AP Corner – May 2021

Original article appears in GuideStar May, 2021.

Measure Your Imaging System

By Don Selle

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The age-old astronomy curse which promises that a new telescope will bring cloudy skies (the bigger the OTA the cloudier for longer) applies not only to visual observers but to we astro-imagers too. Whether (pun intended) this curse is real or generated by our selective memory really doesn’t matter. The reality is that clear transparent moonless nights are a limited commodity, and we should stive to use them as efficiently as possible. Developing and consistently following an imaging workflow is a good way to do this.

It doesn’t matter if you are just getting into astro-photography or if you have been at it a while, it seems almost inevitable that you will acquire new equipment (remember – there’s always an upgrade!). Becoming familiar with and measuring key parameters of your equipment, can and should be done during cloudy or moonlit nights when conditions are not very favorable for actual imaging.

Here are a few things that you can do ahead of time so that you can make the most of your time when skies are good for imaging. I have organized them by equipment type that is new to your imaging system. Partly cloudy skies with lots of moonglow will not be a big hindrance for any of these tasks.

New Mount There are several things you can do with a new mount on a partly cloudy night to save you time and trouble later. These include:

  • Polar align your mount. There is nothing more frustrating than wasting time trying to get polar aligned. You can practice when the skies are not so good, though this will normally require that the skies to your north are relatively clear.

Polar Scope - If your mount has a polar scope and you plan to use it now is a good time to get familiar with it. Follow the instructions that came with the mount. A good tutorial video can be found here: https://www.youtube.com/watch?v=Q1p6yohys_0

If you are using a mount from a different manufacture than shown in the video, an alternative polar alignment app is PolarFinder. It has most polar scope reticles and allows manual input of longitude. As the video explains using a polar scope gives you a reasonable polar alignment, good for shorter exposures. Autoguiding may increase this time a bit, as your target will drift in declination and autoguiding  can make corrections in declination to compensate.

Autoguiding however, cannot compensate for image rotation. If you are doing long exposure imaging, say for narrow band work, or imaging close to the pole, you may need to do a drift alignment in addition to using the polar scope. Drift alignment when done correctly will considerably improve your polar alignment accuracy.

Use your camera or guide camera to do this with the freeware program PhD2. It has built in utilities to assist in getting you dialed in. Some practice required. Read the PhD2 polar alignment utilities documentation here: https://openphdguiding.org/manual/?section=Tools.htm#Polar_alignment_tools.

Polar Alignment Camera. But wait – there is always an upgrade! If your mount came with a built-in or an add-on polar alignment camera, or you are adding one on yourself, you can expect good polar alignment in less time than drift aligning. It is still a good idea to practice polar alignment with this tool when conditions are not so good for imaging. You’ll be more confident and get polar aligned quicker when the skies are good and the pressure is on.

  • Periodic Error – Most mounts use a worm gear / wheel combination for the RA axis. These all will exhibit some amount of periodic error or drift in the RA direction. PE is caused by slight imperfections in the gears that repeats itself in the same way over every rotation of the worm gear. The period of this error will depend on the diameter of the worm gear and the pitch of the worm wheel.

This is quite easy to measure using your camera or guide camera. Center a star and take a 1-2 min image without any autoguider corrections. You should see the star trail in the east - west direction. If you know your image scale (see below) you can measure this drift by counting the pixels. Compare this to the manufacturer’s PE specification for your mount. While this does not happen very often, if the measured PE is way higher than specified, you may have a faulty mount.

  • PEC – Periodic Error Correction – Most mounts marketed to imagers will have PEC built in. It will also have a utility to measure the PE, calculate RA corrections to minimize the PEC, and store these in the memory of the mount controller so they can be applied whenever the mount is tracking at sidereal rate. Use the method specified by the mount manufacturer to measure and add PE corrections. Definitely something to do before you try your mount out for some serious imaging!

New OTA. If you have a new imaging OTA, the clouds will certainly descend on you! (just kidding but if so keep it to yourself or we might just have to sacrifice your OTA to appease the weather gods and clear the skies for the rest of us!). Actual focal length is the main thing you will want to measure on a new OTA or any “new glass” such as a field flattener or reducer you have added to your imaging system. The best way to do this is to use your camera and do a plate solve routine somewhere the clouds are not.

Plate solving is the process by which you take a short image, usually 10 seconds or less, and the resulting image is analyzed in software. Stars in the image are identified, and the spatial relationship between them is measured. These relations are matched with a systemized database, and when a good match is found, the software solves for the position on the sky (RA & Dec) the camera was pointing at. It will also report the angle the camera is rotated away from north, and the actual image scale (arcseconds per camera pixel) of the imaging train.

In many plate solvers, a good starting guess at your image scale may be required. The better this guess is, the quicker the plate solve will be completed. That’s why it is a good idea to measure the actual image scale upfront – it saves time and maybe some frustration.

You can calculate an approximate image scale as follows:

image scale = (206 x pixel size (in mm)) / focal length

Your focal length estimate must include the effect of any focal reducer of field flattener. Keep in mind though that this is only an estimate because the spacing between these accessories and your camera sensor can make a big difference in actual focal length.

Additionally, the focal lengths of these accessories and even of your refractor will vary slightly from manufacturer’s specs. The actual focal length of an SCT is even more variable since it uses a moving mirror to focus. Changes in the actual position of the mirror makes a significant change in the focal length of the OTA.

Most newer imaging programs will have plate solving built in because it makes framing your targets from night to night easier and much more consistent. If not, you can use the website astrometry.net to get a plate solve done.

Plate solving is not too difficult, especially if your imaging software has an all-sky plate solving mode (sometimes called “lost in space” mode). Usually, you will be able to specify an image scale to start with, and if you are close it can shorten the time to find a solution.

Try to use the default settings for light source (star) detection, as changing these can result in lots of frustration. At most you may need to change the duration of the image you are solving.

If your software does not support an all-sky plate solve, you will need to make sure that your imaging software puts the sky coordinates into the image file header to be used as a starting guess on where the telescope is pointed. These coordinates usually come from the planetarium software or mount control program you are using and are passed to the imaging software for inclusion into the image file header.

Once you get a good plate solve, you will know your image scale quite precisely. Knowing the image scale, it is easy to determine the actual focal length of your imaging train using this formula:

focal length = (206 x pixel size (in mm)) / image scale

New Camera. There are a couple of practical things you can do with non-imaging time to make your time with a new camera more productive.

  • Take library dark and bias frames – Exposure times and camera temperatures can be modified to match what your typical light frame exposures are. I personally tend to use the same temperature for the camera in most situations. Therefore, I can expose a single set of darks based on exposure duration. A typical set of exposure times I use are 1, 2, 3, 5, & 10 minutes. I will add 20 and 30 minutes if I expect to be doing narrow band imaging. Additional series of the same durations should be taken for each level of camera gain you expect to use.
  • Star Saturation – I will also take different length exposures of a star field that contains stars with a wide range of magnitudes, so that I can get a feeling for what exposures will saturate the brighter stars. The fewer stars that are saturated in an image the better it looks, and this exercise will help your judgement here.
  • Focus Points – This exercise will not only give you a feel for how your camera focuses, but it will position you to take evening twilight flats. Flat frames are used to even out the illumination of each of your sub-frames – eliminating vignetting and dust donuts and dust spots from your subframes.

While there are other methods, I have personally found twilight flats, when done right, to be very consistent. The also do not require any additional equipment, just an evenly illuminated twilight sky.

To shoot any flat frame, your camera must be at or very close to critical focus if you hope to expose good flats. That is why you will want to measure and record these focus points for your camera and OTA combination.

You should focus on a star or star field where the stars are not so bright as to saturate with a short focus exposure. If you have a one-shot color camera and a software controlled motorized focuser, this can be a relatively quick task. Focus the camera, record the focus point, and then intentionally defocus the camera. Rinse and repeat several times. Average the results.

If you have narrow band filters you use with your OSC camera you will want to repeat the same procedure using each of these filters too. Monochrome cameras require that you carry out this focus procedure for every filter in the camera.

If you do not have an automatic motorized focuser, do not despair. Assuming you have a draw tube focuser, you can focus by hand and measure the extension of the draw tube with a digital caliper. These are readily available at your local home center or hardware store and can be purchased for $20-$30.

If you are imaging with an SCT measuring focus points is not practical without a focus motor to move the mirror. You might try adding a drawtube focuser on the visual back of the OTA, but this will require you to keep the mirror locked. Focus the OTA roughly by moving the mirror, then lock the mirror in place. Finish your fine focusing with the drawtube focuser and measure with a caliper.

  • Focus your guide camera – if you are using an off axis guider (OAG) adding a new camera to your system will most likely change the focus position of your guide camera. Even if you are using a guidescope, Re-focusing your guide camera and recording (or marking) the focus position will save some good sky time later.

Completing the above tasks before you go out to image in earnest will get you started more quickly and with less stress. That way you can put all your mental energy into capturing your best-ever image yet!  

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