My Astronomy




My Telescopes

My Main Telescope - C14 and Paramount ME

My new Paramount MyT and 8-inch Ritchey-Chretien Telescope

MyT Hand Controller

My Meade 12 inch SCT on a CGEM (Classic) Mount

My 4 inch Meade Refractor with Sky Watcher Guidescope and ZWO camera on a CGEM (Classic) Mount

Skywatcher Star Adventurer Mount with Canon 40D


My Solar setup using a DSLR and Mylar Filter on my ETX90

DSLR attached to ETX90. LiveView image of 2015 partial eclipse on Canon 40D

Astronomy Blog Index
About the Site

 I try to log my observing and related activities in a regular blog - sometimes there will be a delay but I usually catch up. An index of all my blogs is on the main menu at the top of the page with daily, weekly or monthly views. My Twitter feed is below. I am also interested in photograping wildlife when I can and there is a menu option above to look at some of my images. I try to keep the news feeds from relevant astronomical sources up to date and you will need to scroll down to find these.

The Celestron 14 is mounted on a Paramount ME that I have been using for about 10 years now - you can see that it is mounted on a tripod so is a portable set up. I still manage to transport it on my own and set it all up even though I have just turned 70! It will run for hours centering galaxies in the 12 minute field even when tripod mounted.


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« Day 184 Monday 29th September 2014 QX Cas is suspected of ceasing its variability - I investigate by measuring its current magnitude. | Main | Comet Siding Spring, Photometry references and NGC 7790 images for determining Transforms. »

Day 183 Sunday 28th September 2014. Using NGC 7790 images to find Transform values for my photometry setup

Still bad weather - cloudy - no observing today.

The negative V image of NGC 7790 (taken last Tuesday on my C14) uploaded to VPhot is shown below

There is a standard chart for this field

These standard V stars are superimposed on the VPhot negative image

Getting a photometric solution with no target will list all of the stars circled in terms of their instrumental V magnitudes. The red magnitude values can be ignored. The signal to noise ratio is significant. The Air Mass indicates how high the object is in the sky - the lower the better.




 Without going into the details the same process is used for the B image producing a table of B instrumental magnitudes for the same stars.

I used Excel to produce graphs as follows based on the data from the B and V lists of instrumental magnitudes.I did not use all of the stars and excluded those with lower signal to noise ratios.

 I produced the graph on the left (above) from the instrumental magnitudes determined from my B and V images and the standard B and V magnitudes for the stars. Plotting b-v against B-V shows the difference between standard values and values that my equipment will produce, allowing a "fiddle factor", sorry Transformation Coefficient to be introduced in future photometric measurements, The graph on the right is produced automatically by a downloadable AAVSO programme based on my results. The Transform Coefficient Tbv which is the inverse slope of the graph is 1.067.


 These charts show how the measured b and B values differ depending upon the colour of the star - the slope of the graph providing another correcting factor - thus one is called Tb and has a value of 0.099 for my equipment. If there was no change with colour then the line would be horizontal.

Similarly for Tv giving a value of 0.025 for my photometric arrangement.