FWHM Measurements V. Image Scale

[Stefan Buda]

Originally Posted by Peter Ward

Stefan has mentioned the elephant in the room IMHO.

Tight & deep sky star profiles need far more than good seeing. A smooth stable and accurate mount being first and foremost. As for the optics I have found that using better than "diffraction limited" helps buffer the effect of atmospheric turbulence.

Larger apertures apart from having higher resolution, also stabilise the image position at the focal plane (i.e. the airy disk might look like rubbish, but it stays still and is not dancing all over the place). Mechanical and thermal stability prevent loss of collimation and focus drift during the exposure (e.g. ZeroDur optics do not expand/contract, hence change focus, with temperature changes).

Taking uber-tight and deep images a bit like Formula one racing, a few percent gained from the tyres, suspension, brakes, engine, aerodynamics etc, often add up to a large overall gain. If the visibility is clear and the track is dry (i.e. good seeing) then all the better.

Yes, one should be aware that there is a connection between seeing and aperture size.

Also, a good planetary seeing occurrence may not necessarily be a good one for deep sky imaging, or vice versa - at least for a given aperture. The data is captured and processed in different ways. Image distortions are dealt with in software in the case of planetary imaging while that is not the case when measuring FWHM.

It is the average size of convection cells in the atmosphere, that disturb the wavefront, relative to the size of the telescope's aperture that is important. Convection cells that are smaller than the aperture, shift the photons from the Airy disc into the diffraction rings - larger spots - and large convection cells, relative to the aperture, mainly make the whole spot dance around.

[ChrisV (Chris)]

I always thought Nyquist was just the minimum to be able to reproduce a signal frequency. Doesn't necessarily mean it is very accurate (like way out in amplitude etc). So better off going higher, 3 4 or more5 ????

[alpal]

Originally Posted by ChrisV

I always thought Nyquist was just the minimum to be able to reproduce a signal frequency. Doesn't necessarily mean it is very accurate (like way out in amplitude etc). So better off going higher, 3 4 or more5 ????

It's sort of used as an analogy in this case.
https://en.wikipedia.org/wiki/Nyquis...mpling_theorem

You'd have to understand Fourier transforms and
a lot of advanced mathematics to really get your head around it.
In Mike's case yes -
he has 0.84 arcsecond/pixel. - 0.42 would be better.




cheers
Allan

[Atmos (Colin)]

I’ve always thought of it as being a mid way point being enough samples (more will always give a more accurate figure) and a high enough SNR (required for accurate samples).

The more samples you have the less accurate each sample becomes. This isn’t as important when measuring a star which are very bright but when imaging a shock wave through a nebulae it’s a lot more important. Over sample by too much and you lose contrast. Under sample too much and you get lots of contrast but you sacrifice detail.

[strongmanmike (Michael)]

In the end, I don't need fancy software to convince me of the quality of the seeing, 100's of nights, watching sub frames come down from the same scope, with same camera and same filters, tends to give you a pretty good understanding of what good seeing looks like ....Over the seven years of collecting data from Wallaroo, sitting with my scope , I got to know the seeing out at Wallaroo pretty intimately and I have to say it was very rarely truly mushy and usually good enough to extract some nice detail from small targets and more than occasionally I enjoyed some nice steady nights, where stars in sub frames were nice and tiny .

It is abundantly clear, that the sub frames I have been seeing coming down each night up here at Eagleview, on my lap top, have put a smile on my face, they are clearly as good as or better, than the handful of the very best nights I experienced over that seven years at Wallaroo and regardless of what MaximDL tells me, I am pretty bloody excited...and now wanting to re-image quite a few objects!

Now that I have it however, yes, I will probably use MacimDL to assess the state of the seeing and will be able to quantify when it is really good and when it is not...then, the detail revealed in the finished image, will ultimately tell the story

Looking good outside up here right now actually ....bloody Full Moon

Mike

[alpal]

Originally Posted by strongmanmike

In the end, I don't need fancy software to convince me of the quality of the seeing, 100's of nights, watching sub frames come down from the same scope, with same camera and same filters, tends to give you a pretty good understanding of what good seeing looks like ....Over the seven years of collecting data from Wallaroo, sitting with my scope , I got to know the seeing out at Wallaroo pretty intimately and I have to say it was very rarely truly mushy and usually good enough to extract some nice detail from small targets and more than occasionally I enjoyed some nice steady nights, where stars in sub frames were nice and tiny .

It is abundantly clear, that the sub frames I have been seeing coming down each night up here at Eagleview, on my lap top, have put a smile on my face, they are clearly as good as or better, than the handful of the very best nights I experienced over that seven years at Wallaroo and regardless of what MaximDL tells me, I am pretty bloody excited...and now wanting to re-image quite a few objects!

Now that I have it however, yes, I will probably use MacimDL to assess the state of the seeing and will be able to quantify when it is really good and when it is not...then, the detail revealed in the finished image, will ultimately tell the story

Looking good outside up here right now actually ....bloody Full Moon

Mike

Still though Mike -
numbers give everyone a warm sense of true scientific knowledge.
I would still be interested in your FWHM comparisons here:
https://pbase.com/strongmanmike2002/image/173190596

What was Wallaroo really getting?

cheers
Allan

[strongmanmike (Michael)]

Originally Posted by alpal

Still though Mike -
numbers give everyone a warm sense of true scientific knowledge.
I would still be interested in your FWHM comparisons here:
https://pbase.com/strongmanmike2002/image/173190596

What was Wallaroo really getting?

cheers
Allan

Yes, bean meaning to do that, just haven't had a chance yet

Mike