andrew s

14 minutes ago, vlaiv said:

I'm not sure what is it that you wanted to say, but somehow it does not seem right

Strehl ratio is ratio of energy or number of photons that should land in ideal optics (meaning perfect wavefront) in Airy disk compared to actual number of photons / energy that is landing inside Airy disk. As such it has nothing to do with physical size of Airy disk - as it represents comparison between ideal scope (of certain Airy disk) and actual scope (of again - same Airy disk). Strehl ratio does not give you any sort of indication of size of aperture and or focal length used. Smaller scope can have both larger and smaller Strehl compared to larger scope (in terms of aperture and/or focal length).

Out of all measures of mirror I would say that two are most important - Strehl ratio, and associated MTF (used for better understanding of performance) and micro smoothness of the mirror / coating.

First one is obvious - how sharp image will look with said mirror. Second one is related to general scatter of light and is important in the same sense baffling and flocking of tube, position of focuser (for newtonian scope) and such - it impacts how dark sky looks thru the scope. We might say that first measure is that of planetary performance, while other one is that of deep sky.

Having said all that - main mirror quality is upper bound of system performance, and in practice it is modulated by other things - collimation and quality of secondary for example, and flocking / baffling / position of focuser (to minimize stray light)...

If you are interested in sharpness of the system - then there is a simple way to measure whole scope as a system. When I say simple - I mean you don't have to take scope apart, have interferometers for mirrors and such - simple planetary camera will do the job. You can choose to use artificial star or do real star testing with it - there is software made for this purpose and it is called WinRoddier, and it enables you to do Roddier analysis of scope, which involves pretty much the same thing as shooting a planetary image - you take in/out focus images, stack them in certain way, load in software and it will give you results.

I agree but what I am saying is that if you have the same focal length then with the same Strehl ratio a larger aperture mirror will give a smaller image than a smaller one.

Thus you can simply compare mirrors of the same size, but when you compare mirrors of different sizes and focal lengths you need to be aware of difference in Airy disk size. That is all.

I could have a 1cm diamter fl 500mm mirror/lens with a Strehl ratio of 99.999 would it be better in resolution that a 100cm diameter fl 500mm mirror/lens with Strehl ratio of 98.9?

Not obvious to me one way or the other without doing the calculations. 

All else being equal the simplest way to increase resolution is to increase the aperture!

Regards Andrew

Just be aware that Strehl ratio is dependant on the size of the Airy disk . For a given focal length a small aperture has a larger Airy disk than a larger one so can have a higher Strehl ratio than the larger one even if the linear image size is the same!

Regards Andrew 

This gives an idea of the scale from here http://www.ls.eso.org/sci/facilities/lasilla/instruments/feros/Projects/ADC/index.html

The left scale is arc seconds and the bottom scale zenith angle.  Blue, green and red are  at 350nm, 500nm, and 920nm

Regards Andrew

Look at this image. The chromatic aberration would be along the the blue dotted arrows. Red towards the arrow head blue on the south pole side.

Regards Andrew

At what altitude were these taken as the atmosphere is chromatic? It gets worse as you approach the horizon. Easy to test by taking images at the same altitude east and west of the meridian assuming an eq mount.

The chromatism is along the parallactic angle.

Regards Andrew 

As with the weather you can run simulations with a range of initial conditions (limited to some region of phase space) and see how it evolves. If you get convergence then you can be reasonably sure that the outcome is stable in that region. If not then it diverges and the system is chaotic in that region.

This is both what weather forecasters and solar system modellers do. If stable we can rely on the forecast if chaotic then not.

If this were not the case we would not be able to predict where are planets will be year on year even if the solar system is dynamically unstable long term.

Regards Andrew 

The best we can do is show that simulations of the evolution of planetary systems can develop to be like ours. They do. So you don't need design just the right range of initial conditions.

Regards Andrew 

3 hours ago, Rodd said:

Ahh--so you are saying--for the second (the first does not apply)  that the F7.7 will have more noise and more signal, and the F3 will have less noise and less signal......or is that reversed?  We're you being clever, or is there an important consideration here?

Rodd

No. The larger aperture is better for collecting photons and will have a better S/N ratio. 

It is just a matter of how much. Collecting photons improves faster than S/N as aperture increases.

Regards Andrew 

10 minutes ago, Rodd said:

If something is theoretically correct, but not practically correct--what good is theory?  If there are 2 theories and only one yields the "correct" value....both can't be correct.   One, while theoretically correct, is in fact not correct in reality.

Both are both practically and theoretically correct. One refers to the mean number of photon collected and the other the variance in the number collected. Two statistics measuring two different things about a signal. Roughly...

Say you were doing photometey rather than imaging then the first gives you (after some calculations)  the magnitude of the star and the second how confident you are in that value. 

For an image the first tell you how intense it will be and the second how much noise it will have.

Regards Andrew 

4 minutes ago, Rodd said:

why is the gain due to aperture less than what one might hope for (which I take to mean what theory says)? 

Rodd

Simply the gain in photons goes d^2 which you thing gives you a parabolic bang for your buck. However due to photon statistics you only get a linear improvement in S/N as it improves as d. Both a theoretically correct

Regards Andrew 

I think image is the correct phrase. We map numbers representing whatever, light I tensity, polarisation, graphs etc. into a form we can sense with our eyes ergo an image.

Regards Andrew 

My only contribution would be to point out that while total photons goes a d^2. S/N ratio goes as d (assuming photon limiting and not read noise etc.) so the gain due to aperture is less than might wish for.

Regards Andrew 

OK @Datalord, it looks like the tools are the problem. With The Sky X you can blind solve down to 10x10 arc minutes with the data base add on. No use to you unfortunately. 

Regards Andrew 

40x40 8s not too extreme. How long are you exposing for and at what arc secs per pixel?

Regards Andrew 

How big is your field of view. This is critical in knowing if you will find enough stars to plate solve.

Regards Andrew 

1 hour ago, Datalord said:

Can you give me a bit more information on this? Are you using PinPoint and/or using an ASA mount and software?

Yeah, no lies there.

The good news is that I have it set up well enough that it literally would make no difference whether my mount was in my living room, backyard or in Spain. This software is incredibly demanding on the exactly right parameters everywhere and the documentation and forums are virtually dead. I just now read a thread from 2016 where an ASA representative was quoted to say that he didn't understand why users who have used the mounts for years needs better documentation. I now understand why they are leaving the amateur segment...

Meanwhile I could just flip the table on the software and just guide it better than I have ever had any mount guide before. 🙄

I have a Paramount ME II and use the native The Sky X plate silver. I don't thing you could use it. I was just pointing out the need for a large database of stars to reliably plate solve.

Regards Andrew 

9 hours ago, Datalord said:

Mine is running right now. It's not going too great. The plate solving is still giving me grief. Even at "just" 2400mm it is very unforgiving to solve. I probably need to figure out a different catalog than GSC.

The GSC does not have enough stars when the field of view gets small. I got the SB data base add-on to reliably solve with The Sky X and my ODK 16.

Regards Andrew 

8 minutes ago, vlaiv said:

Depends how you look at it. Have a read on wiki page that lists voids:

https://en.wikipedia.org/wiki/List_of_voids

KBC void is about x6 as large as Bootes Void by diameter (that would be 6^3 or about x200 in volume)

Funny what a little research can uncover. But pure speculation is somehow more fun!

Regards Andrew 

I wish them well. I looked at the current edition in Smiths today but it was so thin I put it back.

Regards Andrew 

This tread will run and run as there is no right answer.

Modern society is a cooperative activity. None of us could image if we were still a subsistence culture.

Provided you list you equipment and other relevant details, hosting site, source of images, software etc. then it seems to me you have acknowledged the direct contributors to you image and other can judge it on its merits and their own personal philosophy.

What more can you do?

Regards Andrew 

36 minutes ago, Craney said:

I think I read once about an "uni-directional cable"..... ie. it would work one way better than the other  due to the strand coiling and twist.....   could it have been April 1st ??

I decided not to purchase.

It's no joke in some Hi Fi circles even if the signal is ac!

Regards Andrew 

You should see the price of top end Hi Fi cables. Silver, cryo treated snake oil.

Regards Andrew 

3 minutes ago, vlaiv said:

I guess that above example is pretty indicative of how "successful" drizzle is in amateur setups.

 

No. It show how drizzel works with you simulation. Binning is not the same as dithering at lower resolution.  However, I am not sure thus discussion is very productive. 

I know you have strong views back by considerable theoretical understanding I accept we reach different conclusions. 

Regards Andrew 

No image processing  can recover more information than is in the original captures. All it can do is highlight some aspects more than others or present it in a more pleasing form.

Image processing just trades off various factor to suit our visual systems and the eye of the processor.

Yes drizzle is not perfect even with Hubble.

Yes you can use a Barlow but it reduces the field and adds its own aberrations.

Regards Andrew

However  @vlaiv, the processed images are an improvement. While you may be correct from a certain theoretical perspective the process, what ever you might want to call it works in the example gi en.

Regards Andrew