The Ring Nebula M57 in a wide field (19/4/2023)

A week ago on the 19th April we had a clear night and so I set up my Celestron NexStar 102 SLT on a HEQ-5 mount. The main purpose was for me to try do some photometry of Melotte 111 (also known as the Coma Star Cluster) but towards the end of the evening I pointed the scope at a couple of other easy targets and one of these was the Ring Nebula in Lyra (which was rising in the north east). This telescope is not ideal for taking images of this planetary nebula because it is relatively small in the 2.07x1.38 degree field but nevertheless I thought I would have a go. The moon on this night was one day before new and astronomical twilight ended at 21:20 UT (22:20 BST).

 

 

M57 is the tiny blueish ellipse near the center of the field (you can click on this image to see it full size). M57 sits between two prominent stars in Lyra - beta and gamma. Beta Lyrae or Sheliak is the brightest star on this image and is to the upper right. North is up, East is to the left. The two stars of similar brightness at the bottom right are nu 1 and 2. I used Astrometry.net to plate solve this image.

This image consists of 10x30s frames at ISO 3200 taken on a Nikon D90 at prime focus (the first frame was taken at 22:42 UT and the last at 22:49). I processed these images in DeepSkyStacker (4.2.6) just using the light frames and some flat frames. The resulting image has been binned x2 and processed in Photoshop. The picture below is a crop of the original unbinned image:-

 

This shows the nebula more clearly and I liked the nice blue colour with a hint of red around the edges. The shell of gas is emitted by a dying star which isn't detected here but can be seen on deeper images. I have imaged this nebula before in 2018 using my 5.5" Orion OMC-140 Maksutov Cassegrain and I reproduced this image here below:-

Here you can see the central star as this is a deeper image (8 minutes on a 5.5 inch as opposed to 5 minutes on a 4 inch) however I think the colour on my lastest image is more accurate.

All text and images © Duncan Hale-Sutton 2023

Venus, Mercury and the Pleiades (12th April 2023)

This month has seen our two inferior planets well placed in the evening sky. Mercury reached greatest elongation east on April 11th and Venus is still climbing away from the sun to reach the same point on June 4th. Elongation is a measure of the angular distance of a planet from the sun. Evening apparitions of Mercury and Venus are always better in spring because the ecliptic is at a steeper angle to the horizon than at other times of the year.

I took this picture eleven days ago on the 12th April (20:05 UT) when we had a brief spell of clear weather at the end of the day. Venus can be seen in the upper left of the image with the distinctive 'Seven Sisters' stars of the Pleiades next to it. Venus at this time was shining at magnitude -4.1 and had a 74% gibbous phase. The planet is so bright that it is causing a bit of flaring in my camera lens. Mercury is low down and just above the horizon at the bottom right of the picture. It was magnitude -0.2 and so was quite a bit fainter.

I also took a more detailed picture (at 20:03 UT) of Venus and the Pleiades together:-

More details about the equipment and the exposures can be found by 'clicking' on either image.

All text and images © Duncan Hale-Sutton 2023

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