Reducer Myth: On Point

[vlaiv]

3 minutes ago, Rodd said:

Some of us are not as experienced as others.  

 

I am surprised in this contrivance loaded endeavor there is so much resistance.  All parameters can be uploaded to the software--an estimate of noise, etc.  I did not say it would be foolproof--I said it would be useful, and for more visual oriented people (with limited experience) it may prove very useful.  

Rodd  

I would not characterized it as resistance per se. I don't think we are countering you for countering sake. Even for visual comparison you need reference value. How will you know if circle is small enough if you haven't seen how small or big circle can be. And it is simple fact that judging a length of, let say extension tube, is simply less precise than taking ruler and reading off the number. You know that you need 65mm extension, knowing that you need about "this much" (insert eyeballing action here) isn't as helpful.

[vlaiv]

On the other hand, what could be helpful indeed, as you have pointed out - software has access to measure much parameters of image is to use calculations to suggest appropriate resolution for measured FWHM. Or maybe by setting reference value, signal you that you need to improve your guiding (if measured value is larger than preselected reference value - show warning "c'mon you can do better than this"). But that also requires reference value (how good do you expect your gear to work at its best).

[Rodd]

2 minutes ago, vlaiv said:

On the other hand, what could be helpful indeed, as you have pointed out - software has access to measure much parameters of image is to use calculations to suggest appropriate resolution for measured FWHM. Or maybe by setting reference value, signal you that you need to improve your guiding (if measured value is larger than preselected reference value - show warning "c'mon you can do better than this"). But that also requires reference value (how good do you expect your gear to work at its best).

If you input FL, Pixel size, FR, Declination of DSO, Type of filters, OTA type, an estimate of noise based on a few subs, the estimated seeing--or measured seeing,  and what ever else I have missed, the software will present a circle of a size that represents the stars of the identified field at 2,3, 4 arcsec/pix--what ever.  If you want to rely on experience, then you can look at the circle and say--no--1 arcsec/pix FWHM is not realistic due to the seeing conditions.  So I will use the 3 arcsec/pix circle.  You then shoot a sub and see that your stars are  bigger than this circle.  A guide tweak is then performed.  I don't see the trouble--provided all parameters are entered.  Everything in this hobby works this way--enter the paramerers and the software uses them to provide an output

Rodd

[Stub Mandrel]

1 hour ago, Rodd said:

It would be useful; because it would tell you immediately by visual inspection that your stars were too bloated and that you need to refine your guiding.  If the stars are bigger than the circle you need to tweak.  I think that would be eminently useful, and as I intoned, much quicker than looking at numbers.

Rodd

Surely star size is related to seeing and magnitude as well, because on our images they are 2D gaussian curves, that's why brightness translates to size in our images.

I can see this helping with the least-bright stars under ideal conditions.

 

[Rodd]

9 minutes ago, Stub Mandrel said:

Surely star size is related to seeing and magnitude as well, because on our images they are 2D gaussian curves, that's why brightness translates to size in our images.

I can see this helping with the least-bright stars under ideal conditions.

 

Under real conditions.  Jut input the data.  The circle size will be as accurate as any FWHM estimate by software

[vlaiv]

5 minutes ago, Rodd said:

Under real conditions.  Jut input the data.  The circle size will be as accurate as any FWHM estimate by software

What will circle size represent? What measure?

[Rodd]

2 minutes ago, vlaiv said:

What will circle size represent? What measure?

It would represent the target star size for the seeing conditions as a way to assess guiding (and maybe focus)

[vlaiv]

3 minutes ago, Rodd said:

It would represent the target star size for the seeing conditions as a way to assess guiding (and maybe focus)

Here is crop showing 6 different star diameters:

Each one of these images represents the same seeing / guiding, the same FWHM.

In fact it is the same image.

Star size depends on SNR and level of stretch that you have applied. But FWHM is independent of that. Brighter star looks bigger not because it is blurred more or because it was shot in poorer seeing, but because it is brighter and has greater SNR due to greater signal.

Star circle is simply not good measure to use for this.

[Rodd]

12 minutes ago, vlaiv said:

Here is crop showing 6 different star diameters:

Each one of these images represents the same seeing / guiding, the same FWHM.

In fact it is the same image.

Star size depends on SNR and level of stretch that you have applied. But FWHM is independent of that. Brighter star looks bigger not because it is blurred more or because it was shot in poorer seeing, but because it is brighter and has greater SNR due to greater signal.

Star circle is simply not good measure to use for this.

Allyou have to do is input the stretch that maxim or whatever uses to depict downloaded Images--that's how I assess the image quality by eye--I look at the downloaded subs.  In fact the stretch that is applied is ALWAYS the same by default--not a permanent stretch.  If I look at the size of stars to judge--it would be useful if there was a circle for a guide.

[vlaiv]

1 minute ago, Rodd said:

Allyou have to do is input the stretch that maxim or whatever uses to depict downloaded Images--that's how I assess the image quality by eye--I look at the downloaded subs.

And again, you might be shooting 1 minute subs, or you might be shooting 4 minute subs. Each will have different SNR. You can maybe judge background noise (again no simple way to determine what is background if shooting nebula vs star field) and based on that determine how much to stretch. You can even have algorithm to make circle around stars (bright pixels) but it will simply not be reliable measure of seeing / guiding performance. It will depend on focal length and pixel size that you are using, it will depend on exposure length. Using focal reducer will make circles on the screen smaller compared to native resolution even if seeing is slightly worse.

Some time ago PHD used to report guide error in pixels rather than arcseconds. That created a lot of confusion - because error in pixels depends on guider resolution and it is not absolute measure - you can't compare between two setups to judge between figures. Measurement in arcseconds is independent of that. This is the same - measuring in "circles" depends on a lot of factors. Measuring in FWHM is universal and comparable not just between sessions, different star brightness, different exposures, but between totally different imagers and setups.

[Rodd]

3 minutes ago, vlaiv said:

And again, you might be shooting 1 minute subs, or you might be shooting 4 minute subs. Each will have different SNR. You can maybe judge background noise (again no simple way to determine what is background if shooting nebula vs star field) and based on that determine how much to stretch. You can even have algorithm to make circle around stars (bright pixels) but it will simply not be reliable measure of seeing / guiding performance. It will depend on focal length and pixel size that you are using, it will depend on exposure length. Using focal reducer will make circles on the screen smaller compared to native resolution even if seeing is slightly worse.

Some time ago PHD used to report guide error in pixels rather than arcseconds. That created a lot of confusion - because error in pixels depends on guider resolution and it is not absolute measure - you can't compare between two setups to judge between figures. Measurement in arcseconds is independent of that. This is the same - measuring in "circles" depends on a lot of factors. Measuring in FWHM is universal and comparable not just between sessions, different star brightness, different exposures, but between totally different imagers and setups.

Then you input the sub duration......seems like a "no brainer" to me.  I know I could use it (if I new how to create it!)  All important parameters can be input

[vlaiv]

1 minute ago, Rodd said:

Then you input the sub duration......seems like a "no brainer" to me.  I know I could use it (if I new how to create it!)  All important parameters can be input

I've expressed my view of its feasibility, but by all means, you can propose it to developers of your favorite acquisition software - they are the ones that will decide whether it is worth to put effort towards.

[Rodd]

1 hour ago, vlaiv said:

And again, you might be shooting 1 minute subs, or you might be shooting 4 minute subs. Each will have different SNR. You can maybe judge background noise (again no simple way to determine what is background if shooting nebula vs star field) and based on that determine how much to stretch. You can even have algorithm to make circle around stars (bright pixels) but it will simply not be reliable measure of seeing / guiding performance. It will depend on focal length and pixel size that you are using, it will depend on exposure length. Using focal reducer will make circles on the screen smaller compared to native resolution even if seeing is slightly worse.

Some time ago PHD used to report guide error in pixels rather than arcseconds. That created a lot of confusion - because error in pixels depends on guider resolution and it is not absolute measure - you can't compare between two setups to judge between figures. Measurement in arcseconds is independent of that. This is the same - measuring in "circles" depends on a lot of factors. Measuring in FWHM is universal and comparable not just between sessions, different star brightness, different exposures, but between totally different imagers and setups.

BTW--Maxim reports in pixels or arcsec--your choice.  I compare 2 systems all the time--my guide sensor has different pixel size then my image sensor--A simple conversion in my head does it--but if I can spend a few terrible minutes doing that in subfreezing whether, why not have a circle drawn around the stars to aide?  Maxim or PI will tell me the FWHM value of my stars--if the software can do that--it can draw a circle of that value so that subsequent subs can be compared to see if focus or conditions are worsening.  Again...I don't see the problem.  When I m shooting 20 30 min subs--the first sub can be used to draw the circle and it can be used as a guide for the remaining subs.  Each session would require a new initial model.  But we are wasting our time--the fence seems pretty immobile.

Rodd