SkyInspector.co.uk

Astronomy and Sky Website of Martin Lewis

This page is currently being built and should be finished in the next few days (Martin 30-6-2022)

This page describes the methods I use to capture images of the International Space Station and other spacecraft which are shown on my related webpage here.

On this page here you’ll find techniques and background information that will be useful to those wanting to have a go at imaging details of spacecraft using their telescopes and employing the hand-tracking method. By necessity this procedure is biased towards the techniques and equipment I use, however, I hope there is enough here to enable readers to see what is important in the method and to be able to interpret it for their own set-up.

Step-by-Step Guide – Overview

Get pass details

Find out details of upcoming passes – dates, timings, path, max altitude

Prepare scope

Assemble, collimate and allow to cool

Optical set up

Insert Barlows, filter, camera etc into focuser. Connect laptop to camera and roughly focus on a star.

Scope Sweep Check

Check sweep of scope anticipating predicted path of ISS across sky and relocate/adjust scope if necessary.

Check Finder alignment

Check and adjust so that when the ISS is at the centre of the finder field of view it will also be in the middle of the camera sensor attached to the main scope.

Fine Focus

Use a star and critically adjust the focus so that the ISS will be as sharp as it can be when it appears

Camera Settings

Pick appropriate camera settings and await pass commencment

Record pass

Capture a video of the pass – following the ISS as it arcs across the sky using hand-guiding

Check Video

Check you have indeed captured something during the pass!

PIPP

Use PIPP to process video and only keep the frames where the ISS is present

Stack

Stack short sections of the video where you have several good frames and the ISS perspective is hardly changing. This will create one Master Image

Registax wavelets

Process stacaked imge in Registax to draw out details

Photoshop/PaintShop Pro

Tweak image to reduce noise and make image the best it can be xxxxxxxxxxxxxxxxxxxxxx

Step-by-Step Guide – Detailed Guide

Pass Geometry

Refer to the ISS pass information on prediction websites such as Heavens Above. This will give you information about upcoming passes from your location such as appearance time and time of maximum elevation above the horizon, as well as what that maximum elevation is, and its expected brightness.

Whichever website you look at, make sure it uses your correct latitude and longitude as this is used to calculate the geometry of the pass. Also realise that information more than a few days old may be inaccurate as the ISS is low enough to be affected by atmospheric drag and has to regularly boost itself to a higher altitude orbit.

Example of Heavens Above website ISS pass details

From the prediction program work out where in the sky the ISS will first appear, reach its maximum altitude, and disappear, relative to your observing location. You will obviously need to know where N,S E & W are from your location to do this – use Polaris here to aid you here.

ISS making its appearance in the west, rising vertically up from the horizon

If your telescope is portable, place it in a position which allows a clear view of the pass, especially of the part of the pass where it is above ~60° altitude. At this higher altitude the ISS is not only closer and so larger, it is less affected by atmospheric dispersion effects and the seeing is significantly better. Images taken when the ISS is below ~30° rarely show much detail but may be worthwhile if something usual is in progress such as a solar or lunar transit. From your intended imaging location make sure the path of the ISS won’t be blocked by a tree or roof when it will be best placed in the sky.

As the ISS climbs higher in the sky, its size changes, one’s viewing perspective changes, and the direction of solar illumination changes. All these have an effect on an imager’s ability to see structure on the craft. For evening passes the ISS generally appears as a faint star in the west, close to the horizon. It then climbs upwards, very slowly at first, brightening and increasing in speed as it gains altitude. Above about 60° altitude the ISS is moving at over 0.5°/sec and will usually be brighter than Jupiter. Often the view in the evening, taken a bit past it’s time of highest elevation, is the most interesting, as then the sun is more fully illuminating the underside of the craft. Then, however, the ISS is about to disappear into the earth’s shadow shortly after, so this optimal stage can be short.

ISS 20th Oct 2007
Changing perspective and illumination of ISS from an evening pass on 20-10-2007

If you are keen enough to image the ISS during a morning pass then things are a bit different. Morning imaging is harder as there is no slow build-up and it can all happen a bit too fast. The ISS still travels from West to East but it will appear out of the earth’s shadow travelling fast and often high up in the sky. It quickly reaches maximum elevation then starts to drop in altitude, slowing and fading as it nears the eastern horizon.

Setting up your Telescope

The assumption is made that the telescope is optimised for imaging in that it is properly collimated and has left to cool for enought time for tube currents to have died down. Covering these aspects is beyond the scope of this article – you can, however, find lots about these aspects of telescope optimisation on the internet.

Chasing the ISS by hand -guiding during the pass with a Dobsonian reflector

Optical Set-Up – Camera, Filter and Barlow

To capture detail in the ISS, the best method is to use a high speed digital video camera connected to a laptop rather than a DSLR, as you will get far more usable frames with the video camera and this then allows you to stack several frames to improve image quality.

High speed digital video cameras can be one-shot colour (OSC) or monochrome. For an OSC camera you will need a UV/IR filter although some come already fitted with such a filter. For ISS imaging, monochrome cameras are probably more popular than OSC cameras and are often used with red or wide-band IR filter which help improve image quality by effectively improving the seeing. You can read more about factors affecting camera and filter choice under Extras here.

The easiest way to image details on the ISS is to set the camera sensor at prime focus of the telescope with no Barlow and setting the sensor to capture full-frame. This set-up will give the widest field of view, and you will capture the greatest number of frames occupied by the ISS. This set-up is recommended when you are starting out, as manual tracking will be less critical.

As you get more practised at hand-guiding you can increase the magnification during imaging by introducing a 2x or 3x barlow before the camera (between camera and focusser), but the magnification you pick will always be a compromise between resolution, noise and motion smearing. You can read more about the compromise between too high a magnification and too low a magnification in the section at the bottom of the page here.

Once you have decided on which camera, filter and Barlow combination to use, connect these all up and secure then in you focuser and roughly focus on a star.

Mount Placement

With the hand-guiding imaging method you will be attempting to follow the ISS whilst looking through your finder telescope; the idea is that with the finder carefully aligned with the main telescope, when the ISS is centred in the finder, the ISS image will be in the middle of the camera sensor.

With your scope is correctly placed and everything connected up – camera, filter, Barlow (if necessary), laptop, batteries etc., do a trial run, sweeping the scope along the anticipated path of the ISS. If you are not using a Dobsonian telescope then you may need to unlock the axes of your mount to be able to do this.  In this mode you need to be careful if the scope is not well-balanced because if you let go of the tube it may tip up.

Checking the sweep with a Dobsonian whilst looking through the finder

During the trial sweep, check for smoothness of movement of the telescope, potential cable snagging, or lead pull-out, and anything else that might go wrong during the vital imaging stage. Check that, whilst looking through the finder, your body won’t get too contorted or your head knock the camera during the trial. Difficult body positions will have a big effect on your ability to move the scope smoothly and to concentrate on following the ISS.

If you have an equatorial mount do check your sweep won’t be constricted by the orientation of the polar axis and its attached couterweights. As you are not using the motorised tracking, with an equatorial mount you can reorientate the polar axis to any direction you want to best aid you with this.

Dobsonian owners have similar movement restriction issues with the azimuth axis, but these are at the zenith where it becomes increasingly difficult to move the scope quickly in azimuth the closer you are to the overhead position – this is the so-called Dobson’s Hole. Unfortunately, overhead is also where the ISS is at its closest and largest. As there is no option for repositioning the vertical azimuth axis to a more convenient direction to allow easier following, for a Dobsonian the best passes are invariably not overhead passes, but passes where the ISS reaches a maximum elevation of 70°-75°.

Finder

Open the camera’s interface program. For my planetary imaging cameras I use the excellent and feature-rich Firecapture program. Using a brightish star as a target (with my undriven mount I often use Polaris to help with finder alignment as it is virtually stationary during the task), carefully alter the finder’s orientation so that when the star is centred in its cross-wires the star is at the centre of the sensor as shown on the laptop’s preview screen.

Some ISS imagers use reflex type finders like a Telrads tracking the ISS. Although you can see the ISS easily through these types of finder I find there is insufficient positional discrimination due to their zero power nature. I prefer to use a finder with a power of 8x-10x and double cross-wires, so that when centred the ISS is not blocked by the wires.

If you do use a magnifying finder then tracking will be so much easier if it is one with an internal roof prism arrangment. This will give a non-inverted and non-mirrored view matching the naked eye orientation. If the finder gives a mirrored or inverted view this adds an extra layer of complexity between you and success! My personal preference is a 50mm RACI Celestron model of finder scope which is 9x power which is right-angle correct orientation and which has variable brightness illuminated crosswires. I know that if the ISS enters the box in the middle of the crosswires it will be on the sensor.

Illuminated double crosswsire reticle in Celestron RACI finder

Fine Focus

Once the finder and camera field are aligned I then use the same star, for me usually Polaris, to carefully optimise the camera’s focus. You’ll have no chance to focus on the ISS itself during the pass,, so your set-up needs to be already in a state of good focus before the pass commences. For the star you aligned on this may well require altering the camera exposure time to get a decent star image, but do be careful to change it back to the correct exposure for the ISS later. You are looking to set an exposure where the star image on the screen is tight and not overly bright but not so dim you can not readily see it.

First roughly focus the star on screen by eye, then fine tune the focus using a different method. The best method I have found to optimise the focus in preparation for the ISS, is to view the star with a Bahtinov mask over the front aperture of the telescope. With this mask I get a more reliable and quicker best-focus position compared to using the simpler method of just getting the best, tightest star image with no Bahtinov mask. See a fuller description of the use of a Bahtinov mask at the bottom of the page.

Warning. Don’t forget to remove the Bahtinov mask before the pass commences!

Bahtinov mask in place for focussing

Camera/Laptop Settings

Once aligned and focussed, set the following camera settings (for fuller details see under Camera Settings below);

  • Recording format – ideally SER format. See here for more details.
  • Capture duration – longer than the pass duration
  • Frame rate – generally as long as possible
  • Exposure time – typically 0.5to1 msec for hand guiding
  • Gain – typically 20dB to 30dB (200 to 300 in Firecapture for an ASI camera
  • If using Firecapture as you camera interface program, enable the Hisotgram Sounding feature. See here for more details.

As a final pre-run check, ensure the Bahtinov mask is removed and check that your laptop battery level has sufficient charge for the time you will need to record and plug it into the power if it is low on charge.

When you have checked everything, it is useful to record a short video of a bright star at the desired settings to check all is ready to go and the laptop records data okay.

Capturing the Pass

Once set up you are ready to record the pass so make yourself very aware of the time of the upcoming pass and stay alert for it first showing.

When the ISS appears and is approaching a reasonable altitude (>30°) then hit the record button on the laptop. Double check to make sure you really are recording and calmly sweep the scope to pick up the ISS. Once you see it in the finder, start chasing it across the sky, continuously attempting to keep the bright star-like image of the ISS aligned on the finder’s cross-wires.

If you are in Firecapture you will hear the audible histogram change in tone when the ISS is in the camera field and this feedback is very helpful to keep you focussed on this difficult task.

When you have finished the chase, hit the record stop button and RELAX. You can then quickly review your video in something simple like Windows Media player to get a sneak preview of your achievements and see if you have caught anything useful. If you have recorded in the recomended SER format you can use the much more friendly SERPlayer freeware to review the video frame by frame if you wish – it is so much easier to use than Windows Media Player.

If you see something buzzing around like a firefly on your video during review, then you have met with success and you can move onto the next stage.

If you see that you have caught nothing then do not be disheartened, the task is quite testing and everything needs to come together just right. Fortunately there is often a second pass 90mins later and you may be able try again. The important thing is to review what went wrong and take steps to correct it next time.

Processing

Extras

Camera Choice

The best cameras are generally planetary imaging cameras, as these are fast, low noise and high sensitivity. Such cameras are made by ZWO, FLI, QHY and many others.

Some imagers prefer one-shot colour cameras to image the ISS but I prefer imaging using a mono camera combined with a red or IR filter, as the seeing is better at these longer wavelengths, pulling out more detail.

My favoured camera is an ASI174MM made by ZWO which has a large chip and large pixels which increase the dynamic range in each shot. This is a fast CMOS camera capable of 164fps at full frame (in high speed mode) and unusually for a CMOS camera has global shutter which reduces distortion of the ISS during each exposure.

Magnification

The ultimate resolving power of a telescope is determined by diffraction effects. To make the most of this resolving power you should have 3 pixels covering the smallest details the telescope can produce. This so-called Nyquist Criterion is satisfied when the focal ratio (apeture/focal length) is 3-5x the pixel size in microns for a mono camera and 5-7x for an OSC camera.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The size of the ISS on your chip will, depend on the pass geometry, the telescope focal length and the camera pixel size, and for short focal ratio scopes the ISS could be on the small size. Almost certainly you will be undersampling and not Adding a Barlow to increase the effective focal length will increase the image size so you are undersampling less severely, potentially enabling you to resolve more details, but will have the following disadvantages:

  • Sensor field of view decreases, making manual tracking more critical and yielding fewer occupied frames to stack for your final image
  • You magnify any relative movement between tracking and ISS path occuring during the exposure time. This leads to more obvious movement smearing
  • The light of the ISS will be spread over more pixels meaning that each pixel gets less light and shot noise in each frame will increase, reducing the overall signal to noise ratio (SNR)

Focussing with a Bahtinov mask

Use of Filters

Bahtinov mask

The Bahtinov mask is a recent invention by Russian astrophotographer Pavel Bahtinov. The special mask fits to the front your scope and alters the diffraction pattern of a star, allowing you to more easily find best focus.

I made mine from a piece of 1.5mm acrylic cut with an IR laser cutter, but you can readily buy them from many vendors or make your own with card and a sharp knife. If you want to make your own, there is a design facility and more instructions on Astrojargon here.

Trying to determine the position of best focus for a star is a difficult thing and it gets harder the worse the seeing. This is because the best focus position for a star is actually dependent on the exact state of the atmosphere at that time. Poor seeing means fluctuating focus. As you alter the focus close to the optimum position you are just altering the proportion of time that the star spends at best focus. The best focus position is the position where the star spends the most time at best focus but even here, if the seeing is poor, the star may spend only a very small portion of its time actually at focus or even be in a constant state of agitation. Determining what is actually the best position, when the best position is so mushy, is really hard.

The difficulties with determining the position of best focus, especially in poor seeing, have prompted imagers to invent various aids to help. Hartmann masks have been around for years and these have two large, well-separated, circular holes in an opaque mask in front of the scope. At best focus position the two diffraction spots are supposed to merge. If the seeing is poor, however, the image rapidly fluctuates between single spot and separate spots even at the best focus position.

The Hartmann mask method suffers from the same weakness at just simply focussing on a star with no aperture mask. That weakness is that at any one focus position you can’t tell is that is the best position, except by reference to what it looks like at positions just inside and outside of that position. They are relative methods not absolute methods.
 
The real beauty of the Bahtinov mask is that it is an absolute method, which unlike the other methods does not rely on comparisons with how the image looks at focus positions either side. Either the image is, on average, symmetrical, and so in focus, or it is not. In addition, from examination of the asymmetry you can tell which side of focus you are. Poor seeing still causes fluctuating focus which causes the image to change from one type of asymmetry to other but by eye you can easily average the fluctuations to see if there is a bias to an asymmetry on one side or the other side.
For more on this method of focussing see; http://en.wikipedia.org/wiki/Bahtinov_mask

Magnification and Focal Length


For my imaging I use an f5.9 Dobsonian Newtonian with my camera inserted into a 2x barlow (Meade series 4000 Model 160 Tele-negative amplifier) which gives f14.5 and an effective focal length of 3.2m.  This gives an image scale of 0.35″/pixel. This is a much lower magnification than I would really like to use. I recently switched from f23 to f36 for Jupiter with significant benefits (see link) and I image Mars at f46, consequently at f14.5 I know I am losing the potential to catch more detail in the ISS. However, increasing the effective focal length/f-ratio would have the following issues;

Reduced field size which would be very likely to reduce the number of frames that contain a useful image of the ISS.

The field size could be increased again by using a chip with more pixels and a larger area, but then the max.frame rate would fall due to the limitations of the Firewire data transfer rate. For a 640×480 camera and Firewire 400, the max uncompressed frame rate is 60fps whereas for a 1280×960 camera with 4x the area the max frame rate drops to 15fps. Of course there are faster connection speed protocols coming on stream, such as USB3.0 or Firewire 800, which should allow larger area chips and higher frame rates but I have not experimented with these yet mainly due to lack of funds and time!

Increased motion blurring. Increased magnification would mean I would have to use shorter shutter speeds to reduce the increased impact of motion blurring – which is a manifestation of tracking by hand. This is not practical for me as the shutter speed is as short as I dare go at the moment without undue image dimming – see more on this under camera below).

Reduced image brightness.  Increased magnification would mean that I would have to increase the exposure time to compensate for the reduced image brightness.  This is exactly the opposite of what I would want to do to reduce the increased motion blurring.

As you see it is all compromises and trade-off using the hand-guiding method, but the big thing going for it is that it is a simple method. Faster cameras/laptops with larger areas and higher sensitivities would help alleviate some of the difficulties experienced and allow shorter exposures and at the same time higher magnifications but ultimately it is the tracking system, the very thing that makes it simple, that is the weakness.

With a lock-on tracking system one could increase magnification and increase the exposure time without worrying about motion blur. I could then also use an IR filter to improve the seeing without worrying about dimming the object too much.  I may explore the use of such systems like this: http://www.optictracker.com/Home.html when I get more time.

Camera Settings and Filter

Most of my ISS webcam images on the website have been taken with a DMK21AF04AS, in the telescope set-up described above. This is a 640×480 pixel monochrome high speed video camera using a ¼” Sony 098L CCD chip, and is made by The Imaging Source. The camera connects to my laptop using a Firewire 400 cable. The camera is easy to use, readily available, (I got my from www.opticstar.co.uk) and is a lot cheaper than the popular Lumenera monochrome cameras.


For ISS imaging I use this camera with the following settings

Max. frame rate of 60fps
The greater the frame rate, the more useful frames you will have from a pass. I run the camera at the max fps (60fps) as you need as many frames as possible within a period when the ISS’s appearance does not change that much (typically all within a 0.5 to 1sec window). If I could run my camera at higher fps, then I definitely would, as I need as many frames as possible within any short useful period to be able to combine those frames. The more frames you stack the lower the noise and the better the image quality.

Gain of 900 (ie 90% as max is 1024)
This is a compromise between noise, image size and brightness. The higher the gain the more the noise but the larger the image you can have without excessive dimming. Much above 90% gain, however, the noise levels increase significantly

Exposure of 1/1241secs
The longer the exposure the brighter the image, but the more you risk motion blur. This is another compromise and one that needs to be found by experimentation. This has already been discussed above under ‘Magnification and Focal Length’

Gamma of 22 (max gamma)
The high gamma setting I use in my ISS imaging is essential to reduce the very high contrast of the object and allow bright and faint detail to be recorded- this is very different from the low gamma setting needed to image planets where you want to increase rather than decrease contrast.

Avi Duration

I set the avi. record duration to 3mins which should be enough to capture the pass without recording too large an avi if you forget to hit the stop button.

Avi Format

My camera is set to take an uncompressed avi video in Y800 format, to maximise image quality.

Filter 

I did use a UV/IR blocking filter before the camera to help reduce the effects of dispersion. Anything else in the optical path (eg red filter) would have lost me too much light. The recent move to a more sensitive camera (see below), however, has allowed me to add a red filter (with UV/IR blocking) in the light path. Adding a red filter improves seeing as the atmosphere is steadier in the red and it also reduces effects of atmospheric dispersion by narrowing the bandwidth.

At the end of last year I moved to using an Imaging Source DBK21AF04AS camera which has been converted from a colour camera to a mono camera by removing the colour chip and replacing it with a mono Sony ICX618 chip. This was expertly carried out by Andy Ellis of Astronomiser (http://www.astronomiser.co.uk/index.htm). This, like the chip in the unmodified DMK21AF04AS, is 640×480 pixels, but the sensitivity is significantly higher than the 098L chip in that camera. The increased sensitivity allows me now to use a red filter in the optical path (with IR and UV block) without hitting problems of insufficient light. For this more sensitive camera I use essentially the same camera settings as for the DMK camera.
 
Although both Imaging Source cameras are known to suffer from ‘ringing’ artefacts at 60fps, giving a ‘cheese rind’ effect to the sharpest limb of planets, I have never seen an issue on the ISS. This may be because the gamma is set to maximum, reducing the contrast to a minimum.