ideal imaging resolution per pixel

[red dwalf]

what is or would be the ideal imaging resolution i should be looking for under a bortle class 6 sky ?

currently imaging with a William Optics gt 81 without the reducer fitted and an Atik 460 mono which gives me a resolution of 1.96" per pixel, for greater detail should i be aiming to get this lower to around 1" per pixel or does my light polluted sky set a limit to the amount of detail obtainable ? 

or should i be looking for a smaller pixel camera with bigger imaging chip and a larger aperture telescope ?

was thinking of a Skywatcher esprit 100 ed with a smaller pixel camera but i`m getting confused by it all and can`t make my mind up, any help gratefully received.

[vlaiv]

LP has no impact on attainable resolution.

There are couple of things that impact system's ability to resolve detail, I'll name major three:

- seeing conditions (very variable and in general very unpredictable, but you can get a sense for average conditions on your location, as well as "best" - few times a year events)

- scope aperture

- mount performance (tracking / guiding)

There are other aspects as well, but are not easily quantified (like Ha narrow band imaging - lower intrinsic resolution due to longer wavelength but less susceptible to seeing, telescope Strehl in particular wavelength - some scopes like ED doublets are not completely color free so some parts of the spectrum loose resolution because of this, ....)

Best way to asses if you need higher resolution is to examine your average FWHM that you get in your subs. You can safely use following formula to assume close to optimum sampling rate for your system / conditions on a given night (I know that is going in "reverse" - finding out optimum resolution after you've done imaging - but it is good for future reference to help you decide if it is feasible to switch to different resolution) - FWHM / 1.6 = sampling rate.

Aperture has impact on resolved detail, and some general guidelines about that can be as follows:

below 80mm - around 2"/px

80mm - 100mm - somewhere between 1.5"/px and 2"/px

100mm - 150mm - in range of 1.5"/px - 1.2"/px

If you want to try to approach 1"/px you will need 200+mm aperture. In reality, for most mounts, most seeing conditions and most amateur scopes you should not go below 1"/px.

Having said all of that, if you plan for example to change to CMOS style camera, you can use one with smaller pixels but it requires certain changes in workflow. You will be sampling below 1"/px in some cases, but there are ways to "adjust" your data to particular resolution after the capture - software binning - which is feasible with low read noise cameras like CMOS. Currently you can do integer binning in software, but there is also way to do fractional binning - although not implemented in software that I'm aware of. This would let you "dial in" sampling rate of your subs prior to stacking and processing to be as close match to captured detail as possible. This is "advanced topic" but we can discuss it in detail if you are interested.

[red dwalf]

many thanks for the reply, sorry for the late reply by me, busy weekend here.

to quote one of your comments,

"Best way to asses if you need higher resolution is to examine your average FWHM that you get in your subs. You can safely use following formula to assume close to optimum sampling rate for your system / conditions on a given night (I know that is going in "reverse" - finding out optimum resolution after you've done imaging - but it is good for future reference to help you decide if it is feasible to switch to different resolution) - FWHM / 1.6 = sampling rate."

i`d like to try this but have no idea on the way to do it, could you, if not too much trouble, please go into detail on how to get the average FWHM in the subs to achieve the sampling rate, as you say at least i`ll know where i am and if any more resolution is obtainable, cheers

 

[vlaiv]

This info is usually available in software that you are using, so it depends on software for stacking / processing.

For example, DSS will list this when registering subs (for each sub) in list of subs:

Note that DSS will give this values in "pixels" and not arc seconds. In above example, let's say that you are working with 0.5"/px, actual values will be:

3.44"
3.35"
...

(so half of what is written, or FWHM in pixels * sampling rate)

PI also has this functionality. I don't have a license for PI so can't say where that option is but I know that it's there and you can use FWHM values in sub selector for example to choose which subs you want to stack.

AstroImageJ will also provide this info, here is one of my Ha subs:

It says that FWHM is 3.19 and this image is sampled around 0.96"/px, so true FWHM value is about 3". Optimum sampling rate in this case would be 3/1.6 = 1.875, so I'm oversampling at 0.96"/px. However this is based on single star, you want some sort of average FWHM on your sub, or even better average across subs to get idea what your sampling rate should be.

[red dwalf]

Yes indeed, I use DSS for stacking and can remember seeing these details for fwhm now, didn't really pay much attention too them in the past, I usually use the score to determine the best subs, many thanks, I'll do a few subs and try to use formula.

[red dwalf]

finally had a minute to re process some earlier images in deep sky stacker, 3 minute lumiance subs, and here are the results for fwhm, what would be my sampling rate ?

 

 

[vlaiv]

1 hour ago, red dwalf said:

finally had a minute to re process some earlier images in deep sky stacker, 3 minute lumiance subs, and here are the results for fwhm, what would be my sampling rate ?

If you are already at close to 2"/px (1.96"/px) then these FWHM values are quite high - and you should not be trying to get higher sampling rate.

If DSS is correct about these values, this means that your FWHM is 3.3px x 1.96"/px = ~6.47". Under these conditions your sampling rate should be about 4"/px. My guess is that on this particular night seeing was poor and guiding was not very tight as a consequence.

 

[red dwalf]

had a little time last night for a few test images as not been out much this summer with virtually no real darkness, 1 minute subs on M31 with much the same values as the other, checked and did a nice polar alignment and guiding seemed to be good.

 

 

[vlaiv]

2 minutes ago, red dwalf said:

had a little time last night for a few test images as not been out much this summer with virtually no real darkness, 1 minute subs on M31 with much the same values as the other, checked and did a nice polar alignment and guiding seemed to be good.

These show improvement in comparison to last post - now you are down to 3.1px average.

Out of interest, what is your guide total RMS (in arc seconds)? And also, could you post a single sub - one with known FWHM (from DSS - for example 1min lum 31-1_006.fit - that has FWHM of 3.0) - I would like to compare it to other FWHM measuring method to see if it is indeed 3.0px?

Btw, if DSS has good FWHM measurement you need about 1.6 FWHM figures to fully exploit resolution that you already have.

[red dwalf]

here is tonight's phd graph and and the single sub

 

1 min lum m31-1_006.fit

[red dwalf]

i can see some light cloud around this evening, graph looked a little flatter earlier, i usually go for 3 or 4 second exposures but last night phd guiding assistant recommend 2 seconds 

[vlaiv]

Let's see what we can gather from this.

First - DSS seems to over estimate FWHM by just a tad - most stars AstroImageJ measures to be around 2.8-2.9 pixels FWHM, but larger ones are 3+px FWHM, so I would say that average is about 2.9 - which is not far off 3.0px that DSS gave (maybe DSS is only looking at the brighter stars).

This gives rather large FWHM in arc seconds - 2.9px x 1.96"/px = ~5.7".

We do have info that you had about 1" RMS guide error, and that you are using 80mm scope. Let's assume somewhat poorer seeing to see if we can get close to that 5.7" FWHM.

According to calculation, you should be getting quite a bit lower FWHM for your stars with this setup in regular seeing conditions (assumed 80mm scope, 1" RMS guide error and 2" seeing) - about 3.4"-3.5" which is ideal for sampling rate of about 2"/px, but for some reason you are getting higher value of 5.7".

Might be that seeing is indeed poor - I see you are doing M31, and I'm guessing it is still in the east when you shoot it? What sort of landscape do you have due east? Might be that there are heat sources there that create local seeing effects (houses, road, large body of water) - that impacts your FWHM significantly?

For the time being, under your circumstances, about 2"/px is probably optimal sampling rate. If you want to move "up" in resolution, it might be worth considering 6" or 8" scope and aim for 1.5"/px, but only if you can get about 1.8-2px FWHM in your subs in DSS with the gear that you already have.

 

[kirkster501]

Remember that when imaging with a wider field of view setup - such as I have been lately with Samyang 135mm lens - your pixel scale is much higher.  I am at 8.5 arcsec/pixel with that set up (and camera) which makes guiding etc easy.  It is also less sensitive to atmospheric effects.

Conversely, I can be at as low as 0.3 arcsec/pixel with my Meade 14" ACF which makes tracking and guiding very much more difficult.  A night of bad seeing (which is usually the case in the UK) and I can forget imaging with such a high resolution setup.

 

[ollypenrice]

1 hour ago, kirkster501 said:

Remember that when imaging with a wider field of view setup - such as I have been lately with Samyang 135mm lens - your pixel scale is much higher.  I am at 8.5 arcsec/pixel with that set up (and camera) which makes guiding etc easy.  It is also less sensitive to atmospheric effects.

Conversely, I can be at as low as 0.3 arcsec/pixel with my Meade 14" ACF which makes tracking and guiding very much more difficult.  A night of bad seeing (which is usually the case in the UK) and I can forget imaging with such a high resolution setup.

 

I think your chances of usefully imaging at 0.3"PP are zero. I'd also say they were close to zero at 0.6"PP.  The Mesu should deliver 0.3"RMS which means about 0.6"PP useful scale at the imager. Then there's the atmosphere: uh-oh!!

Olly

Edited August 27, 2019 by ollypenrice

[kirkster501]

1 hour ago, ollypenrice said:

I think your chances of usefully imaging at 0.3"PP are zero. I'd also say they were close to zero at 0.6"PP.  The Mesu should deliver 0.3"RMS which means about 0.6"PP useful scale at the imager. Then there's the atmosphere: uh-oh!!

Olly

Agreed Olly

My Meade 14" ACF is a visual and lunar/planetary scope.  And what a scope it is too for that purpose.

[red dwalf]

4 hours ago, vlaiv said:

Might be that seeing is indeed poor - I see you are doing M31, and I'm guessing it is still in the east when you shoot it? What sort of landscape do you have due east? Might be that there are heat sources there that create local seeing effects (houses, road, large body of water) - that impacts your FWHM significantly?

 

 

i`m unfortunately in the middle of a housing estate between two big cities, Nottingham in the east and Derby in the south west, plenty of houses around here and in this hot weather i`m sure there are plenty of hot tiles on the roofs, in fact i had trouble cooling the Atik 460 down to -10 degrees even at 2300 hours.

Edited August 27, 2019 by red dwalf

[MartinB]

Whilst imaging scale is far from irrelevant I think it is easy to get too hung up about it.  Think of all the fantastic images that have been taken with 9 micron pixel chips and a Tak FSQ106.   If you are planning  to produce large poster size images then it's definitely worth squeezing the pips,  similarly if you like zooming in to the max to analyze pixellated bits of the image  then fair enough, but, there are other things that may matter more.  For instance, larger pixels will collect more light over a given exposure time meaning lower noise so you will have more faint detail even if it isn't immaculately resolved.  Most of the time you can't get any where near the theoretical optimal sampling rate anyway!  

[red dwalf]

very true, i think we are all striving for perfection under less than perfect skies

[red dwalf]

here is last nights endeavours under another less favourable sky

 

[vlaiv]

14 minutes ago, red dwalf said:

here is last nights endeavours under another less favourable sky

I can see an improvement there, average about 2.5px or so? If you reach something like 1.8px on average - that will be the limit of what your sampling rate can capture, for tighter stars it's worth going with finer resolution than 1.96"/px.

[Xplode]

8 hours ago, ollypenrice said:

I think your chances of usefully imaging at 0.3"PP are zero. I'd also say they were close to zero at 0.6"PP.  The Mesu should deliver 0.3"RMS which means about 0.6"PP useful scale at the imager. Then there's the atmosphere: uh-oh!!

Olly

I have to agree with this, i image at 0.667" from a remote observatory in Spain where conditions are much better than further north like up in the UK and northern Europe.
There's large differences in FWHM and guiding performance from night to night (which i expected)
I've actually thought about adaptive optics to get more out of the nights that are less than optimal.

Something else with larger scope is that they are very hard too cool properly to not get heat plumes.
It's a 14,5" open reflector with cooling fans, the roof is opened almost 2 hours before imaging starts.
The primary mirror still takes a couple hours after imaging starts before it gets to a nice temp....I wonder how long it would take with a pretty much closed construction like SCT/ACF etc...

Edited August 27, 2019 by Xplode

[red dwalf]

last nights were 10 minute subs in HA, so much of the rubbish that Luminance might pick up, HA should reject.

many thanks for the help, much appreciated

[ollypenrice]

11 hours ago, Xplode said:

The primary mirror still takes a couple hours after imaging starts before it gets to a nice temp....I wonder how long it would take with a pretty much closed construction like SCT/ACF etc...

The alternative solution, and one advocated by many experts, is to close the tube thoroughly and insulate it.

Olly