Double stars - Mizar and Alcor (1st May 2024)

Having got my telescope set up correctly on this evening (see previous post) unfortunately the weather wasn't going to play ball and it was pretty hazy. So I decided to observe something where the weather didn't matter too much and these were double stars. The SynScan handset of the HEQ-5 has a number of double stars preprogrammed into it and so I began to take a tour of them and to photograph them as I went. The first was Mizar and Alcor. This is an extremely wide but very well-known pairing but the interesting part is that Mizar is itself a double which I had only realized recently. Here is my photograph:-

This was a 1s exposure at ISO 1600 on a Nikon D90 at the prime focus of a 140mm Maksutov Cassegrain (an Orion OMC-140). The picture can be better seen by 'clicking' on it. The image is being viewed at full scale (in other words it has not been binned up). Mizar A and B are marked and are magnitude 2.23 and 3.88 respectively. Off to the left is Alcor which is magnitude 3.99. The other star, Sidus Ludoviciana is magnitude 7.58.

To get an idea of the image scale here we can determine the angular separation of Mizar A and Alcor using their coordinates. Mizar A is at RA 13h 23m 55.543s Dec. +54 deg. 55' 31.30" and Alcor is RA 13h 25m 13.538s Dec. +54 deg. 59' 16.65". Using the tool here, the separation is found to be 0.1968 degrees. Their separation in pixels is 1209.25 which gives 0.5859 arcseconds per pixel. 

It should be noted that this image here has been cropped and is 1600x1066 pixels. The full image is 4288x2844 pixels and this corresponds to 0.698x0.463 degrees. To get the right orientation with North upwards and West to the right I used this article for reference (the article is worth a read in itself) where there is a diagram showing the relative positions of the stars relative to North and South. I then rotated my image so that the stars were orientated correctly.

For a double star, a couple of measurements are given. The first is their separation of their components in arcseconds and the second is their position angle. The latter gives the position of the fainter star relative to the brighter and is an angular measure from the north anticlockwise through the east. I measured the separation of Mizar A and B to be 25.08 pixels which corresponds to 14.7 arcseconds. I must be to within about a pixel so + or - 0.59 arcsecond. The measured value is 14.4 arcseconds which is fortuitously close. The measured postion angle is 153 degrees and so by my image I don't think I am too far off in my orientation.

As a footnote, if you read Bob King's article highlighted above, you will discover that all three stars Alcor, Mizar A and Mizar B are doubles in themselves making this a sextuple system and all bound together gravitationally (in the case of Mizar and Alcor, just!).

All text and images © Duncan Hale-Sutton 2024

Telescope set up on a HEQ-5 mount (1st May 2024)

On Wednesday this week I was anticipating that it might be clear in the evening and I thought it would be a good opportunity to try and set up my 140mm Maksutov Cassegrain correctly on my HEQ-5 mount. I now have three stone blocks set up in the garden to rest the feet of my tripod on. Recently, I have been having trouble with polar aligning the scope and so, idly searching the net for help, I came across this video which gave me some ideas about how I could do things better.

I will describe what I did so that it may be of use to others. Firstly, I placed the tripod on the stone blocks in roughly the right position for true north. Although the three blocks are roughly levelled between themselves I adjusted the tripod legs so that top of it, using a spirit level, was properly level. I marked the location of the legs on the stone blocks so that I could replace the tripod in exactly the same place on another night.

Next I attached the HEQ-5 head and then made sure the azimuth adjustment screws were screwed in equally on both sides, so that the head was centred. I added the counterweight, then the telescope, DSLR camera and red dot pointer. I had been using a 50mm finder with crosshairs but I found it too heavy and it was awkwardly messing up the balance of the scope. The red dot pointer is much lighter and smaller. I then balanced the telescope in RA and Dec by adjusting the position of the telescope in the dovetail slot and by moving the counterweight.

I then waited until the stars were visible, lined up on Arcturus and made sure that the red dot pointer and telescope were aligned. I did need to take the camera off briefly to do this and replace it with a star diagonal and eyepiece but once the camera was back on I could focus the camera by using liveview and seeing the star on the screen.

Now, something perhaps I have not been careful about before was putting the telescope in the 'park' position before switching on the drive. This means that the counterweight is at its lowest position and the scope is pointing roughly in the direction of the pole. This helps when carrying out the alignment. Next came entering the usual date, time and location information on the handset (sometimes I get the date format wrong which can lead to all sorts of trouble!). When this is done the handset shows the location of polaris in the polar finder. This is in the format of HH:MM. From what I understand this is a 12 hour pointing system with 12 pointing directly up, 6 pointing directly down and the other locations increasing clockwise as on a face of a clock).

If you look through the polar finder you will see a small circle marked where you have to place polaris (northern hemisphere only). Suppose the handset said a time of 03:00, then you would rotate the polar scope until that circle is at the 3 o'clock position. Now using the azimuth and altitude adjustments on the HEQ-5 head only, move the head until polaris is seen in that small circle. Don't move the feet of the tripod as you might mess up the levelling you did earlier.

You should be roughly polar aligned. Then carry out a 3 star alignment. Not a 1 star or 2 star but a 3 star. This is something else I have learned. Do a 3 star as it is much more likely that the telescope will more reliably point to an object. Another thing I have learned is that you can do this ok with the camera in position. Just use liveview to centre the stars each time. Then you can be sure that the telescope remains well balanced.

I found that carrying this all out was entirely sufficient for my needs. Each time I used the controller to send the telescope to a new object it was reliably in the centre of the field. The video goes on to show how you can improve the polar alignment using certain functions on the controller which I might try sometime.

Finally, when dismounting the telescope I made a note of where the telescope was in the dovetail connection and where the counterweight was on its arm. When releasing the head from the tripod I undid the azimuth screws equally by one full turn each. Also I left the adjusted feet of the tripod as they were. Hopefully, this means that on another night when I go to set up I can return the telescope to exactly the same place and level and have the axis of the mount pointing in the same direction. It should speed up the whole set up process.

All text and images © Duncan Hale-Sutton 2024