Mobile unguided, feasibility discussion

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elgol20
elgol20 Regular Member • Posts: 287
Mobile unguided, feasibility discussion
1

hello,

maybe a new thread is good which deals with the question of how far we can go being mobile and use no guiding (for different reasons).

It would be nice to keep this experienced based without gearfights. I have used all kinds of equipment but like to be fast and light and still do deep sky astrophotography. Rudy and others gave nice examples of how precious time under clear skies is to them and we all have different circumstances, reasons and needs to get there. so how good does ist get, more with less?

with astrotracks and lighttracks (II, which I have) there are effects like

PE (periodical error)

wind,

refraction,

stellar aberration,

differential flexure,

polar alignment method (accuracy),

seeing

etc.

which might play a role. the measured result can be a distorted star shape. softwares are able to "measure" FWHM, maximum and minimun. means if max and min are the same the star is round but may be out of focus ... the aim is the get as small and equal FHWM as possible with dependency on focal lenghts, wheight (aperture, sensorsize etc)

The goal is of course to gather as much light as possible so you have limited chances to do this. Increase aperture, exposure time, sensor capacity (area and QE). Then to be able to make DS pictures you easily must go beyond 150mm foal length. some say 400mm is still wide field. with 300 or 400 mm there a lots of objects possible and pixelpitches lead to resolutions of 2 to 3 arcsec per pixel.

For me I want to be able so be as precise as 2 pixels of my D810A using 400mm, talking 5 arcsec. Using a New Atlux in Namibia using 400mm and 10 min exposure were easily done guided even though there were still lost frames due to whatever. I did 2 to 6 hours total exposure per object time with the dslr. and still I see noise...

so for me the question is, how good can it be and this I want to find out with tests. or just do 30 sec or 1 min exposures forever?!

the effects namend above can be discussed, like Roger has already done, me too a bit.

here are some examples of how things can be mounted.

due to bad weather I had 2 chances so far to test things. with a nikkor 300 f/4 PF pics were super sharp doing 2 minutes with no wind (cirrus). so 300mm 2 min is easy but the corners are no good so another lens (used gimbal)!

next time I tried my 400 f/2.8 which wheighs 4.6 kg, with counterwheights, see above and another thread. Did this before a storm so no real test, but 6 min had "OK" results not quite so sharp though, round stars so I guess due to seeing and wind.

guessed or known  quantity of the effects:

PE: Lighttrack 2 arcsec, Astrotrack 5 arcsec

wind: unpredictable, but could be that using wheights is less prone to blur

refraction: depends on height obove horizon and exposure time, increases and limits accuracy, like 1 to 2 arcsec per minute possible not too close to horizon. astronomical refraction is 0 in zenith and about 60 arcsec at 45 degress above horizon, 5.3 arcsec at 10 degrees and can be approximated to 20 degrees above horizon with R=tan (angle obove horizon). for example when doing exposure you start at 45 degrees and end at 44 you get 2 arcsecs of error. hope that is correct ;), for tan(45) is 1 so 1 arcmin. tan(45)-tan(44)=0.034 arcmin=2.0... arcsec

stellar aberration, can be ignored, small enough

differential flexure, well... could be a big problem and depends on many things. I saw it when using the lighttrack  when moving the scope, it was quite strong with the gimbal head, less with the counter wheights. that means i saw a change in position of the polar star which can only be a part of the total diff. flexure of the setup. limits of wedges used, bar for wheights...lots of contributions...

polar alignment method (accuracy), how good is the polar aligment, depends... for me I calculated for the rotational calibration of the polar scope plus viewfinder scale: it can reach to like 10 arcmin accuracy which would result in like 1 arcsec per minute for my setup. better I think is impossible or pure luck. alternative buy a polarmaster...

seeing: widens the stars and can be checked, Pickering steps go up to more than 5 arcsec to no test when seeing no diffration rings in the telescope at all.

etc., ...

-- hide signature --

sorry for the typos 
_____
Stefan

Nikon D810A
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