11 O'clock position and is pretty much the same colour as the primary.

Cheers

Ian

[The Warthog]

So, Ian, you had a good night!

What does it take to split Castor? I tried last night, but couldn't. I am reasonably sure I was looking at the right star. I should be able to split it with a 4.5" scope, right?

[lunator]

WH

Whem I 1st starting observing double stars I had a 4.5" scope and had some trouble splitting it.

I found that it took about x150+ and on some nights x200.

Once you have split it, it becomes easier.

Collimation is importnat on Castor as I found that the secondary was right on a difraction spike so merged into the primary if collimation was off.

I hope this helps

Cheers

Ian

[The Warthog]

Helps very much, thanks. My collimation is right on at the moment. I have a one-legged spider, so diffraction spikes don't appear to be much of a problem, but I am going to make an apodizing mask anyway, once I awake from my winter torpor.

Thanks!