An attempt to obtain a photometric magnitude of Z Ursae Majoris

On the 2nd April I took an extra frame of Z Ursae Majoris with the intention of attempting some photometry with it. This was the frame:-

 

 

This is a single 30s exposure at ISO 800 (taken at 20:44 UT). It looks a bit odd because I have deliberately prevented the brighter stars on it (including Z - marked) from saturating. The stars are also defocused (perhaps by slightly too much) as this will help the photometric measurements. This is actually a binned (x4) jpeg image shown here, but for my measurements I used an uncompressed 16 bit tif.

So how can we go about measuring the magnitude of Z UMa? On the image I have marked three other comparison stars; 84, X and Y. 84 is the star that is referred to on the AAVSO chart X28625AEL (see my previous post). Stars X and Y (my designation) will be referred to below. The method will be to measure the brightness of each of these comparison stars and compare it to the brightness of Z in the green G channel. The green channel is used as this better matches the visual response of our eyes.

Beginning with Z I used Photoshop to open the tif file and place a circular selection around the star of diameter 70 pixels. This was big enough to encompass all the light from the star but not so big as to start adding more light from the background. Using the histogram tool I used the green channel and found that the mean count per pixel was 92.02 over 3852 pixels which gives a total count of 92.02 x 3852 = 354461.04.

I then placed a larger circular selection of 120 pixels diameter centred on the star and measured the green colour count in that. The mean count per pixel in this case was 47.33 over 11304 pixels giving a total count of 47.33 x 11304 = 535018.32. I made sure (by temporarily stretching the image) that this measurement wasn't affected by any hot pixels or background stars. This total count includes light from Z and from the sky background. So the sky background surrounding the star is 535018.32 - 354461.04 = 180557.28. This is spread over an area of pi x (120/2)^2 - pi x (70/2)^2 = pi x 2375 = 7461 pixels. Therefore the mean count per pixel for the sky surrounding Z is 180557.28 / 7461 = 24.20. If we want to obtain the count per pixel that is just due to the star we have to subtract this from what we recorded, namely 92.02 - 24.20 = 67.82 per pixel. 

This is the first main measurement. We now repeat this process for the comparison stars. I used exactly the same circular selections of 70 and 120 pixels for each measurement.

Beginning with star 84. This star has another designation HIP58303. This is a Hipparcos catalogue number. According to the AAVSO they assign this star as having a V magnitude of 8.438. I measured the sky background around this star to be 22.45 counts per pixel and the sky subtracted brightness for the star to be 23.27. Unfortunately, the brightness of the star at the position of 84 on the frame will not be quite the same as the brightness of the star at the position of Z due to vignetting (and for other reasons, such as atmospheric extinction). You can see this to a certain extent by the fact that the sky at the position of 84 is fainter than at the position of Z (22.45 vs 24.20, respectively).

We need to make a correction for this. Below is an image which shows the vignetting of this telescope/camera combination:-

You can see that the image is darker at the edges of the frame. To find how to correct the brightnesses, I put a square selection of 100x100 pixels centred at the position of Z and at the position of star 84. The counts per pixel were, respectively, 61.80 and 56.84. This means that the corrected count per pixel for star 84 should be (61.80/56.84) x 23.27 = 25.30.

Now we estimate the magnitude of Z. The difference in magnitudes between Z and star 84 is given by

m (84) -  m (Z) = 2.5 log10 (brightness of star Z / brightness of star 84)

which implies 

8.438 - m (Z) = 2.5 log10 (67.82/25.30)

and m (Z) = 7.37 (to 2 d.p.).

Now we can make similar calculations for the other comparison stars X and Y.

Star X is HIP57820 (also known as Aniara) and this has a V magnitude of 7.858. The sky brightness at the position of this star is 20.46 per pixel and the sky subtracted count for this star is 36.78. Correcting for vignetting this is 42.01. From this I obtain a magnitude of 7.34 for Z.

Star Y is HIP58302 and has a V magnitude of 8.375. The sky brightness at the position of this star is 19.51 per pixel and the sky subtracted count for this star is 24.99. Correcting for vignetting this is 28.73. From this I obtain a magnitude of 7.44 for Z.

So in all we have three estimates for Z of 7.37, 7.34 and 7.44 which gives a mean of 7.38 +/- 0.05. I think these values are pretty consistent considering I am not using software to produce the results. The problem is that I know that Z had a visual magnitude of about 7 on this date (2nd April 2023) and so my value is too faint.

The problem that needs to be addressed, I think, is that the green G channel of my camera (a Nikon D90) is going to respond differently to the eye (and to Johnson V) and this needs to be corrected for. I need to do this calibration and I need to adopt some photometric software to carry out the processing.

All text and images © Duncan Hale-Sutton 2023

M51 The Whirlpool Galaxy in Canes Venatici (2nd April 2023)

Whilst I had my Celestron 102 SLT set up the other week I thought I would quickly have a go at M51. It wasn't the best conditions with the moon being up but I thought I would see what I could get.

I was quite pleased with the result (you can "click" on the image to see a full-size version). This a combination of 10x30s exposures at ISO 1600. This is a rotated and cropped version of the full image and I have displayed it this way as I wanted to compare it to another similar image that I took in 2016:-

This was also 5 minutes in duration using the same telescope but on Celestron's alt-az mount rather than the HEQ-5. I think my new image is better in that it is much sharper and shows considerably more detail in the spiral arms of the galaxy. I reckon this is due to the better mount and better focusing. 

The only thing that I am not happy about is that in my new image the colour of the galaxy is a bit orangey brown when the spiral arms should be blue/white. However, considering that I took this in moonlight I really think there is a vast improvement (even the background noise seems less). I don't think there is a colour cast that has caused the orange colour as the background is pretty neutral (i.e. close to grey/black). In the older photo there is more of a strong green cast in the background.

All text and images © Duncan Hale-Sutton 2023

Z Ursae Majoris near maximum (2nd April 2023)

The semi-regular variable Z Ursae Majoris is currently near its maximum brightness and a week ago we had a clear evening when I thought I could try and get an image of this star with my Celestron 102 SLT. Unfortunately, the moon was in the sky and four days from full but I thought it was worth doing. I mounted the telescope on the HEQ-5 and was able to get this set up reasonably quickly. It involves pointing the axis of the drive at the pole and then aligning on a couple of stars but this went smoothly. 

The above image (taken on a Nikon D90) combines 9x30s frames (at ISO 1600) using DeepSkyStacker. I have rotated the image so that north is at the top and east is to the left. I have also binned the image x2 and cropped it so it more or less matches the field in the AAVSO chart X28625AEL. This chart is appropriate for use with a telescope rather than the larger field chart for this star produced by the BAA (and useful for when observing with binoculars). Z Uma is the bright orange star marked in the centre of the field. I have also marked two other stars which are included on the chart as photometry reference stars. These are star 84 (which has a V magnitude of 8.438) and star 99 (which has a V magnitude of 9.898). Z appears much brighter than these two stars but this is in part due to the red colour of this variable star. At this time Z had a visual magnitude of about 7.

There is a good correlation between the stars you can see in this image and those in the chart. The chart has stars plotted down to magnitude 14.5 and so the limiting magnitude of this image must be about the same. If you look closely there are a few faint nebulous patches in the image which are probably faint galaxies.

All text and images © Duncan Hale-Sutton 2023

Z and RY Ursae Majoris and TX and AH Draconis 27th March 2023

It has been a while but at the end of March we had a clear evening when I could get back to do some variable star observing. I haven't looked at these stars since the end of last year and so I was keen to get some new estimates. Unfortunately, the moon was in the sky and it was 2 days before first quarter. Also, the evenings are getting lighter and astronomical twilight didn't end until about 20:20 UT (21:20 BST).

I began with my current favourite Z UMa. Using chart 217.02 from the BAA and my Nikon 7x50 binoculars, I was able to see star H on the chart near to Z and this was a good sign as it meant my limiting magnitude was fainter than H which is 8.7. I could see that Z was brighter than star D (=7.9) and C(=7.5) and at 20:19 UT I estimated that Z was between stars A (=6.3) and B (=7.3). I found that Z was 2 points from A and 1 point from B, namely A(2)V(1)B which made it magnitude 7.0 (to 1d.p.). This is in good agreement with estimates from other BAA observers.

So this indicates that this star is approaching its peak luminosity. I have written a thread about this star on the BAA Forum. What makes this star interesting at the moment is that the recent rises in luminosity have been double peaked and it may be this is due to the two pulsation periods that contribute to the brightness changes of this star. In my latest post I considered whether this was a reasonable explanation.

Using the same chart I then went on to look at RY. I saw that this variable was fainter than comparison star 1 (=6.7). In fact, I thought it was roughly equal in brightness at 20:33 UT to star 2 which is magnitude 7.4. Other observers in the BAA had this star at magnitude 7.6 and 7.7 on this night, so perhaps I was slightly on the bright side with my recording.

I then went onto chart 106.4 to have a look at TX Draconis. This star was lower in the sky than the previous two and rising in the East. At 21:34 UT TX was fainter than star K (=7.0) but marginally brighter than star N (=7.7). I estimated it to be 3 points from K and 1 from N, that is K(3)V(1)N. This made magnitude 7.5 (to 1d.p.). I think this estimate was fine.

Finally, from the same chart I had a look at another semi-regular variable AH Dra. At 21:44 I noted that this star was fainter than star 1 (=7.0) on the chart. At 21:49 I thought it was between star 2 (=7.3) and star 6 (=7.8) in brightness, namely 2(1)V(1)6, which made it magnitude 7.6 (to 1 d.p.). Other observations of this star are a bit sparse at the moment so I am not sure if I was on the ball with this one.

All text and images © Duncan Hale-Sutton 2023

M42 and the Running Man Nebula

On the same evening that I was taking more pictures of the comet C/2022 E3 I decided to have a quick look at M42 in Orion as it was nicely placed. As I was using my Celestron 102SLT I wanted to do the wide shot which included the Running Man Nebula which lies to the north of M42. I thought I had captured this image since I came down to Norfolk but it seems that I haven't and the last time I attempted this was in 2006 when I was using a 200mm lens in Yorkshire. This time I managed 20x30s at ISO3200 and then I combined them in DeepSkyStacker using auto adaptive weighted average, binned the final image x2 and modified it using levels in Photoshop. This was the result (you can click on any of the images to see an enlarged version):-

I was pleased with the result. The stars have come out quite sharp and there is plenty of detail in the nebulae. M42 is the pink fan shaped nebula to the right with its four bright stars of the Trapezium in its centre. Below and very close to M42 is another nebula designated by Messier and this is M43. It is seen as an almost circular nebula, with a central star, cut off from M42 by a dark lane of dust and gas. The Running Man Nebula, as it is called, is to the left of the image and appears as a faint blueish nebula with dark lanes running through it. I have reproduced a larger version of it here where I have labelled the contributing parts to it:-

With this orientation the dark lanes that lie in this reflection nebula do really look like a man with his legs splayed as if running.

To compare these images with my previous attempts here is an image that I took from 2006 with a 200mm focal length lens:-

All text and images © Duncan Hale-Sutton 2023

Comet C/2022 E3 (ZTF) begins to fade

Last Wednesday evening (8th February 2023) we had some more clear weather before the waning gibbous Moon rose at 19:58 UT. The seasons are moving on now and astronomical twilight lasts until nearly 19:00 UT. Comet C/2022 E3 is now beginning to fade and is moving fast across the sky. On the 8th it was in the constellation of Auriga not far from iota Aurigae and the border with Perseus and Taurus. COBS indicates that it was about 6th magnitude. I again set up my Celestron 102SLT on the HEQ-5 mount but I found setting up was more complicated this time because I lost power to the drive momentarily whilst doing the sky alignment. Anyway, I managed 26x30s frames of the comet (ISO 3200) before I accidentally nudged the tripod. Here is one such frame taken at 19:00 UT (binned x2 and modified in Photoshop using levels):-

I then combined all 26 frames in DeepSkyStacker (comet stacking, star trails, auto adaptive weighted average). The resulting frame was then modified in Photoshop using curves and levels, binned x2 and then cropped slightly. This was the result:-

The green colour of the comet's coma is nicely seen. The dust tail is obvious but the ion tail was too faint to be seen here. The first frame was taken at 18:51 UT and the last at 19:09. I think the flattening of the image to remove vignetting of the telescope went slightly better this time as I took some flats about sunset. Basically these are out of focus shots of clear patches of blue sky.

All text and images © Duncan Hale-Sutton 2023

Further pictures of Comet C/2022 E3 (ZTF)

Just over a week ago on a Sunday evening (the 29th January) we had some good clear weather but with a (one day past) first quarter moon to contend with. At this time Comet C/2022 E3 was approaching its nearest distance to us and its brightest magnitude (about magnitude 4.5). Since then it has begun to fade and its distance from us is increasing.

I decided to set up my Celestron NexStar 102 SLT on my borrowed HEQ-5 mount to observe it. The setting up all went much better than on previous occasions and I successfully managed to align the polar scope on the pole (it is an equatorial mount). Part of my decision to do this was to see if I could extend exposure times beyond 30s but more of that later. The NexStar has a reasonably wide field of view which makes it ideal to capture the tail of the comet. I began by taking 30s frames of the comet with my D90 camera attached at the prime focus. Here is one such frame:-

I was pleased by how sharp the stars are in this image. The comet at this time was in the constellation of Camelopardalis and just over the border from Draco. It was at a very high declination of nearly +80 degrees and so not far from the pole. This image is about 2.05x1.37 degrees. North is towards the top left-hand corner and East to the top right-hand. It was surprising how fast the comet is moving across the sky during the 13 minutes I was observing it.

 

 

The above picture is combination of the first and last frames that I took over this period. The first was taken at 21:28 UT and the last at 21:41. The comet has moved by about 55 pixels during this time and, at approximately 3.44 arc seconds per pixel, this corresponds to 3.2 arc minutes. So it was moving at 0.24 arc minutes per minute or 14.6 arc minutes per hour.

In all I took 19x30s frames (just short of ten minutes, ISO 1600) and combined them in DeepSkyStacker:-

The background stars have disappeared because I have centred the stacking process on the comet and used the median rather than the average. This has helped bring out details in the tail. All the images I have shown are binned x2 and have been modified in Photoshop to bring out the fainter detail. In this image above it is possible to see two tails. The dust tail causes the wedge of brightness that emanates from the comet at the 7 o'clock position. The ion tail can be seen as a thin line that emanates from the comet nucleus at the 3 o'clock position. Closer inspection (click on the image) reveals that this ion tail extends right to the edge of the frame and splits into three parts.

Finally, I did try to get more photographs of the comet a few days later on the 31st January using the same setup. This time I extended the exposure time to a minute with the use of a digital remote control for the camera. This worked well enough but by then the moon was waxing gibbous and the faint detail from the comet was missing. This is a one minute exposure (ISO 800) I took at 20:14 UT:-

The comet was still in the constellation of Camelopardalis at this time having crossed a bit of Draco again.

All text and images © Duncan Hale-Sutton 2023