andrew s

Half in jest we used to say Physics is just maths with boundary conditions.

If you just give the "variables" fancy names e.g. mass, position, field strength, then it is Physics! 

Regards Andrew 

To be pedantic a plane parallel glass element doesn't change the focal length it shifts the focal plane.

Regards Andrew 

Mirrorless cameras have built in lens correction including vignetting so it may be due to that. Not sure if it's done in all formats or if you can turn it off.

Regards Andrew 

Prism diagonals can't be CA free. Try without it.  Regards Andrew 

9 minutes ago, dph1nm said:

Well yes and no - using a larger annulus for the sky can produce a better estimate of the mean sky under the star  (assuming the sky is flat, otherwise you are in trouble), which is important to avoid systematic photometry errors, but the photon noise from the sky, which is the dominant source of noise for faint photometry, is determined by the aperture used for the star, and so is unaffected by the size of the sky annulus. I seem to remember that an aperture of diameter ~1.4x full width half maximum is optimal in terms of s/n.

NigelM

Yes it's a complex issue and depends on many variables and as with all things a balance is required. 

Which noise component dominates is an interesting question. Many of the "standard" answers were derived in the early CCD days and don't reflect the reality of modern CMOS detectors with their small pixels. They suffer significant telegraph noise which is not included in the standard noise equation used by, for example, astroimagej. 

I have never even seen this discussed recently other than on C Buils spectroscopy web site. It is, however mentioned in the pioneering work of Jansick but was a minor issue compared to others at the time.

Another area is scintillation noise in short exposures. 

Regards Andrew 

Astroimagej is very good and I use it for exoplanet photometry. 

Not the simplest to get into but very easy to use once set up.

Regards  Andrew 

4 hours ago, vlaiv said:

For best photometric performance - one wants for stars to be as tight as possible (occupy as few pixels as possible) - while sky background to be stretched as much possible - or occupy as much pixels per arc second squared as possible.

This is not always best practice.

If you have bright stars they are often defocused to avoid saturation rather than shortening the exposure. 

In addition for optimally exposed stars fixed pattern noise dominates and can be exacerbated by switching between a few pixels if guiding is not perfect.  It can be reduced by having the star cover more pixels to average it out. 

Sky background noise can be reduced by by using a good sized annulus. 

Regards Andrew 

11 minutes ago, Rodd said:

I have eliminated the filter positioning as I always shoot flats before any filter change--so the filter wheel is removed from the equation.  Not sure I understand about different light paths.  I will try and collect flats at different elevations, including horizontal, though it wont be easy to devise a way.  

I should have made myself clearer.  The light from the sky is essentially from infinity and follows the design light path through the optics. The light from the light panel is from just above the corrector and is somewhat diffuse this will will illuminate the camera field  and dust particles differently to the sky. 

This results in different vignetting as well as differences in how the dust shades the camera and hence differences in the motes.

I am not clear if you see shifts in the position of the motes or just not fully removed. 

I would still do the first test just to prove the processing worked as expected. Gumption traps abound in my world.

Regards Andrew 

If this were my problem I would try the following.

Take a set of flats with the ota pointing up. Make a master flats. Then take a few flats in each filter and process them as if they were target images images.

Are the vignetting and dust motes removed? If no then either the filter wheel is not accurately positioning the filters or the processing is incorrect. 

If ok take flats with the ota horizontal in at least two ways to maximise and drop or flop. Again process separately as lights if vignetting and motes not removed movement may be an issue.

If vignetting and drop removed then my bet would be different light path through ota for sky and panel as I proposed before. I would investigate this next even if it's not a longterm option.

Regards Andrew 

13 hours ago, Rodd said:

Well, I already know there is mirror flop. I see it when I focus. The focus star shifts.  The mirror is locked down using 3 mirror lock knobs. I added a focuser to the back. I focus using a crayford shorty and the mirror remains locked.  Not sure why there is flop. But it’s definite. Not sure what the fix is, seeing that I already use the fix. But the fix doesn’t work

Why should the focus star shift if you are using a add-on focuser to focus? It is more likely to be play in the focuser or camera side of it than the mirror moving especially as it's locked down. 

Regards Andrew 

1 hour ago, Rodd said:

Not really possible to put it far away.  It is designed to sit on top of the tube.  I don't have a an observatory wall to hand it on.  Besides, then I would need a panel 3x the size.

Then I don't think you will solve your problem.  Why need it be 3 times the size? The angular size of the c11 field is quite small.

Try some twilight flats or just experiment.  If you won't change anything how do you expect to solve the problem?

The high focal ratio of the c11 makes it sensitive to changes in light path.

Regards Andrew 

Rod , I feel your frustration!  For flats to work the flat light path needs to match the light path from the sky as closely as possible.  I suspect you are getting additional light from the Flatman panel either from reflections or flooding in the C11.

Not sure where you have the panel in relation to the corrector but the further away the better to match the sky.

Regards Andrew 

Advise him against getting one it's an incurable disease that leads to licking, sniffing and other disgusting habits as @JeremyS will testify to. You/they have been warned. Regards Andrew 🙃

It's important to remember the statistical spread is not due to the "classical" measurement error but due to quantum nature of the microscopic world.

Regards Andrew 

Not sure what calibration you used but noise in images goes through 3 major stages. 1) read noise, 2) photon shot noise, 3) fixed pattern noise and finally saturation. 

1 and 2 have reduced effect with increasing signal but 3 is proportional to the signal and they only way to remove it is with a good flat.

Not sure it's relevant.

Regards Andrew 

In a thread I can't now locate I was discussing with @ollypenrice that spacetime was seen as geometry. I came across this  

which may be of interest. He uses the term model for what I would call an empirical model as contrasted to a theory based model.

He is a mathematical physicist specialising in string theory but I won't hold that against him.

Regards Andrew 

1 hour ago, vlaiv said:

I'm not entirely sure that it will make a difference in case where you are optimally sampling.

Sure, if you take two images with finite sized pixels, with positional shift between them - then sample values will be different, but those sample values will tied to different locations (sampling positions) so it's normal for them to be different. Question is, when you restore original function using both sets of data and ideal interpolation function (sinc) - will you get same or different thing?

My guess is that you will get the same thing - it will be original function produced by optical system convolved with function representing pixel shape and sensitivity over that shape, and if I'm not mistaken - convolution is shift invariant.

I had a good paper on this which of course I can't find now. 

Shift invariance is fundamental to all linear imaging theory. Hence the point sampling of Nyquist. Things like MTF, convolution and optimal sampling don't formally apply with integrating areal detectors.

I do have a paper showing this via simulation for slit spectrographs if your interested.

I doubt that with pixels getting ever smaller and the over sampling it makes possible that there will be any practical difference. 

Regards Andrew 

PS "Theoretical Bases And Measurement Of The MTF Of Integrated Image Sensors" is behind a pay wall. However the abstract makes my point 

"By analogy with optics, the spatial resolution of image sensors is generally characterized by the Modulation Transfer Function (MTF). This notion assumes the system being a linear filter, which is not the case in integrated image sensors, since they have a discrete photoelement structure. These sensors must in fact be considered as integral samplers. Their response to any irradiance distribution can thus be computed, knowing the pitch of photoelements and using a characteristic function. This function is more or less similar to the MTF. Once exact theoretical foundations have been defined, a computer simulation enables the various MTF measuring methods to be compared this makes it possible to rule out er-rors inherent to experiments. The most accurate and reliable method appears to be the knife edge method, applied with a relative displacement of the sensor and of the image. This avoids the occurence of aliasing phenomenon. Experimentation of this method for measurement of the CCD sensors characteristic function, which we call MTF as agreed, is described. This method also makes it possible to evaluate the transfer inefficiency of shift registers."

2 hours ago, ollypenrice said:

A more general point: we know that stars are 'exceptional cases' in the world of optics. I wonder to what extent we can rely, therefore, on stellar FWHM as an indicator of the resolution of non-stellar details?

Olly

Optical theory would say yes you can. Regards Andrew 

PS On reflection that's not quite true as a CMOS or CCD sensor has finite sized pixels and is thus not shift invariant.  That it it depends exactly where the point source (star) fall on the pixel.

However, for all practical purposes it does as other methods like MTF are similarly impacted.

I think you should get yourself a planesphere, a beginner's star atlas or the maps from an astronomy magazin or a similar app based map.  That way you can identify them yourself and begin to learn your way a round the sky.

Regards Andrew 

The expansion of the Universe does indeed result in a tension pulling things apart.  However,  it is orders of magnitude weaker than the electromagnetic force holding atoms and rulers and other objects together.  

Similarly,  gravitational bound systems like the solar system and cluters of galaxies are bound by orders of magnitude more strongly than the expansion induced tension.

It's only when object are much much further apart that the tension is greater than the gravitational attraction and they move apart with the expansion. 

Regards Andrew 

3 hours ago, Alien 13 said:

I had always thought that the Universe was tiny, smaller than a photon or electron...

Alan

I like this from William Blake:

"To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour."

Regards Andrew 

4 hours ago, VirgoCluster25 said:

if the universe is already infinate then is the observable universe expanding at the speed of light or do you mean that you universe itself is infinate because its infinatly expanding? Because if its because its infinatly expanding then i mean at the moment its 93 billion ly in diamiter. Its not exact but its estimated.

If the Universe is spatially infinite then it always was infinite and this is the case in the currently accepted concordance LCMD model.

If you think of @vlaiv grid then it always extended to infinity. What is changing is the distance between the dots. This is called  metrical expansion. 

Regards Andrew 

36 minutes ago, ollypenrice said:

I like dining out!

More seriously, I'm not sure that Newton regarded space simply as geometry. I may be quite wrong but did he not conceive of it as having a fixed existence, rather as we conceive of grid lines on an OS map? Yes, these are expressions of geometry but they have an absolute relationship with church spires and crossroads, etc. Didn't Euler challenge Newton with a question like, 'What would happen if the Universe moved two feet to the left?' or something like that? It's a very long time since I read Westfall and I'm hardly on home turf with any of this stuff.

Olly

I have no idea what Newton actually thought but his theory is one in which space and time can be coordinated with a Euclidean geometry.  In retrospect I was reflecting the  modern view of Newtonian spacetime rather than his.

I am not familiar with Westfall but I know his absolute spacetime was challenged at the time from many angles but its success at predicting the motion of the planets rather confounded them.

Even Einstein's formulation of Special relativity and General Relativity along with Bohr's quantum mechanics are now seen as outdated.

It reminds me that what we think of as fixed changes and morphs with time. The idea of the election of JJ Thompson in 1897 is very different to that of QED.

This thread has weak anti-parallels with Plato and his theory of forms. From Wiki 

"The theory of Forms or theory of Ideas is a philosophical theory attributed to Plato, that the physical world is not as real or true as timeless, absolute, unchangeable ideas."

Regards Andrew 

Newton viewed space and time as absolutely (the same for all observers) Einstein viewed them as relative (different for observers in motion with respect  to each other) however, both are geometry.  They are models, useful representations in different circumstances. No fabric to rip or weave just a branch of mathematics. 

In this sense the physicists spacetime is as real or unreal as the mathematicians pi. 

Philosophy has dined out for decades on debating what terms like real and nothing mean without any accepted progress. 

Regards Andrew 

PS how can nothing be contained by anything? For if it could it would be the something contained by the anything and so not nothing any more.

13 minutes ago, ollypenrice said:

So that NOTHING! Brilliant, we got there. But... erm, what does it look like???

lly

 

I already answered that.  It doesn't look like anything 😕

Regards Andrew