Astrophotography – equipment

Solar Image processing. Myths and Legends.

I have just read an (unattributed) article in April’s Sky at Night magazine on solar imaging, and I have to say, from my own experience I disagree with a lot of it!

It says

“— requires a monochrome high frame rate camera set-up” and “use of a colour camera is inefficient”

Who cares? There is plenty of light from the Sun, efficiency isn’t a problem!

It also says that Ha features change quite quickly. True. As do the atmospheric “cells” that cause image wobble. It suggests you take a 1000-1500 frame avi. The time this takes immediately cancels any advantage there might be from a high frame rate. When you stack all these, you get a blurred image. The only reason you would take so many frames is to reduce noise. Again there is plenty of light from the Sun, so this isn’t an issue.

It suggests that you might need a flat frame (possible) and that you take a defocussed 500-1000 frame avi to achieve this. Why? It is much easier and more accurate if you need a flat frame to simply blur an image you have already acquired.

My images use a £50 colour camera with a not particularly high frame rate. I find a good compromise is 200 frames. This takes around 7 seconds.

Click on “Solar” on the blog and judge for yourselves!

While still in Victor Meldrew  mode, in the same magazine there is a review of a new Skywatcher 20” goto dob for £5499. I am sure that this is a splendid scope, but following my earlier post it is worth remembering that it is only 1 stop faster than Rob’s new 14”! I am pretty sure Rob didn’t spend that amount on it! In fact, in the review there are pictures of M42 and the Trapezium. There is also a picture of M51 of recent discussion. They look nice, but I would invite you to compare the pics with these window-sill images with a scope costing £100 ish.

Moral – Just because something is in print does not necessarily mean it is correct. This is a hobby, it is whatever floats your boat. You can spend a fortune if that is what you want to do, but you don’t HAVE to!

Some Musings on Aperture and DSOs

Rob’s recent post on his new scope led me to reflect (!) on my own experiences. My main interest is deep-sky objects, or “faint fuzzies”. I have a rule of thumb that says that to see any appreciable difference in object  brightness, you have to go up 2 stops in focal ratio (f-numbers). This translates to a doubling in aperture (doubling the aperture increases the light grasp by 4X). So when I upgraded from my old 4”, I went to 8”. Doing the same again would suggest a 16” would be needed. For me, a 16” would be unmanageable, so I went to a 12”. This is only 1 stop advantage to the 8”, and visual results were a bit disappointing. This more-or-less coincided with getting the PD camera, and even using this “live” as an electronic eyepiece gave views so superior to the 12” that the 12” virtually never got used.

So I wondered why this was, so here is a bit of basic physics.

Firstly, the eyeball. This iris opens to about 7mm when fully adapted to the dark. Or maybe 5mm in an old fogey like me. Given the size of the eyeball this is around f/3. See

To get the brightest image from a scope, the magnification from the scope must produce an exit   pupil (the beam width leaving the eyepiece) pupil less than 5-7 mm. Otherwise some of the light leaving the scope doesn’t enter the eye and is wasted. This defines the minimum magnification you can use for a given aperture. For example with the 8” and a 5mm exit pupil, this minimum magnification is 200/5 = X40. (The SCT has a focal length of 2000mm, so this is an eyepiece of 50mm FL). A lower magnification than this is not harmful, it is just that you are then not making use of all the available aperture. 7X50 binoculars are known as “night glasses” for exactly this reason – their exit pupil is 7mm, making the best use of all night-time light. Therefore, going up in aperture does not necessarily make the view brighter, but rather allows you to us a higher magnification for the same brightness. This might be a huge advantage for small objects like planetary nebulae, but less so for extended objects. Bigger aperture also improves resolution, allowing you to split closer binaries, but this is usually not the critical issue for faint fuzzies. The other issues affecting brightness are the eye sensitivity (more of this later) and its “integration time”, or the time period over which the eye sums the image it sees. This is its “shutter speed” and there is some literature that suggests in the dark, this is about 0.2 seconds. Again. see

Now, from here, I have NEVER conclusively seen any galactic spiral arms visually, although sometimes I have persuaded myself I can. So when I first coupled up the PD camera to the 8”, and turned it on M51, this is what I saw, live, with no processing at all:

Bingo! Spiral arms!

So if we are now talking about imaging, rather than the eyeball, what comes into play for image brightness?

3 things.

  • Focal ratio (not aperture)
  • Integration time. The PD single image is 1/50 second. Its “senseup” parameter allows it to internally stack up to 1024 single images, giving an integration time of about 20 seconds, or about 100 times the eyeball.
  • If the CCD had the same sensitivity as the eye, the brightness of the CCD image would be the same as the eye if a senseup of 4 were used. An experiment is called for!. I attached a lens to the camera, set it to around f/3 (similar to the eye), then in a darkened room compared the image from it to that I could see with my eye for various senseups. Although this is a very crude experiment, I reckoned that about senseup=2 was about right-not miles away from the predicted value. So a senseup of 1024 suggests a sensitivity about 8 stops faster than the eyeball.

There is also another factor involved, and that is contrast. This is the brightness of the faint fuzzy compared to the sky “background”, and I have found that with our local skies, that is the controlling factor. You can improve this visually using filters (UHC or OIII for example). These tend to dim the whole view, but the right one can improve contrast. Light pollution filters used to be good in the days of low-pressure sodium street lighting but are not much use with LED lights. On the other hand, the camera has a “gamma” setting that allows you to “stretch” the contrast, live, or if you post-process, the sky is the limit, as they say! For example, stacked 11 of the basic frames of M51 (11264 frames in total), processed it with GIMP, and here is the result. It is flipped vertically to get the orientation right (See for the details). Stacking 11 frames with a senseup of 1024 gives another 3 ½ stops faster than the eyeball or 11 or so altogether.

Interesting as all this might be, let’s remember that this is a hobby – and you do whatever you enjoy!

As a final thought, Lord Rosse used a 72″ aperture reflector to first identify the spiral nature of M51!

PD camera musings

At the last meeting, I had several discussions with people as to whether you could see galaxy spiral arms “live” with the PD camera.

Here is a raw frame of M51 I did 3 years ago:

Although not that prominent, they are there. This was with a “Senseup” of 1024, that equates to a stack of 1024 1/50th second frames, giving a 20 second exposure. So it is “live” and refreshes every 20 seconds.

Below is a stack of 11 of these frames , processed in GIMP

See for info. on how this was done.

It also seems you can’t get the PD camera any more. PD’s mods to the camera seem quite simple, so I did a search for the Huviron camera upon which the PD is based.

Here it is:

It is discontinued, unfortunately.

I had a quick search on other Huviron cameras that might use the Sony Superhad II chip and a senseup of 1024, but couldn’t find anything appropriate


Warm Astronomy – first attempt.

So, the clocks have gone back, the dark evenings are here and (when the clouds and rain leave us alone) there’s some great stargazing to be done. Within reason I’m quite happy to get my thermals on and put up with a bit of cold to enjoy the show, but I also like to share (inflict?) my hobby with my nearest and dearest and my chances of getting them outside in this weather for any length of time are pretty slim. Over the summer I’ve been good friends with eBay and have picked up some bits and pieces to enable me to bring the hobby indoors.

The kit I’m using is: HEQ5 Mount, Skywatcher 130P-DS, ZWO ASI224 Camera (with an LP filter) and a laptop running Cartes du Ciel (I know lots of club members like Stellarium, but my geriatric laptop doesn’t!) to control the mount, SharpCap to run the camera and TeamViewer to control the laptop remotely (I tried using Remote Desktop, but Windows wasn’t having any of it).

First chance to use it came up on Sunday night- full moon or not! Altogether it took about half an hour to set up (hopefully this will drop with practice). Pictures below show the setup (complete with frost) and then images of what we could see in the session. Whilst we were running I saved the captures and later stacked them along with a dark stack (not sure I’ve got this bit right) and did a histogram stretch- these are shown alongside.
Altogether, it worked well- both being able to easily show images and the novelty of pointing at something on screen and then images of it appearing a minute or two later. Some friends popped over and they were quite taken with being able to all see it at the same time and discuss rather than taking turns at the eyepiece and being unsure of what they were seeing.

The brighter objects were certainly better- the targets we looked at were:
Albireo – right image is 15 x 10s exposure.
M57 Ring Nebula – right image is 15 x 30s exposure.
M27 Dumbbell – right image is 30 x 30s exposure.
M15 – Globular – right image is 35 x 30s exposure.

Gain was set to 300 throughout. I also tried the Double-double, which became the Single-single and M81 which just came out as a blob- I think this and the Dumbbell might work out better with a bit less moonlight. Overall it was a really successful evening- the setup time is a bit of a pain compared with the 5 mins it takes to set the Dob up (and that includes making a cup of tea!), and it lacks the magic of finding it yourself and seeing with your own eyes. But for sharing with others it’s brilliant, and later on I even managed to get my month-end books done with the scope still on which made that task less of a drag than usual!

Oh- and thanks for the earlier post in the blog about using old storage boxes to protect your laptop/shield the light from its screen- worked a treat!


Observing Log Friday 27/10/2017 7-9:30 pm

The forecast was correct, clear skies, a chance to used the skywatcher ST102 bought earlier in year and only used for solar work so far. ( see pic.1)

I started under the carport ,as moon was not visible from back garden, not quite first quarter, used it to complete lining up red dot finder, took some getting use to smaller image after the 8″ Newtonian or the 9.25″ SCT. When at IAS I bought a smartphone adapter to take afocal images using the wifes’ new smartphone, now was on opportunity to try it out, pic.2 shows adapter and phone, pic.3  image of moon, notice the chromatic aberration, however visually it was not noticeable. The image was taken with a 30mm plossl eyepiece with this 500mm focal length refractor this gives a mag of x17. The crater marked with a red dot in the centre is Ptolemaeus, at slightly higher magnifications the centre of crater appeared to have horizontal bands across it, is this an artefact, blemish on objective/ diagonal?? at a mag of x83 (6mm plossl) all was revealed there were shadow bands from the peaks on the Eastern crater wall, the wall reaches heights of 3000m (9000+ ft) and with the sun relatively low on the moons horizon the peaks cast some long shadows, it was fascinating watching the shadows shorten even over a relatively short period of time , Liz had taken her phone back, so I have attempted a sketch of the shadows cast over the crater floor ( see pic.4),  the floor is relatively smooth, having been flooded with lava, some very minor impact craters formed since, the darker shading on the west is due to floor slumping towards crater wall. This was the first time I have seen such marked shadows on a crater floor formed by the crater walls, shadows from central  peaks are usually observed and just blanket shadow from the wall, the continual changing of relative positions of sun and moon makes the terminator a dynamic visual environment, there is always something new to see, even in one evening.

I then relocated to the back garden, starting in the SW with Albireo in Cygnus, the 10mm plossl  ( x50) clearly showed B1 cyg ( Alberio) as a orange red K class star and B2 cyg B class blue star. Taking a line down to zeta Aq from Albireo, bisecting the line from Vega to Altair, just slightly left the Coat hanger asterism fell neatly into the field of view using a 40mm plossl ( x12.5) , normally I would use binoculars for this target, but the wider field of view afforded by this small refractor enables it to be seen in its entirety. Up into Lyra,aiming between Sulafat and Shellak to locate the Ring Nebula ( M57), fuzzy ring but no hint of central white dwarf in this planetary nebula. Continuing west into Hercules M13 and then up to M92, even with 6mm plossl ( x 83) not a lot of detail. Better with the double cluster in Perseus and as I headed to M31, Andromeda galaxy the cloud had rolled in bring the session to a close at around 9:30.

It was nice to get out with some clear skies and I found the AZ3 mount that came with the ST102 easy to use and manoeuvre and although the refractor shows some chromatic abberation as shown by the photograph of moon , visually it was not noticeable enough to be a problem.

here’s to more clear skies!!!

Pete H

Scale for focussing tube

Hi, it’s useful to know the focussing position especially when imaging and using different combinations of filters/ lenses with the camera, although my sky watcher 120 Evostar has a graduated Crayford focuser, the newly acquired ST102 doesn’t, so rather than replace the rack & pinion focuser with a calibrated Crayford I managed to source some transparent adhesive scale tape, which I have attached to barrel, after cleaning the area with some isopropyl alcohol, its still on but have yet to put it to use. The tape was relatively cheap, and I do have an excess of repeating 40cm lengths, so if any one fancies adding a scale to their focussing tube get in contact.


Pete Hill

Solar – Mikrokular with or without barlow?

One of the reasons I like the Mikrokular is that it is full HD. It should therefore stand more magnification before being limited by pixels (the PD and Toucam are 640X480). So as an experiment, I took the last solar image I posted, cropped out the “no barlow” part including the prominences, desaturated it, re-sized it by 200%, “unsharp masked” it slightly, and set it alongside the X2 barlow version for comparison.

Although I think the barlow version is better, there is not much in it! (Just look at the prominences detail, not the disc)