Need a bit of help from seasoned observers ...

[vlaiv]

I'm trying to model the view thru telescope - not only size of objects vs field of view (most common simulation type) but also contrast and what would be visible given sky conditions (transparency, light pollution), type of scope and target position in the sky.

Quite a bit of "science" / number crunching went into this, and images that I'm going to present were in no way processed other than by means of number crunching (no brightness / contrast adjustments or curves in photoshop or such).

Overall, I'm pleased with how images turned to be, and I believe they are quite a bit close to what one can expect from telescope view, but I would like confirmation from other observers. In your experience do these images match what you would expect to see under given circumstances?

These simulate best possible view - meaning target at zenith, best ever transparency (actually transparency and target position were not taken into account, so these images are slight over estimates what can be possibly seen but difference is small), with x60 magnification (20mm plossl) and 8" standard dob (26% CO, 94% mirror coatings) under:

a) SQM 18.5 (which is red zone closer to white, Bortle 7 for those not familiar with SQM readings)

b) SQM 21 (green zone almost bordering on yellow, Bortle 4):

Above are best viewed under normal lighting conditions (accurate color reproduction in sRGB color space requires regular illumination - dark room will show brighter view), also your monitor needs to be calibrated properly.

Values used for simulation were from my experience, and images match very well indeed. I think they are slightly over estimating visibility, but I'm now looking them under lower illumination levels than advised (and I know when I'm doing astrophotography processing - image looks a bit dimmer during the day in daylight than it does with soft dim illumination at night). In any case - first image shows hint of spiral structure - something that I never saw from my LP back yard - but just two cores and number of stars is remarkably accurate as is contrast between sky and field stop (I did not do color but blue tint should be more accurate on background sky).

With second image, fact that you can spot the bridge and darkening in it matches what I saw from SQM 21.3 site (according to light pollution info, but could be closer to SQM 21 in reality). Extent of galaxies also matches and so do stars.

In any case, I would appreciate feedback if these two match the reality and your experience.

[bingevader]

I'm having a bit of a Spinal Tap moment at the minute!

How much more black could it be? None more!

I think I see three white dots in the pictures.

Is that right?

My Bortle 4 skies seem to show a few more stars to be honest.

[estwing]

I see good spirals in the first of m5

1..

[vlaiv]

41 minutes ago, bingevader said:

I'm having a bit of a Spinal Tap moment at the minute!

How much more black could it be? None more!

I think I see three white dots in the pictures.

Is that right?

My Bortle 4 skies seem to show a few more stars to be honest.

I'll need a bit of clarifying that - I have no idea what Spinal Tap moment means

In any case, if you are seeing very dark images - it might be calibration of your monitor.

With a casual glance of Bortle 4 image on my monitor - I can see 20+ stars on that image.

I did worry about star rendition, as these tend to be tough to do on images vs real life experience, but this calculator:

http://www.cruxis.com/scope/limitingmagnitude.htm

coupled with star magnitude from Stellarium seem to give good results on above images.

This sort of image will poorly represent some stars because human vision and sensor don't have same sensitivity over spectrum and sensors tend to capture more light in blue / red regions then humans can see in low light conditions.

21 minutes ago, estwing said:

I see good spirals in the first of m5

1..

I do to sort of see them, but how does that compare to your observing experience? 8" scope in red zone. My experience so far was only cores and no hints of arms, so above image is rather optimistic in that regard.

[JamesF]

17 minutes ago, vlaiv said:

I'll need a bit of clarifying that - I have no idea what Spinal Tap moment means

If you weren't aware, "This Is Spinal Tap" is a spoof documentary film about a 70s rock band.  One of the best-known quotes from the film is when the guitarist (Nigel Tufnel) is talking about his amplifiers.  Whilst everyone else has a volume control that goes up to 10, he thinks his are better because they go up to 11.  I think that's what Ben is referring to: the images are supposed to be different because of the SQM values, but actually they're the same.

As the images look different to me, I have to assume Ben's display is not rendering them well enough for him to be able to see the difference :)

(If you want to know more: https://en.wikipedia.org/wiki/This_Is_Spinal_Tap )

James

[jetstream]

In the first case drop any hint of spiral arms and in the second M51 will have a glow about it more so than distinct separated spirals at 21 SQM IMHO. Under 21.5+ it all gets larger, brighter and the 2 spirals arms emerge from the glow nicely.

I use an 8" f3.8 31% CO and a 17mm EP and also a 10" ff4.8 25% CO with a 20mm EP. Increasing the mag will reduce the glow and pull out the spirals better with good eyepieces ie 12.5mm Doc, 10 Delos and the super 10mm BCO. Any high transmission eyepiece works well.

M51 is a bright galaxy pair.

[PeterW]

Interesting, sounds a bit like what Eye&Telescope looked to do http://www.eyeandtelescope.com/eandt.asp%3Fcontent=start.html

I believe that SkyTools has some observer/sky modelling in it to judge likely visibility.

PEter

[vlaiv]

7 hours ago, jetstream said:

In the first case drop any hint of spiral arms and in the second M51 will have a glow about it more so than distinct separated spirals at 21 SQM IMHO. Under 21.5+ it all gets larger, brighter and the 2 spirals arms emerge from the glow nicely.

I use an 8" f3.8 31% CO and a 17mm EP and also a 10" ff4.8 25% CO with a 20mm EP. Increasing the mag will reduce the glow and pull out the spirals better with good eyepieces ie 12.5mm Doc, 10 Delos and the super 10mm BCO. Any high transmission eyepiece works well.

M51 is a bright galaxy pair.

So you are saying that you think first image of 18.5 mag is "over estimated" - in sense it shows more of galaxy then you would expect and in second case it is under estimated - it is less bright view than you would expect?

[bingevader]

Sorry, much better now.

The reference was to the Black Album.

I've never seen that much detail from my bortle 4 site.

[vlaiv]

1 minute ago, bingevader said:

Sorry, much better now.

The reference was to the Black Album.

I've never seen that much detail from my bortle 4 site.

Since I did not change anything - what changed? Your viewing conditions?

What scope did you use on your Bortle 4 site, same as sim - 8" dob?

 

[pete_l]

As you say, the images are highly dependent on monitor calibration (and whatever is reflected onto the monitor from the ambient light in the room). Is it possible to add a calibration strip into some of the dead space in a corner. That way we could all set our monitors to show the same quality of image

[vlaiv]

Well, for one thing - threshold is over estimated in above case. I took 7 photons per one arc minute squared to be threshold for light sensitivity (anything less than that will be effectively black in the image) - but that might not be true figure.

According to this https://www.ukessays.com/essays/psychology/the-experiment-of-hecht-shlaer-and-pirenne-psychology-essay.php

Quote

The results of this experiment revealed that the participants perceived flashes of light that ranged in intensity from 54 to 148 quanta (Hecht et al., 1942).

This is for 10 arc minute diameter disk and 100ms flashes. 10 arc minute diameter disk actually has ~78.54 arc minutes squared. If we take upper bound from above experiment and say 150 photons and divide the two, we get 1.9 photons per arc minute squared should be threshold - that is per 100ms or 0.1s, so it translates into photon flux of 19 photons per second per arc minute squared.

In fact - we can do empirical analysis of that - we can track the following:

at what magnification, telescope aperture and SQM reading we no longer see difference between sky background color and field stop (neither will be completely black, but that has to do with brain and not number of photons - see https://en.wikipedia.org/wiki/Eigengrau for explanation).

 

[vlaiv]

6 minutes ago, pete_l said:

As you say, the images are highly dependent on monitor calibration (and whatever is reflected onto the monitor from the ambient light in the room). Is it possible to add a calibration strip into some of the dead space in a corner. That way we could all set our monitors to show the same quality of image

Sure, I'll look into adding calibration information into image - there is space in field stop (that is supposed to be black, but I might include Eigengrau effect into it?). I might throw in gamma checker as well?

 

[Wiu-Wiu]

They are quite dark, especially for Bortle 7 skies  

I usually mix a bit of red hue into the background when digitalising sketches from lightpolluted areas. Raising the blue tones might also help to make it a bit more lively. (I am also switching to more blue as the LED invasion has started here, and skies are getting grayer)

 

[vlaiv]

I think I figured out where problem might be with above images. Besides photon floor estimation - I assumed that monitor intensity levels are threshold levels for detecting brightness difference.

That means that smallest difference between color #000000 and #010101 (or black and grey of #1 out of 255) is equal to difference in brightness that we can see. But now thinking about it - there is no solid reason to believe that. I just made image composed out of three colors - 0, 1 and 2 pixel values (out of 255) and difference is clearly seen - I did not have to really strain to see so might not be just noticeable difference.

In fact contrast ratio of human eye is said to be 1000:1 (most good displays have at least that much of static contrast ratio). I guess I should go with that number instead - smallest detectable difference (around 3% in photon flux) should be equal to 0.001 after gamma correction (1/1000 instead of 1/256).

Let's see what will I get when I take into account these corrections. I've found a sketch online with good info (source cloudynights) - will try to match that level of detail:

I don't have info on altitude of object or transparency - but I guess we can work with that. That is 16" aperture with SQM20.4.

Maybe I could also switch to SDSS data or some other data source so we can have more objects to compare with actual observing reports.

 

[Wiu-Wiu]

It's a very interesting experiment, I always wing it, but having a more reliable baseline would be awesome. 

I recently got myself a SQ meter, and will do a measurement with every sketch, just to get that basic figure in. 

 

You might want to browse deepskylog.org to compare sketches. it also takes 'contrast reserve' into account for your location, that could help calculating... 

For M51: there are 67 sketches in the database: here

 

[estwing]

An old member on here Mike73 did some fantastic sketches ...

[vlaiv]

Ok, I'm going thru this again, and it does not make sense.

Above we concluded that we should be able to detect 19 photons per arc minute per second kind of flux as a threshold flux or rather we should not be able to tell less than that.

Let's see what it means for 8" dob under SQM 21 skies.

SQM21 skies mean photon flux of 0.0039 photons per second per cm^2 surface from 1x1 arcsec (based on mag0 star emitting roughly 980,000 photons per second per cm^2 - often quoted as roughly one million photons). For 8" dob with 94% reflectivity and 26% CO that boils down to 266.65 cm^2 that means 1.039935 photon / s / arcsec^2.

At x60 magnification - 1 arc second will be equal to 1 arc minute (there is 60" in 1'). This means that at x60 magnification background sky will glow with about 1.04 photons / s / arc minute. That is x19 less photons than we said was a threshold value, yet I believe one can see that above skies are brighter than no light (field stop).

Mag 18.5 skies are x10 in photon count (2.5 magnitudes - that is x10 in intensity) - that means 10.4 photons/s/arcmin, still less than 19photons/s/arcmin - yet we can clearly see background sky as being bright at x60.

Either I'm wrong in above calculations, or initial premise is wrong about number of photons being threshold.

 

[jetstream]

4 hours ago, vlaiv said:

So you are saying that you think first image of 18.5 mag is "over estimated" - in sense it shows more of galaxy then you would expect and in second case it is under estimated - it is less bright view than you would expect?

Yes, in both cases. and

this is overly optimistic at 20.4 in a 16" dob. One issue with some observers and sketchers is reporting things better than reality dictates IMHO and I hope I didn't offend anyone.

[Littleguy80]

I think this biggest challenge is factoring in all the things that can affect the views. Gerry, @jetstream, pointed me at Mel Bartel's list of factors that affect visibility at the eyepiece: https://www.bbastrodesigns.com/visual.html. If you take into consideration that any of these can lead to a reduction in how much can be seen then it starts to make sense that modelling without all of these accounted for would lead to something that it more optimistic than most would see. For example, "Observer experience is worth 2 magnitudes". Not easy to model for but 2 magnitudes is a big difference! 

Perhaps it would help to set out a series of assumptions and use these to calibrate your model against real world experience. E.g. Intermediate experience observers have a 5% drop in what is visible from the model baseline.

[estwing]

Tbh how much is involved and the chance of it all coming together when you drag your scope to give you the views we are promised are slim...I've had endless so so nights.

[vlaiv]

Just returned from very nice grab&go session (first this year, and I'm rather happy that I had such an early start - it's second of January and I already have first light and one lunar session ), and I made an observation that is relevant to this:

Using 4" scope - Mak102 and 5.5mm eyepiece under my light polluted skies - which are usually SQM 18.5 but it was quarter moon out, so we need to modify that SQM rating - without dark adaptation I was able to see difference between field stop black and sky "black" (very close but distinguishable).

Let's say that full moon knocks down about 2 magnitudes, so 7 days old moon (21% bright as full moon) is going to knock down about 0.4mag. We can say that SQM reading was about mag18.

Mag 18 = ~0.06183382 photons per second per cm^2.

Scope is 102mm, CO is 31mm, two mirrors at 94% I presume, and two glass/air surfaces at 99.5% give clear aperture of ~64.9 cm^2.

This means that SQM18 produces ~4.0117 photon per second per arcsec^2 with this scope. FL is 1300 and eyepiece is 5.5 - that gives x236.4 magnification, so one arc second is magnified to 236.4 arc seconds or 3.94 arc minutes.

Flux per arc minute at eyepiece is therefore 4.0117 / (3.94)^2 = ~0.2584 photons per second per arc minute.

This is rather interesting that eye can detect such small photon flux at eyepiece.

Btw, Mak102 is super sharp. I love both ES82 6.7mm and ES62 5.5mm and AzGti is very nice in use. There is little backlash in both az and alt (it is actually quite large, but I'm sure I'll be able to tune it out or at least reduce it to reasonable level).

 

[jetstream]

8 hours ago, vlaiv said:

two mirrors at 94%

I was told by a reputable coater that many reflectivity specs are "optimistic" or "nominal". My top notch mirror coatings are a true 93%, the sec 96%. They also will vary in reflectivity across the surface a little. Mass produced mirrors rarely reach this level and many corrector plates are not near the level you quote at 99.5%... IMHO.

If you take a realistic mass produced mirror and corrector % it might be .90x.90x .95= .77 or 77% transmission. Roland Christian has an essay somewhere noting this stuff. It get worse with poor eyepieces too.... My 24" dob runs .93x.96=89% reflectivity.

@John do you still have Rolands essay link?

[vlaiv]

1 minute ago, jetstream said:

I was told by a reputable coater that many reflectivity specs are "optimistic" or "nominal". My top notch mirror coatings are a true 93%, the sec 96%. They also will vary in reflectivity across the surface a little. Mass produced mirrors rarely reach this level and many corrector plates are not near the level you quote at 99.5%... IMHO.

If you take a realistic mass produced mirror and corrector % it might be .90x.90x .95= .77 or 77% transmission. Roland Christian has an essay somewhere noting this stuff. It get worse with poor eyepieces too.... My 24" dob runs .93x.96=89% reflectivity.

@John do you still have Rolands essay link?

In any case - that just makes things "worse" - less photons reach eye, yet we are able to see background illuminated for some reason, although above quoted experiment says subjects were able to detect much higher levels only something like 60% of the time.

[jetstream]

from Vogel, not sure if you saw this Vlaiv, very interesting to me the stages of dark adaptation. When fully dark adapted my 21.8 sky is grey... and I can see much outside unless in a thick forest etc.

" All cats are gray in the dark "

http://www.reinervogel.net/index_e.html