Methods

Methods and Equipment

I have taken the pictures mainly in my backyard in two kilometers distance from center of small city Kirkkonummi. There is quite much light pollution around. When taking deep sky images, the sky has to be cloudless and there should be no moon. This limits the good deep sky imaging session to about 30 nights per year in Kirkkonummi.

In image below is the equipment year 2023. Telescopes Celestron 8, TS Photon 200/800, Sky Watcher 80/600 ED and the tele zoom lens EF100-400L.  Cameras Canon Eos 80Da and 60Da are specially designed for astrophotography. They pass the infra-red light trough to CMOS Sensor. Normal cameras filter away the infra-red light.

Essential thing when photographing stars is that the telescope must follow the rotation of sky with high accuracy. It is essential to make good polar alignment of the tripod before the photographing sessions. Aligning the tripod to North star is not enough, since it is half a degree from the celestial pole. In the HEQ5 mount  there is a built in alignment process and it takes about 15 minutes to execute it before starting imaging session.  With TSL60 auto guider telescope and ASI120 guider camera, connected to a laptop with PHD2 tracking software, the mount will track with high precision the stars, making very long exposures possible.

When taking exposure series for stacking the TC-80N3, timer remote controller, is a good equipment. With image stacking methods the quality of astro images is improved a lot. In the method a large set of exposures is stacked together to remove noise from the image.

The optical combinations which I use with Canon Eos 80Da camera are in list below.

Below is comparison images of field of view with different optical setups.

TS Photon , March 2023

On March 2023 I got new equipment for astrophotography from Telescope Service. TS Optics Photon, 8 inch F4 telescope.  The kollimation of the mirrors I do with help of a laser beam. First secondary mirror adjustment, then primary mirror. With coma corrector the star field is sharp to the edges of the image. Taking flat frames for image processing is essential with this setup.

Autoguider

Year 2020 I upgraded to auto guiding solution with Zwo ASI120 mini   guiding camera and TS Optics TSL60D guider telescope. I ordered them from  Telescope Service . ASI120 is a small camera connected to computer and to HEQ5 mount. On laptop I have and  PHD2 Guide software  to take care of quiding. I started autoguiding experiments year 2013 with the Orion StarShoot auto guider. At that time the guiding telescope was quite heavy, and the total weigth was a little too much for CG-5 mount, which I used that time. With 10 meter USB cable, I can keep the laptop warm inside on winter time.

Using Deep Sky Stacker

I use Deep Sky Stacker  (DSS) software to reduce noise from pictures. Noise is caused by heat from camera sensor and from light pollution. This random noise can be reduced when several images are taken of the target and they are compared to each other. The stacking improves the signal-noise relation of the image. And then it is easier to make afterwards image improvements. Stacking method is useful especially for galaxies and nebulae.  For planets, sun and moon there is another method implemented for example in Autostakkert or Registax software.

Signal-noise relation is improved in relation to square root of the amount of the stacked images. So a stack of 4 images has 2 times better signal-noise relation than a single image. The images have to be taken with same exposure times and ISO sensitivity. The latest version of DSS supports the RAW format(CR2 files) of Canon Eos 60Da, so it is possible to stack directly those. But I often convert the RAW images to TIFF format with Canon Digital Photo Professional (DPP) software, and then I stack the TIFF images.

Concepts in stacking:

Light frame  The original image of the target, galaxy or nebula

Dark frame  Image taken with lens cover on, with same ISO sensitivity and exposure time as the light frame. This will contain the noise patterns of the camera sensor and this will be reduced in stacking process from the stack of light frames. It is essential to have same temperature as when ligth frames were taken. I usually take 4-6 dark frames at the end of imaging session.

Bias frame Used to remove the noise signal caused by camera reading the sensor. Taken with lens cover on, with shortest possible exposure time, for example 1/8000s. About 10-20 images needed. I do not use this in my astro images.

Flat frame Used to remove vignetting effect. Taken with putting a white blanket in front of the optics and take images of it in daylight. Optical setup, ISO sensitivity and focusing has to be same as in light frame. Use Av mode in camera, so that the camera decides the exposure time. I take 6 flat frames for stacking.

DSS has also a mode for stacking comet images so that comet is centered and stars become trails. For each image the center of comet nucleus is marked by pushing right shift key down and clicking the center with mouse.

Autostakkert for Moon, Sun and Planets

On March 2020 I learned to use  Autostakkert   software for stacking images on Moon and planets.. It calculates a average image from large set of frames, so that the distortion caused by earth atmosphere is reduced. Autostackerts gives easily much better results than Registax,  Autostakkert can handle Full HD size AVI format video, but not MP4 format. With Video editor Pinnacle Studio I convert original MP4 HD video to AVI format. The procedure in Autostakkert is:

1.Open videofile "1)Open".

2.Select "Surface" for Moon and Sun or "Planet" for planets. In the window with opened video, set the Image stabilization anchor.

3.Click "2)Analyze". This takes a little time.

4.In "Frame percentage to stack" set how many percent of frames to stack. If the atmosphere is restless and a lot of blurry images in the video, set 2-5%.

5.In the window with opened video, set alignment points. For Moon and Sun the "Place AP grid" sets them well. For planet videos, I set them manually one by one by clicking on the image.

6.Click "3)Stack". This takes a little time. The final image is a TIFF format file, which can be converted to jpeg format for example with GIMP software.

ISO sensitivity of the camera

In image below i tested the ISO sensitivity of the camera and how it affects the noise patterns in the image. Target was globular cluster M13 in Hercules. From the original 5200 x 3500 pixel image a small area of  350x260 pixels has been chosen to see the noise patterns. Images below in first row are raw images without any modifications. In second row are stacked images. It can be concluded that the noise is smaller with low ISO sensitivity.

Light pollution

Below is a test of light pollution. Same target in urban area and on country side with same camera settings. Difference is huge. In practice the level of light pollution puts a limit how long exposure times it is possible to use. Sometimes I use Baader UHC-S filter to cut down the light pollution.

Sky-Watcher Merlin travel tripod

Below is a picture of a compact tripod with sky tracking capability. It is easy to take this system with e.g. when traveling abroad with plane. I used this equipment in a trip to Cran Canaria 2013, to take images if Milky Way galaxy center areas.

Oscillation in HEQ5 tracking 

It is typical to have all kinds of problems when doing astrophotography. Here is solution to a oscillation problem in R.A axis of HEQ5 mount which was just 6 months old. Apparently in the factory the bolts holding the motor and the gears were not set tight enough in the factory. They were loose and the gaps between gears had grown too large, causing a large oscillation movement in R.A axis.

 In image below right, there is 3 gears A, B and C for R.A axis. They are tightened like this:

(1) Loosen the 3 bolts marked with letter T. Push down the metallic plate so that gears B and C are tightly together. Then tighten the T bolts.

(2) Loosen the 3 bolts marked with letter M. Press at same time the 3 M bolts downwards so that gears A and B are tightly together. Then tighten the M bolts.