> 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.

This is a comparison of my not modified Pentax k5 DSLR and the same, but modified one without IR-cut filter.

Astrotracer is a great way to take advantage of the Pentax DSLR camera in-body shake reduction mechanism. It is available since Pentax k5 and track celestial bodies by moving the sensor. Here is what is it ablout:

Staying at my home inspires me to make some documentation of my experience. I got a dedicated drive for my astrophotos and start reorganize them by objects. Here is an article of the most recent process I use on my photos. Enjoy: )

For a few months from now Betelgeuse (Alpha Orion) dim so much, that it looks as bright as the stars in the Orion belt. This is not so disturbing and does not mean that cataclysmic events are happening with it.

For my last trip to the Rodopa mountain I decided to cross the limits and to target an object that is hard to believe that a non-professional equipment could catch.