You can buy pre-made solar filters, but they are expensive and relatively hard to find. You can also buy solar film and then mount it onto your camera lens or telescope yourself, but it is relatively fragile. If it gets a scratch, you could potentially blind yourself. Crinkled foil could also spoil the image quality.

I wanted a solution that was effective, durable and wouldn’t break the bank. Fortunately, one of my friends works in a chemistry lab and has access to a really cool machine that can coat glass filters with various kinds of metal. (This is the same chap who has the frickin’ laser that I wrote about not long ago).

Choosing a donor filter

I needed a 72mm filter for the telephoto lens I planned to use. I bought two cheap UV filters, although plain glass blanks would have been equally good.

What’s important is that the glass can be removed from the frame. Cheap filters often have the glass glued into the frame. Semi-decent ones have the glass dropped into the outer ring, and then the inner retaining ring is screwed in to hold it. Usually these screw-mounted filters have a screw thread on each side of the filter so you can stack two or more on the same lens.

The filter in this picture is the wrong type, where the glass is glued into the metal frame.

Glued filter

And here’s a close up of the kind with a screw-in retaining ring inside. This makes disassembly really easy with a pair of screwdrivers (be careful!).

Filter with retaining ring

I bought two UV filters made by Neewer, who are a relatively well-respected budget filter maker. Ordering from China, I paid less than £2 for each filter.  It was then easy to remove the glass from the frame.

Coating

In order to achieve an optical density of approximately 5, you need a total metal coating thickness of around 80nm. Trouble is, these metal coatings are very easily scratched – a fingernail would easily remove them. So I decided to coat each of the two filters with 40nm of metal and assemble the filter with the two metal coatings sandwiched together between the two pieces of glass.

The coating process is interesting. It’s done by boiling your chosen metal using an electric filament until it becomes a gas. At this stage, it will float around and stick to everything. However, you have to do this in a sealed bell jar that contains a vacuum.

First I cleaned the filters using a jet of compressed air, followed by a wiping with butanol and acetone. Any dust or grease on the filters will prevent the coating from sticking properly.

Then with the clean filters placed on the coating platform, I loaded a filament made of tungsten, wound with nichrome wire, and placed the bell jar over the whole lot. The small lump to the upper-right of the filters is a thickness sensor.

With the equipment set up, I started pumping the air out of the chamber. There’s a conventional rotary pump that can get the pressure down to a thousandth of an atmosphere, but after that you have to use a diffusion pump. For the coating to work properly, you need a pressure of just 10^-6 millibars – that’s a billionth of an atmosphere. This takes about 20 minutes.

When the pressure is low enough, the chamber is sealed, and we can turn the filament on to gently begin heating the nichrome. You can’t heat it too fast, or it will all melt and drip off the filament and be wasted.

When the nichrome starts to evaporate, we crank up the current in the filament to some 50 amps. Now the whole filament acts like a light bulb, and is too bright to look at. No photos of the filament in this state, I’m afraid. I was too busy shielding my eyes.

There’s an instrument on the desk that’s hooked up to the thickness sensor I mentioned earlier. This tells me how thick the coating is, in nanometres. Here the readout is showing almost 25nm, over halfway to my 40nm target. I didn’t time how long it took to build up 40nm but it was something like 20-30 minutes.

After a while we can once again look at the bell jar, since the inside of it is partially coated with metal and quite dim. You can see how the filters now appear to be mirrors.

When the thickness meter reckons we’ve got the right amount of metal on the glass surface, we turn off the filament and gently let the air back into the bell jar. Cautiously we remove the filters from the platform using gloves (because they are hot, and because we don’t want to get dust on them).

Assembly

Now it’s just a simple matter of placing the two filters with their coated faces together, and screwing them back into the metal filter mount. Try to avoid the two filters sliding or rotating against each other. If you have dust between them, it could easily cause a scratch. Scratching one of the coated surfaces isn’t a huge problem, but if you scratch both faces then you’ll get a pinhole which allows unfiltered sunlight through, and will do a pretty good job of zapping you in the eye.

The coating is not exposed to the outside world, so this filter is no more fragile than a normal filter. The outer glass surfaces are just regular UV filters and can be cleaned in the usual way.

I found that with two pieces of glass together in one filter frame, the thin retaining ring didn’t seem that secure, so I simply used the spare frame to hold the glass in place. This means my filter is twice as thick as a normal filter, but that’s no problem.

The finished filter

And here is the filter in situ, mounted on my Tokina RMC 400mm f/5.6.

The filter in place

Testing

Unfortunately as it’s winter, it hasn’t yet been daylight during any of my free time, so I haven’t been able to use this filter with the sun. However, by comparing photos of a lightbulb in my house with and without the filter, I was able to estimate the optical density.

Without the filter, I shot the lightbulb at ISO1600, f/5.6 and a shutter speed of 1/4000s. To achieve the same exposure with the filter (keeping the ISO and aperture the same) needed about 120 seconds. That’s 200,000 times more exposure.

• Transmittance: 0.0005%
• Neutral density: ND 200,000
• Optical density: OD 5.3

Given that I was aiming for an OD of 5.0, I’m pleased with OD 5.3. If my measurements are correct, that will be totally safe for viewing by eye through the camera’s viewfinder. Anyone fancy a sweepstake?

Soon, I hope to be able to publish some pictures of the sun taken with this filter – obviously depending on there being sun during the day at the weekend while I’m free. Cross referencing my personal calendar with the forecast for Bristol, it might be a few weeks…

For now there are automatic redirects in place, and my RSS feed and all historic links will continue to work. But please do change your bookmarks, and if you followed both of my old blogs, you now only need to follow this one.

My camera really struggled in the dimly-lit church and most of the photos were blurry or grainy. This picture of my brother Edmund and his girlfriend Lara was the best of the bunch. They posed for a few pictures with the usual stiff smiles you expect in family photos, so I announced that I had finished, waited for them to relax and shot one more. It’s got a nice candid feel to it.

Edmund & Lara

The numbers challenge is simple. You put a film in your favourite camera, and take photos of numbers in order. On a 36-exposure film, you’d take pictures of the numbers 1-36 in order, as shown on your film counter.

Then, when you’ve finished the film, develop it and make a contact print of the whole lot, rather than scanning each frame individually. The final result is a unique piece of art.

I managed to squeeze an extra frame out of my film, and shot the numbers 0-36. But I could only fit 30 negatives on a sheet of 10×8″ photographic paper, so I had to make do with that. Some time soon I hope to get round to printing the whole series on a larger piece of paper.

Unfortunately the exposure is a bit “off” in some of these. Individually, they are not nearly as over- or under-exposed as they look. It’s just that some are below average and some are above average exposure, and this doesn’t turn out well in the printing process. If I scanned or printed them individually, they’d all come out fine

If you’re interested in the subject matter, here are the notes I’ve been keeping since September.

1. Road sign in BS1, Bristol
2. Lamppost
3. No 3A, Christmas Steps
4. No 4, Christmas Steps
5. No 5, Christmas Steps
6. Business plaque
7. Can of 7-up
8. No 8, Osborne Villas
9. No 9, Tottenham Place
10. Price tag in Wilkos
11. Graffiti on the wall
12. Multipack of Hula Hoops
13. Petrol price at Tesco
14. Computer screen
15. Coins
16. Multipack of Twix
17. Glowsticks
18. Car park space at Durdham Hall
19. No 19, Woodland Rd
20. 20mph speed limit
21. Birthday card
22. Number on whiteboard
23. Birthday candles
24. 24-hour sign at ASDA
25. 25% off poster
26. Steamy mirror
27. Chart show countdown
28. Calendar
29. Alarm clock LCD
30. 30mph speed limit

Lessons learnt

• Use fast film
• Choose high-contrast subjects (the steamy mirror at #26 didn’t work out at all)
• Be extremely careful with your exposure
• Use a macro lens, if you have one. Most numbers you find are quite small!

Clevedon Pier

As we returned to solid land, the people in the gatehouse looked surprised. They said they didn’t know we were out there and assuming the pier was empty, had closed it because it was too windy to walk safely.

Clevedon Pier

Fast forward to February 2011, and the RB67 had another outing, this time for a walk on Troopers Hill.

Troopers Hill steps

Troopers Hill chimney

Troopers Hill chimney

River

Troopers Hill Road

Given Playboy‘s reputation, the magazines are surprisingly non-raunchy. However, the adverts were interesting. They were almost all for tobacco and spirits, but there were also adverts for men’s clothes, shoes, cars, technology and executive toys. I scanned my favourites, and here they are.

I’ve also promised to send these to Chris Wild at Retronaut, so I hope you’ll see them there soon. If you don’t follow Retronaut already, I strongly recommend you follow them on Facebook or Twitter. There’s always something interesting to read!

With time, I’m sure she’ll grow to trust us. In the meantime, here’s a few pictures.

Capturing the best source image

It’s important to capture the best raw image data that we can, which will make the whole process easier. In this guide, I am assuming that you have a DSLR with a removable lens. Most of the advice also applies to film SLRs and digital compact cameras though, so don’t worry if that’s what you have.

Equipment

The lens has a lot more to do with the picture than the camera. For shooting the Moon, I recommend you use a telescope or a telephoto lens. The longer the focal length, the better. For most people, this means using a 70-300mm zoom lens, although if you have a 500mm camera lens, or a telescope which can be anywhere from 700mm up to 2000mm, then you can use that. On most crop-sensor DSLRs, a focal length of about 1400mm will make the Moon fill the viewfinder.

Tokina 400mm f/5.6 on Canon 450D

If you want to get a bit more reach, you could use a teleconverter. This fits in between your lens and your camera, and effectively multiplies the focal length of your lens – at the cost of losing some of the light and sacrificing some sharpness and quality. Common teleconverter sizes are 1.4×, 2× and 3×.

Kenko Teleplus MC7 2x Teleconverter

At these long focal lengths, the tiniest vibration will make your picture blurry. A tripod is absolutely essential – the sturdier the better.

Always use a cable remote to trigger the shutter without touching the camera. All SLRs support these, but most compacts probably won’t. If you can’t use a cable remote, the self-timer is your second choice. Set the timer, carefully press the button, and hope that the vibrations have died down by the time the photo is taken!

Canon Remote Switch RS60-E3

If your camera has mirror lock-up, you should always use it. This means the first time you press the button, the mirror flips up and the viewfinder blacks out. Then you wait a few seconds for the vibrations to die down, and then press the button again to fire the shutter. Mirror movements are a common source of vibrations and are probably the biggest cause of blurry Moon photos.

Technique

Forget autofocus. If your camera has a manual focus option, use it. It will be more accurate, faster, and will prevent your camera from re-focusing on each shot you take. If your SLR offers live view, use that and magnify the view if possible. Once you’ve set the focus, leave it alone

The Moon is bright (brighter than you think) and it is set against a dark sky. This really confuses the camera’s auto exposure, so it’s best to use full manual mode (usually marked M on your camera dial). You’ll need to tinker with the settings, but if you set the ISO to 200 then some reasonable starting settings for the full Moon might be a shutter speed of 1/250 and an aperture of f/11. Take a few snapshots until you get an exposure that looks about right. The most common mistake is to overexpose the Moon. We often think of it as being white, but it should be grey in the camera.

Keep your shutter speed fast. If you let it get too long, you will start to get motion blur. Shoot at least as fast as 1/250.

Choose a middling value for your aperture. Most telephoto lenses have a maximum aperture of around f/5.6 at full zoom. Usually this gives poor image quality, so it helps to stop down a few stops. Usually f/8 or f/11 is OK. Much smaller than that, and you start to lose sharpness again due to diffraction. If you’re not sure where your lens’s sharpest aperture is, check some reviews. Failing that, a good rule of thumb is the the sharpest aperture is 2-3 stops down from the widest.

Feel free to set the ISO as high as you like. You may be aware that higher ISOs cause more noise in the picture – especially in low light conditions. This is absolutely true, but in this case it doesn’t matter. We will discuss effective noise reduction techniques for astrophotography later in this guide. If choosing ISO 1600 enables you to keep a fast shutter speed and to use the sharpest aperture, so be it. Don’t be disheartened by the grainy pictures that come out – these are not the end product.

For reasons that will become clear in a minute, once you’ve found the ideal focus and exposure, you’ll need to take a few near-identical pictures. The Moon will naturally drift across the viewfinder – this is fine. You might want to place the Moon in one corner of the viewfinder and repeatedly take photos until it reaches the other side. Anywhere between 3 and 10 pictures is fine – just don’t forget that when shooting repeatedly, you still need to give vibrations time to die down after each mirror lock-up.

This next picture shows how fast the Moon moves across the sky. These exposures were taken just three minutes apart each, using a 300mm lens. Even leaving time to fiddle with the mirror lock-up between each exposure, you ought to be able to shoot at least one picture a minute, which will give you quite a few pictures at the end of the session.

Moon moving across the sky

Post-processing

Now that you’ve taken a handful of source images, we need to work on them to bring the best out.

Stacking

Stacking means taking a set of similar images, shifting and rotating them so they line up, and adding them together. This has the effect of averaging out noise from your camera, and distortions from atmospheric turbulence. The best free piece of software for Windows is called RegiStax. Those using Linux might want to consider ALE.

I wrote about ALE on this very blog not so long ago, but if you need a helping hand with RegiStax then I recommend you read this RegiStax tutorial. For the mostpart, you just follow through the steps it gives you – but there are a lot of scary options.

No matter which program you decide to use, after stacking, you will end up with a single image file which will look like a slightly improved version of a single frame. Now we move on to post-process this image in a more conventional photo editor. If you have Adobe Photoshop and you are familiar with it, then use that. I prefer to use GIMP which is similar to Photoshop but also free. It runs on Windows, Mac or Linux.

Colour channels

If you are shooting the Moon, it is effectively black & white, so we can do a trick with colour channels to improve sharpness at the cost of converting the image to actual black & white. (This doesn’t work if you want to end up with a colour photograph, by the way. If you’re shooting colour images of planets or similar, skip this step). Open your stacked image in GIMP.

Go to the Colours menu, Components submenu and choose Decompose. Make sure colour model is set to RGB, uncheck “Decompose to layers” and press OK.

This will split your colour image into its red, green and blue components, each of which opens as a new monochrome image. Now you can close the original image to save confusion.

Examine the three monochrome images you’ve got. They should be similar, but subtly different. Which one is sharpest depends on how much light pollution there is in your area, what colour it is, how well your lens/telescope performs at different colours and a million other factors. Zoom into each picture at 100% (do this by pressing 1) and have a look at the craters for comparison. When you’ve chosen the sharpest image, close the other two.

Unsharp mask

One of the best techniques for sharpening a slightly blurry picture is to use an unsharp mask. I won’t go into the theory here, but the basis of a blur is that a tiny dot becomes a small circle. Unsharp masking studies the image, and tries to convert the small circles of blur back into dots.

For this to work, we need to estimate the radius of the blur. Zoom into your image as far as possible (1600%) so you can clearly see the individual pixels as squares. Find an area of high contrast – either the edge of the Moon, or the edge of a crater. The edge of a crater or the lit edge of the Moon should be a sharp, defined line, but you’ll see that it is actually a gradual change, a few pixels wide. Count the number of pixels that it takes to cross the boundary.

In this example the middle red line (roughly) shows where the true edge of the Moon is. The outer two lines approximately show where the blur extends to. The distance between the two outer lines is roughly 5 pixels in this example. Work out the equivalent number for your image, and remember it.

Go to the Filters menu, Enhance submenu, and choose Unsharp Mask. Set Radius to the number you found in the previous step. Amount is set to 0.5 by default but you can change this if you wish. Numbers between 0.5 and 1.0 seem to work best.

Scroll around in the preview window to look at interesting parts of the image. Repeatedly tick and untick the Preview box so you can see what effect the unsharp mask will have. When you’re happy, press OK.

Colour curves

The name is a bit misleading – colour curves don’t have anything to do with colour in this context. They are a good way of enhancing contrast, though. Bring up the curves window by going to the Colours menu and clicking on Curves. The default “curve” is actually a diagonal line.

Arrange the curves window and your image alongside each other so you can see both at the same time. Drag the shape of the curve into a gentle S-shape. The exact shape and amount of the curve depends on your needs, but have a look at the next two screenshots to see what effect the curve has had. Click on the images to view them larger and use the arrow keys to go back and forth. You can immediately see that the dark patches are darker and the pale patches are paler.

Saving

That’s it! You’ve now finished all the basic editing in this tutorial. Save your image, but make sure you do Save As and choose a different name, so you don’t overwrite your original.

Summary

This guide touches upon a few of the most common techniques in astrophotography. It is by no means the ultimate guide. If you’ve got any questions, extra tips or if you spot any mistakes in this guide, please comment and let me know. Also, I encourage you to post your Moon photos at the bottom of this page, share your work and show off what you’ve done.

Further reading

On this blog

• Lenses for astrophotography – a comparison
• Astrophotography and stacking software
• Long telephoto lenses
• Mirror lenses: worth it?

Elsewhere

• RegiStax tutorial
• How to Photograph the Moon and Planets with Your Digital Camera
• Digital SLR Astrophotography