本帖最后由 qhyphoto 于 2016-4-13 00:13 编辑
这几张照片的作者对短曝多叠的优缺点和具体方法做了一个介绍：
Here is a little summary of my technique of short exposures imaging applied to Deep Sky
Objects - DSOs. It's a mix between 'Lucky Imaging' technique for planetary objects and
long exposure for Deep Sky Objects.
Main advantages to use short exposures for DSO are :
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- No need of auto-guiding system which may be difficult to calibrate, 不需要自动导星
- No need of super heavy, super expensive or super sturdy mounts, 不需要很贵很重的赤道仪
- A 'planetary setup' is enough,
- Shorter exposures = less atmospheric disturbance effects on each frame. 减少大气抖动的影响
So a better resolution can be expected in good or even in medium seeing 可以获得更高的细节和分辨率
conditions.
Main drawbacks :
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- Few light compared to long exposure so the limit magnitude is lower than
traditional long exposure mode. In fact the gap is not as huge as we could
expect if adding a lot of frames.
What are the limits of 'short exposure' ?
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Good question...
For planetary target, limit is Neptune with an Infra-red filter, let's say
0.5 second in worst case. In most cases, less than 100 ms.
For DSOs, few objects (some bright planetary nebulas, open clusters or glubular
clusters) could be imaged with 100 to 500 ms exposition time with the most recent
sensors.
For galaxies, lower limit exposure time will be 1 second to... 'more'. And 'more' is
quickly limited by mount tracking, atmospheric disturbances, wind and dark current
of the camera when uncooled. For me a realistic value for 'more' is 5 seconds.
So short exposure for DSOs : between 100 ms to 5 sec.
As you can see on my first post, for uncooled IMX 174 which have high dark
current it's better to decrease the upper limit to 3 seconds.
Frames acquisition :
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First, align correctly your mount as done for long exposure.
I currently use a software for planetary/lunar/solar acquisition like SharpCap 2.8
and when it will be released, Firecapture.
Use only 16 bit Image format and save as TIFF or PNG.
Use high ain value : 60 - 90. Higher than 90% will give very noisy frames.
Less than 60%, the readout noise will increase.
To reduce the noise pattern that may appear in background of final image, let the
target move a little bit on the sensor. With the Hand Pad, or if your mount has a
bad Periodic Error curve, it will do the job.
Don't forget to make 'Dark' images after choosen target imaging session, as done
for DSO with long exposure. The more frames, the best.
I don't use Bias nor Flat with IMX174. For bigger size sensors I guess that Flat
will be necessary.
Processing :
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Before registration, I manually sort out the good images from the bad ones.
I don't use automatic sort out. Of course it takes lot of time but I'm
sure it's efficient ! You can use automatic process if you prefer.
For registration, I use Registax 5.0 in monopoint mode ('Aligment Method' : 'Default').
An other solution is Autostakkert 2.6 which have optimisation options to reduce
horizontal noise pattern like IMX 174.
For post-processing I use Iris and Photoshop to adjust the curves and contrast.
Albéric
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