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Wednesday October 18th 2017

Posts Tagged ‘astrophotography tips’

Eyepiece Projection



Eyepiece projection is a great way to take detailed shoots of moon and planets. Photographed objects in these images are considerably larger and show more detail than such taken with prime focus shots. Prime focus techniques replace the camera lens with a telescope OTA (no diagonal, no eyepiece), but eyepiece projection adds an eyepiece into the optical path, increasing focal length and magnification considerably. The image below shows the typical eyepiece projection setup.

Greater magnification and increased focal length come however at a price.  Higher focal length (at the same aperture) results in a higher focal ratio number (1/f). The higher the focal ratio number the fainter the image becomes. This demands longer exposure times or higher ISO speeds to achieve a decent image brightness. Furthermore, constantly moving air layers diffract incoming light. That means, with stronger magnification distortion is magnified as well. The same is true for any mount and telescope shake or vibration.

Eyepiece projection imaging with refractor telescope and DLSR camera Typical eyepiece projection setup with refractor telescope an DSLR camera.

How to do it?

The following paragraphs describe equipment that is needed and such which is additionally recommended to make photographer’s life easier. I will share some experiences that I had to learn the hard way; it will help you getting good results sooner.

Mount

  • The mount needs to be strong and sturdy. It has to carry all the weight of telescope, camera and all accessories, furthermore it has to stand steady, even with light breezes.
  • Many manufacturers are quite “generous” when listing weight capabilities of mounts and tripods in their data sheets. Unfortunately, this leads often to unsatisfactory imaging experiences.
  • Never max out a mount load. The old astrophotographers’ rule still applies:  actual equipment weight should not exceed half of the mounts specified load capability.
  • Many astrophotographers do not extend the tripod legs for better stability and minimal vibration.
  • Balance the mount very carefully with camera and all accessories attached.
  • Polar align German Equatorial Mounts (GEM) with great care. It helps “keeping the object in the field of view”, even with highest magnification.

Telescope & Accessories

  • Finder scope and main scope axis need to be perfectly aligned. This helps to “find” the object and framing it in the very narrow field of view (FOV).
  • Screwed accessory connections,like tube extensions, are preferred over slide-in joints. Screwed connections offer better stability, less flex and are less receptive to shake and vibrations.
  • Eyepiece projection requires usually significant focuser back travel, particularly with refractors. The required length can exceed the telescope’s focuser travel, which will render the projection out of focus. One or two 2” extension tubes provide the required additional focusing way. My telescope has sufficient travel way but I still use extension tubes because it keeps the, relatively heavy, focuser tube more inserted. This has the advantage that the telescope’s weight distribution is somewhat closer to the center of the mount (less vibrations).
Astrophotography: Typical Eyepiece Projection Assembly with DSLR Astrophotography: Typical Eyepiece Projection Assembly with DSLR T-Adaptor

Note: M42 and T-thread accessories have different threads. While the diameter is the same their thread pitches are different (M42: M42x1mm and T2: M42x0.75mm). Accessories with M42 and T-threads should never be mated.

The Camera

  • Remote control for the camera is strongly suggested. Pressing the shutter release manually will cause shake and vibrations. If your camera does not have remote capability use your longest shutter release delay, minimum is 10 seconds. Some cameras offer only 2 seconds shutter delay. This time is usually too short because many mounts are still shaking 2 seconds after the shutter button has been pressed.
  • Most cameras allow shooting movie clips (avi). Even if the movie mode may provide less pixel resolution, shoot movie clips, particularly for planetary imaging. Movie clips consist of many single frames and software  like RegiStax convert the movie clip into a string of single images, which can be stacked. With a frame per second rate (fps) of typically 10 fps to 30fps, a 10 second clip results in a large number of single frames. This is important because air movement and other distortions will blur many images. The probability of getting a few good ones increases with the number of available images.
  • Stacking good images helps to pronounce object features and texture.
  • If your camera has no movie (avi) feature take at least 30, better 50 (or even more) images to increase the probability hitting  some really good ones with little of no air movement.
  • DSLR cameras use mirrors that flip up during the exposure. If shooting images (not movie clips) use mirror lock if available. Even if the mirror is very light, the fast movement can create enough momentum to cause shake, which again blurs the image.
    Jupiter is the fifth and largest planet in our solar system. It is a gas giant which is primarily composed of hydrogen and helium (very similar to our sun). Jupiter may also have a rocky core of heavier elements.
    Jupiter – Image taken with eyepiece projection technique (telescope: 900/120mm, eyepiece: 20mm)

Object Position

  • Take shots at planets when they are high in the sky rather than low at the horizon. Positions high in the sky minimize air refraction distortion. Light that travels through the atmosphere is scattered by aerosol droplets and absorbed by dust. These effects cause diffraction rings and reduce the image brightness. High in the sky, light’s atmospheric path is much shorter, reducing distortion effects significantly.
  • There are also disadvantages of high object positions. Particularly when shooting with a large refractor, the camera position is very low. Also, a large refractor with extension tubes and camera mounted may hit the tripod legs in this position. Make sure enough space is left when moving the telescope to the desired object.

 Computer

  • Remote controlling the camera with a computer is strongly suggested, particularly with a large refractor. Looking in upright position at the computer screen is simply much (!) more convenient than crawling on the ground trying to peek in the – very low hanging – camera screen or finder.
  • The image on a much larger computer screen allows more precise focusing.
  • Take your time when focusing. High magnifications combined with moving air layers can make this quite a challenge.

 Imaging

  • Re-check with some test shots that the focus is still optimal.
  • Check the histogram and ensure that neither end (black or white) is clipped. If data is lost (clipped) it is lost for good, and can no longer be used to build the image. Even the best post processing effort can not bring lost information back.
  • Shoot several movie clips. My recommendation is 10 by 10. Ten clips each ten seconds long. Depending on the fps rate this  will provide you 1000 to 3000 single frames, a good base to work with.
  • Some photographers prefer much longer clips to increase the probability of catching better results. With very long clips it is more likely that shake, vibrations and drift errors are introduced as well. CCD chips get hotter and start to introduce additional noise and hot pixels. Besides, long movie clips result in very large files, making processing somewhat cumbersome.

Post Processing

  • Powerful software like RegiStax (freeware) converts the movie clip (avi) into single images. Furthermore, it aligns the images, selects the best ones and stacks them for best detail. It allows improving the resulting image even more with a great set of post processing features.

Question for Power

It is possible to calculate how much more magnification we get with eyepiece projection over a simple prime focus setup. To determine this, we need to know some dimensions: focal length of telescope and eyepiece, and the telescope aperture. Furthermore we have to measure the distance from the eyepiece lens to the camera’s CCD chip.

The dimensions used in the following example are from an actual eyepiece projection setup that was used when I shot the Jupiter image: Orion EON 120ED refractor with 20mm Eyepiece, 2 extension tubes each 2 inch ( about 50mm) and a Canon EOS T1i DSLR camera.

Focal length of telescope (FLtele): 900mm
Focal length of eyepiece (FLep): 20mm
Distance eyepiece to CCD (Depccd): 100mm
Telescope aperture (TA): 120mm

Eyepiece Projection Magnification - Dimensions to calculate magnification

Magnification over prime focus set up (Mopf)
Mopf= (Depccd-FLep)/FLep
Mopf= (100mm-20mm)/20mm = 4
The image is 4 times larger than that of a prime focus setup.

Focal Length overall EP setup (FLoEPs)
FLoEPs = Mopf * FLtele
FLoEPs = 4 x 900mm = 3600mm
This setup has a focal length of whopping 3.6 meters (141 inches)! The number shows that eyepiece projection focusing can really be a challenge and has to be done carefully in minute steps.

Focal ratio overall EP setup (1/f oEPs)
1/f oEPs = FLoEPs / TA = 3600 / 120 = 30
The original telescope focal ratio of 7.5 has now become 30. The image will be much darker than that of a prime focus setup. Higher ISO speeds particularly for planetary images may be necessary.

Is it worth the challenge?

Most definitely: YES. Eyepiece projection astrophotography is for more advanced star shooters. It is easily among the most challenging processes in amateur astrophotography, not because of the setup but because of the effects that have to be considered and factored in. But with the right equipment and some practice it can be mastered – and the results speak for themselves: clearly visible features of the moon landscape, surface coloration and visible ice caps of Mars or detailed cloud bands of Jupiter make eyepiece projection imaging indeed quite rewarding.



 

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Astro-Imaging for $100

Astrophotography on a Budget - Orion Image captured with Canon SX120 (10 exposures, 4 seconds each, ISO400)
Constellation Orion - Image captured with Canon SX120 (10 exposures, 4 seconds each, ISO400)

Is is possible to make astro-images with entry level digital point-and-shoot cameras?

The answer to that question is a reluctant “somewhat”. With a basic camera it is indeed possible to shoot decent astro-images but the objects are rather limited: the moon and star constellations. 

It is not about pixels

Basic astrophotography is not about Mega-pixels. Good images can be taken with cameras of 4 MP, or even less.  What is important are three vital camera features, without them astrophotography will become a gamble. These features are:

  • Manual focus
  • Capability to preset exposure time (Tv)
  • Capability to delay exposures. 

Another component is of great importance: a solid tripod. The magic word here is “solid”.

Manual Focus

Manual focus is important because of the way cameras perform auto focus. Some compare contrast changes. Sharp images have more pronounced contrast changes between adjacent pixels, unsharp images deliver more gradual changes. Other cameras compare bit patterns in specials sensors (phase detection). The patterns are shifted when the image is out of focus.

Either way, both methods are not really helpful when imaging a quasi black dark sky. Furthermore, if there is any contour visible in the image (f.e. a tree in the foreground), automatic focus will jump right at it, putting the  actual celestial object out of focus. Astrophotography objects need to be focused manually to infinite.

Manual Time Setting (Tv)

Moon
Moon – Image taken with Canon SX120, post-processed with GIMP

Time setting is important because the amount of light gathered by the CCD is only a tiny fragment compared to that of daylight images. This means, the exposure time need to be long. Typical exposure times for imaging stars are between 1 second and 30 seconds.

Tripod – Solid

A solid tripod will keep the exposing camera steady in position. With long exposure images, any vibration will be clearly visible in the images. The camera has to be absolutely still.  For noise reduction purposes we need to take a series of at least 10 images, ideally 30-50. More on this subject later.

Exposure Delay

Even if the camera is firmly mounted on a tripod, pushing the exposure button will cause slight vibrations. The result is star streaks in the image. Exposure delay prevents this effects. Many cameras have a built in 2 seconds or 10 seconds delay. When the button is pushed, there are still initial vibrations, but the delay allows mount and camera to stabilize. The result will be significantly sharper images.

Zoom – Better Not

Some cameras have a zoom feature. Unless you are shooting the relatively bright moon with a very short exposure time – just forget the zoom feature of the camera. Why? Because the Earth rotates. This will show badly in the images in form of elongated stars. Please try to follow the short calculation below – it is indeed eyeopening.

The earth rotates once in 24hours, one rotation equals 360 degrees. That means, in one hour the rotation angle is 360/24=15 degrees, and in one minute it is 15/60 = 0.25 degrees, right?  A quarter of a degree does not sound a lot.

True, but… Lets say we want to expose a the constellation Orion for 12 seconds. The angle the earth moves during this period is 0.25/5=0.05 degrees. A 10x zoom would increase the apparent angle by the same factor of 10. Within 12 seconds the image would shift by 0.5 degree.

One might think, that still seems negligible. Does it really have an effect?  – Yes it does, and very much so. Picture the moon. The angle of the moon is, well, 0.5 degrees. That’s right, within 12 seconds exposure using 10x zoom, stars in our image would become as long as the diameter of the moon is; definitely not what we are looking for.

So, how are images with a high power telescopes possible?

Astrophotography with high power telescopes requires special mounts; they are called German Equatorial Mounts (GEM). These mounts have gear and electronically controlled motors that move the telescope exactly so that it perfectly compensates for the Earth’s rotation.  You have probably guessed it: these mounts are rather expensive. Price depends on their carrying capability and accuracy. Entry level models that can be used for basic astrophotography start at about $500  ($300 used), and with growing demands, mounts can reach quickly true astronomical prices.  

First Photos

Preparations:

  • Make sure the battery is fully charged and the memory card offers enough space.
  • Reduce the brightness of the camera display to minimum. This helps to keep / maintain the night vision.
  • Mount the camera on the tripod.

 Shooting:

  • Choose your object
  • If possible set your camera to the highest ISO speed.
  • Manually focus to infinity.
  • Set exposure delay to 2 (or more) seconds.
  • Set exposure time (Tv) to 5 sec.
  • Take your first test shot. You can see if the object is framed right and the image is in focus.
  • You might need to play with ISO speed and exposure time to optimize image exposure.
  • Once done and you are satisfied, take a series of at least 10 images of your object (recommended 30-50). 

Note: photos of stars look always quite dark in the camera monitor. It is often advised to increase the brightness of the image later during post processing.  

Post processing (very basic):

This following description is for images with stars (not applicable for moon shots).

  • Load your images to your computer and inspect every single image
  • Sort out wiggly images, and such that have unwanted artifacts like plane or satellite trails
  • Stack the remaining images with DeepSkyStacker (DSS) – setting: average
  • Once DSS has created an image, optimize it with the build-in post processing tool

The advantage of stacking a series of astro-images (rather than using just one image), is that the noise portion will be significantly reduced, and the lunimance and saturation of the actual objects (stars) are emphasized. Since we are working usually with high ISO speeds, noise is much more present in astro-images than it is in daylight images.  

Further reading

  • Catching the Light – Great site on Astrophotography with a DLSR by Jerry Lodriguss. Noise reduction in astronomy images

Coming soon:

  • Deep Sky Stacker tutorial
  • GIMP tutorial
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Solar Dynamics Observatory

Solar Dynamics Observatory 2017-10-18T12:54:09Z
Observatory: SDO
Instrument: AIA
Detector: AIA
Measurement: 171

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