> Some astrophotography myths busted

  • Posted on: 10 March 2019
  • By: tihomiry

Reading a new product that claims to be 100x time more powerful then conventional telescope, I realise that this is just an astrograph with a digital camera that is doing stacked images and show them trought eyepiece or App. So what we can expect from this instrument is something that I already do, but it is put together in one compact place and added connectivity functionality that can contribute to scientific projects.
It has diameter of 114mm and f3.9 and small Sony Xmor sensor IMX224 with high QA. https://unistellaroptics.com

This instrument reminds me that some myths are already busted due to improving architecture of the sensors:
1. The larger the pixel size is the more QA and more light sensitive the sensor is:
This is no more so true as latest sensors are back illuminated and also have microlenses to collect all the photons that hit the sensor.
Old CCD architecture was commonly build interline and this rule was true for them. For example APS-C 10Mpix CCD of Pentax KM has less QA then same size CMOS of Pentax K5 with 16Mpix.
This improvement is especially visible in the recent mobile camera market development.

2. The larger the sensor is the better:
This is true when we want to have a wide filed images but in most cases it is better to have a smaller sensor with higher pixel density and to use it on a faster telescope. The faster telescopes from other site has more compact and lightweighting design. In such way we can also collect faster the data we need or to collect much more of it. This is so called crop factor and it is a good way to get higher equivalent magnification and details without loosing relative aperture. A f5 telescope is a still f5 instrument no matter what size sensor you will put on it. Of course the atmospheric turbulence and the optics quality are becoming critical with the crop factor increase. But if we need to put additional optics to boost the magnification we will also introduce more aberations and light lost. That is way the DSLR lenses with less elements are better then those with more. For example SMC Pentax DA 18-55 was improved in the past to have 11 optic elements instead of 12.

3. Long exposure is better then short and stacked images:
The staked images eliminates noise issues, guiding imperfections, vibration, even atmospheric turbulence and it increases dynamic range.

4. Limited magnitude for a scope is fixed value that is estimated only by the aperture:
Unistellaroptics claims that their 114mm instrument could see up to 16 magnitude. For visual observation this value is around 14.
With better sensor it could collect more light and detect even fainter objects. So the limit is actually the sky background glow and the integration time we use. Of course the larger aperture we got, the easier is to collect light.

To check the polar alignment of your telescope look trough the polar scope. When turning the Ra axis the polar star should move in the exact circle in the eyepiece without any deviation.

Removing the DSLR build in IR-cut filter will give you at least 50% more sensitivity for hydrogen alpha emissions of the nebulas. Also the camera will be more light sensitive at dark allowing you to shoot more faint objects.

We can check what is like to process day shot with similar to astrophotography techniques. It is cloudy in Bulgaria for the next week or so and I took shots of the nearby mountain trough the fog with 200mm lens.

Most of the modern LED backlight screens including Laptops, TV and smart phones use PAM technique for Pulse-amplitude modulation. This means that they switch on and off state very rapidly.

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