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Grant Privett

I've been doing astronomy for 50 years (eek), so I am probably not giving it up any time soon.

Astronomy has changed and over time my interests have evolved from planetary observing, into meteors, through variable stars and, finally, to the deep sky. These days I'm especially interested in imaging of various sorts, but find making pretty pictures increasingly dull. Its just not a challenge anymore - todays's kit pretty much does it for you. 

I generally use a 250mm f4.3 Newtonian on an EQ6 and Starlight SX694 camera from a site in Wiltshire near the Dorset border where the sky is mag 21. I also have sporadic access to a Celestron 11" RASA.

Fortunately, given the UK weather, I like programming and so write my own MS VB6 code to run a Starlight camera and interface it with TheSkyX software. Its a worthwhile skill for when I fully retire and get the chance to observe more - I want to make the most out of every night. I've also taught myself Python 3, which is useful but the language is a mess and was designed by an idiot. That said, so many people have written external libraries for it that is is now possible to do a lot with it if you work round the language's glaring faults.

My current software project is writing code to resample images of variable nebulae in such a way as to allow quantitative  comparisons to be made, while my observing interest is in seeing how far a 250mm can be pushed and, also, satellite imaging.

2019 Jan 11

22:52 UTC

The run of bad nights has sent me back to some old data to have some fun.

I dug out 3 sets of images I took with a 10" f4.3 Newtonian and a Starlight SX7. For 2 of the 3 sets I used a linear polariser - pretty low quality - and on the final set captured a  normal unfiltered image. I used the unfiltered image as the luminance and green band while the 0 degrees and 90 degrees filtered images were used to synthesise the red and blue. 

The effect is interesting, so that any colour you see in the frame isnt down to the wavelength of light, but the polarsiation state of light leaving the nebula. With a highly ionised gas and a magnetic field there was bound to be contrast. Theres other games you can play using the degree of linear polarisation to things clearer but this is a start.

2018 Nov 22

2018 Nov 18

14:26 UTC

As winter is clearly on the way I decided to have a go at some variable nebulae again.

Tried McNeil's nebula (any excuse to image M78) with 34x120s and 2x binning on a Trius 694 using a 0.279m f2.2 Celestron RASA. As has been said, therea bit there but its only just visible - I estimated it at 1.5% of sky count. I forgot to autoguide but still got a tolerable result. 

I also had a go at Hind's nebula (NGC1555) adjacent to T-Tauri and found it broadly similar to last spring.I was worried the sampling wouldnt be up to it but it looks good enough to me. I set up too late for a flat field unfortunately.

In addition I had a bash at Gyulbudaghian's nebula. That was looking particularly nice. It looks slightly more dramatic when it has the full fan shape, but this was pretty good and I especially liked the fine detail. Reminded me of V900 Mon - which I didnt get round to.

And yes, I do know some images need mirroring...

2018 Nov 1

20:37 UTC

The Pelican nebula captured with the Celestron 11" RASA. Love the detail in this object and wanted something to picture after an evening messing about with the CCD/corrector plate separation. About 18 years ago I imaged it with a different system - an 8" f4.3 Newtonian and a Texas 255 based CCD boasting a swanky 344x244 pixels. The field was a lot smaller, the noise much greater and it was much harder work to get something respectable.

And did I mention the Super Polaris mount, the peak QE of 0.35 and that it predated PHD?

Cue: Montry Pythons Yorkshiremen sketch... rolled up newspaper...tell the kids of today that...etc etc

2018 Sep 11

10:25 UTC

I've finally managed to unequivocally see the Zodiacal Light and photograph it. I've been to several quite dark places but usually theres been too much going on, or the weather has been bad, the Moon was up,  or I didnt have a camera. This time things worked out okay. I had a new Samyang 14mm f2.8 lens with me and a Canon 1100D to hang it on. I got the attached picture high up on the island of La Palma in the Canaries at new moon just before astronomical twilight ended . 

Once I was decently light adjusted, it really did look like there must be a town nearby - but all of those were below the cloud layer. 

In the dark clear skies of La Palma it was a lovely sight!

2018 Aug 18

15:19 UTC

Its often said that the Samyang 14mm f2.8 lens manual lens is a bargain for doing wide field images of starfields and landscapes. With the smaller frame size of a Canon 1100D or Canon 700D series cameras it gives a field 90 degrees across, while on a full frame Canon 5D it gives a whopping 110degree field of view. Its a very clever design of more than 10 lenses with multiple glass types, coatings and even aspheric lenses. Typically, they appear on the second hand market for a bit over £200 - the cost of lenses has risen significantly since the referendum.

The problem is that people tend to forget that these are mass production items - assembling that many components accurately and maintaining build quality is non-trivial. There will be poor batches, there will be poor random examples and the camera they sit  on may be duff too - the CCD being non-perpendicular to the optics by as little as 50microns can be enough to mess things up. 

So, when I bought one second hand  - with guarantee happily - I wasnt suprised that things were not quite right. I took a 10s exposure of the summer triangle and the surrounding sky and then looked closely at the shape of star images off the central axis.

The image attached shows the view at the left top corner of the image and at the top right of the image. The difference is very obvious. Pretty much the whole of the left side of the image was superb. Really good for the cost of the lens - the Canon equivalent costs 7x as much.

Suffice to say its going back to the supplier. Though I am looking for another as I borrowed a Samyang from Mark Radice a few months ago and that gave a great response across the whole field. 

Moral of story - don't believe in perfect manufacturing. When you get a new lens - test it immediately.

2018 Aug 17

23:18 UTC

After the M101 image I spent sometime wondering why the stars are so blobby. Especially as it wasnt a hideously turbulent night. Eventually, I realised the RASA really is sensitive to the CCD-Corrector separation and I only guessed it initially - being more concerned with the pointing accuracy.

I added a couple more spacer rings and the star images tightened up a lot. More work needed but gradually getting there. So to test it I took a few frames of the Bubble Nebula and M52. The moon was still up but that didt stop me seeing the tighter PSF.  Looks like the IR and visible is now nearly in focus at the same position.

2018 May 6

2018 May 4

19:40 UTC

Recently I've been using a 11" Celestron RASA which operates at f/2.2 - which means that objects that in the 90's and early 00's used to cause problems due to their size now fit comfortably in the frame - even when using an imager rather smaller than a full frame DSLR. An example was NGC4565. I recall this in the 90's being too big for the Texas 255 (3.4mmx2.4mm) CCD I used then. In fact it sprawled all the way across the diagonal and off the edges.  

Today things are rather different with the definition of "What is a large deep sky object" shifting somewhat.

So, attached is an image showing NGC4565. Its the full frame Trius 694 image. Created by summing 25x 60s frames. Should have dithered them to get the noise down a bit but the pic is still usable. Cannot imagine what it would be like on a Starlight Trius 35. Feel free to lend me one :)

2018 Mar 29

19:31 UTC

A while back I was at the 2017 solar eclipse in the US. I took some images at 800ISO, 1/40th sec with a humble Canon 1100D and combined them to get a luminance frame. I then looked at the images I had used a linear polarisation filter on and used those to code the R, G and B responses of an LRGB composite - one for each angle of filter rotation. The result will be shown here shortly.

The main difference between this and an image I put up soon after the eclipse is that in this one the images have been manual aligned. 

It aint perfect and I could have done better with a higher quality lens and camera, but as I didnt buy the lens until the day before we travelled, and it was held at 300mm zoom using gaffer tape, I don't feel too ashamed. 

2018 Mar 22

21:49 UTC

Managed to get outside again and get some imaging done. This time I was mainly testing some software changes.  I ran some code I had written, which chased down 10 or more geostationary satellites in 5 minutes -which was kind of fun. Pretty easy to do with TheSkyX and VB6 but I don't like TheSkyX controlling the camera so I let my own code handle all that. 

Anyway... went after NGC2437 - the cluster in Puppis - and the embedded NGC2438 planetary nebula. Was pleased to see I caught them quite well despite their low altitude (Declination -14degrees). It is a rather nice simple pairing. Had  expected the stars to be more bloated.

2018 Feb 24

21:21 UTC

Recently, I've been messing about with a Celestron RASA 11" and a Starlight H18 - its a nice combination. Mostly I look at nebulae, but I got curious about how clear the dark lanes would be in M31 and so took an image. The scope wasnt autoguided but I tried a 240s exposure while I was waiting for my chosen target to clear a tree.

What amused me wasnt the amount of detail in a single frame - quite respectable - but the fact that the galaxy core saturated. Really quite strongly. I had to back off to a 60s frame and even then it was borderline. Might have helped myself by not using the camera in 2x binned mode, but thats what I had a flat for :)

Worth remembering if you are considering one of these.

Example shown. 

I also slipped in 16 minutes worth of imaging of Hind's variable nebula. Alas, I was still on 2x2 binning, but it does at least show the broad features.


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