Lots of galaxies are magnificent, but dark after a long travel. Camera's long exposure will collect more star light than our eyes. Stars walk along the sky, so camera and telescope should move at the same time.
A 350mm telescope(SkyRover 70SA), a Sky-Watcher HEQ5 Pro mount, a Sony 7a DSLR, a 190mm guide scope and QHY-5FIIM guiding camera. Kstars controls all these devices to capture sky with a raspberry pi 4B. (Thanks for my friends and SUSTecher who enable me to access to telescope and camera.)
Alignment
Use eqmod to control HEQ5 instead of hand controller. Why align coordinates, then do polar alignment?
https://www.myastroscience.com/polaralignment Tutorial on How To Align Polar Scope to RA axis of your mount
Use KStars polar alignment assistant. Tutorial see Youtube Correct vector.
Focusing
Focusing should be precise, and focuser allows us to adjust imaging train position with high accuracy. The required accuracy mainly depends on focal ratio of telescope. One of qualities called Critical Focus Zone that, simply saying, describes the distance where the image is sharp. The below table shows how CFZ varies depending on telescope focal ratio:
| Focal ratio | Critical focus zone in microns |
|---|---|
| f/3 | 22 |
| f/4 | 40 |
| f/5 | 52 |
| f/6 | 90 |
| f/7.5 | 140 |
| f/10 | 249 |
| f/15 | 560 |
Focuser must provide at least 3-4 steps for the distance of CFZ. So if you have for example f/4 instrument with CFZ 40um, your focuser step should be smaller than 10-15 um. For most of focusers torque required to rotate microfocusing knob is about 1-2 Ncm, and for main focuser knob required torque is typically around 10 Ncm.
In practical, I connect a stepper motor(design for 3d printer) to microfocuser shaft. I prefer that focuser can be power in 5v, so USB port can be power and data communication.
An Arduino nano interprets Moonlite's serial communication protocol to step motor command.
Ref: 1. https://indilib.org/forum/ekos/8997-tips-for-auto-focus.html#68380 2. https://astrojolo.com/astrolink-4-0-mini/focusing-stepper-motor-solutions/ 3. http://www.goldastro.com/goldfocus/cfz.php 4. https://github.com/vehnae/arduino-focuser 5. https://indilib.org/forum/general/283-moonlite-focuser-protocol.html 6. https://github.com/fehlfarbe/arduino-motorfocus 7. https://github.com/benkot/PnP-Focus/tree/master/PnPFocus-V2.0 8. https://github.com/jbrazio/ardufocus 9. https://github.com/Hansastro/Focuser (!)
Guiding and Dither
At KStars, set camera to QHY5-LL-M, source
to EQMod. If short drift error occurs, please check source option.
Setting source to camera means that camera
should connect to mount's with ST-4 cable.
How to choose suitable guide camera and camera lens? For main camera, long exposure, based on Nyguist sampling principle, guiding draft should less than half of main camera pixel scale.1
Dither 一能错开和感光元件有关的杂询，二则提高解析力。
- https://www.drlzf.club/post/161.html
- https://pixinsight.com/doc/tools/Debayer/Debayer.html
Imaging
To remove unexpected noise and enhance signal to noise ratio, I take four kind of image: bias field, dark field, flat field.
Flat field: ISO don't effect. The color should do nothing.[8] T-shirt Dark field: Temperature cause dark current. It should be same time as light frame. Bias field: readout noise and bias
Mono camera VS color camera.
Debayer
Pre-processing, Registration, Stacking
If camera's gian is \(a\), bias is \(b\), the pixel vaule \(P\) after \(t\) exposure:
\[P=atI+b\]
\[calibrated\ image = \frac{light-masterDark-masterBias}{masterFlat-masterBias}\]
Shot noise obey poisson distribution. Indeed, pre-processing images with Dark, Bias and Flat do not remove noise but unwanted signal. Only stacking light frame images reduce noise. Dark current doesn't belong to noise,due to it can be predicted.
Before RGB compositon, different channel should be linear match and registration. Processing stacked image: 1. Cropping the image 2. Removing background gradient(or before stacking, not necessary) 3. Colour calibration using photometry 4. Starting to stretch the histogram with asinh function 5. Completing histogram stretch with the histogram tool 6. Remove green noise(not necessary) 7. Change colour saturation
Sigma Clipping is an rejection algorithm. At each iteration, the central value and dispersion (or sigma) of the pixel stack are estimated, and all pixels whose distances from the central value are larger than a prescribed value in sigma units are rejected. Winsorization, named after statistician Charles P. Winsor, is a process for robust estimation of sample parameters in presence of outliers.
Reference from Pixinisight and free-astro.
Ref: https://free-astro.org/index.php?title=Siril:Tutorial_preprocessing
PC setting
Compile and install indi.
Power
For power support unit, I choose bacterial. It need to consider: total capacity, maximum power, supported voltage, weight, charging method, recharge cycle, battery material.
Now I'm using a charging pocket: 187Wh, 12V/3A.
In order to make most use of old charging pocket, how can I
combine multiple battery and keep constant output?
Single Image Processing
Background extraction: When an image contains an intense light
pollution, it is useful to remove it. Siril provides a "Background
Extraction" tool in order to remove the gradient of the sky background.
Siril uses a polynomial up to the fourth degree. The 4th degree
polynomial is generally a good choice, for this model, though the tool
allows for first to fourth degree.
Ref:https://free-astro.org/index.php?title=Siril:Manual
Color calibration, Histogram transfer, Remove green noise, RGB combination.
Mosaics
Before starting the Mosaic Job Creator in Ekos Scheduler, you need to select a target and a sequence file. The Sequence File contains all the information necessary to capture an image including exposure time, filters, temperature setting...etc. Start the Mosaic Job Creator by clicking on the icon next to the Find button in Ekos Module.
On first use, you need to enter your equipment settings including your telescope focal length in addition to camera's width, height, and pixel dimensions. Finally, you need to enter the rotation of the camera with respect to north, or the position angle. If you don't know this value, start Ekos and slew to to your desired target then use the Align module to solve the image and obtain the position angle.
Next, enter the desired number of horizontal and vertical panels (e.g. 2x2, 3x3...etc) and then click Update. The target FOV shall be calculated given the number of panels and your camera's FOV and the mosaic overlap shall be displayed. By default, the percentage of the overlap among images is 5%, but you can change this value to your desired value. You can also move the complete mosaic structure around to fine tune the position of the mosaic panels. When satisfied, click Create Jobs and Ekos shall create an observation job and a corresponding customized sequence file for each panel. All the jobs shall be saved to an Ekos Scheduler List (.esl) file that you can load on any suitable observing night and it will pick off where you left. Before starting the Mosaic Job Creator, check that all the observation job conditions, constraints, and startup/shutdown procedures are as per your requirements since these settings shall be copied to all the jobs generated by the Mosaic tool.[12]
Reference
- https://www.myastroscience.com/
- https://pixls.us/articles/processing-a-nightscape-in-siril/
- http://www.astrosurf.com/comolli/flatfield2.htm
- https://www.rc-astro.com/index.php
- http://www.stargazing.net/David/QSI/ccdperfflats.html
- http://www.renzodelrosso.com/utilita/flatbox.htm
- https://www.zhihu.com/column/HGAstronomyStudio
- https://zhuanlan.zhihu.com/p/57265735
- https://crazygame12345.wordpress.com/tutorial/
- http://www.astro-imaging.com/Tutorials.html
- https://www.youtube.com/watch?v=qYXd0rrAzjg
- Create Super Mosaics with Ekos
- The Holland Observatory
- Manual pre-processing in Siril
- Whole image processing in Siril
- RGB composition in Siril
- https://www.astropix.com/html/astrophotography/toc_ap.html
- Heuristic Processing