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Noise or Stars?

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#1 Blackbelt76

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Posted 02 July 2020 - 06:29 AM

Been having a ball with a un-modded DSLR.

I'm seeing inconsistent results however.

 

This is a unprocessed image crop after DSS.

I can easily identify some stars, but am wondering if I'm seeing "noise" in what appears to be hundreds of dull stars?

Began dithering lately every 3rd frame which has eliminated walking noise.

 

The odd thing is I shot a portion of the Milky Way at 80mm the same night and did not see this effect.

 

Nikon D-300s w/ 80-200mm; shot at f/4 200mm

iso: 800

Lights: 2min x 30 exposures

Darks: 20

Bias: 20

Flats: 0

Ambient temp: 70F

test1.jpg


Edited by Blackbelt76, 02 July 2020 - 06:30 AM.


#2 Tapio

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Posted 02 July 2020 - 06:37 AM

There seems to be some problems here.

First, focus is off, then it looks like monochrome image so a raw/debayering problem.



#3 endlessky

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Posted 02 July 2020 - 06:38 AM

I just solved your image using Astrometry.net, then opened the solved image in WorldWide Telescope:

 

http://www.worldwide...54692&wtml=true

 

If you play with the slider "Image Crossfade" in the bottom, you can see that those are actual stars. You are shooting into the Milky Way, so it makes sense!


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#4 Blackbelt76

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Posted 02 July 2020 - 06:47 AM

I just solved your image using Astrometry.net, then opened the solved image in WorldWide Telescope:

 

http://www.worldwide...54692&wtml=true

 

If you play with the slider "Image Crossfade" in the bottom, you can see that those are actual stars. You are shooting into the Milky Way, so it makes sense!

wow! OK & thanks.

..and yes, I know they are somewhat OOF but thought :"hmm?..Unresolved stars."

I didn't expect to capture that many. :)


Edited by Blackbelt76, 02 July 2020 - 06:49 AM.


#5 Blackbelt76

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Posted 02 July 2020 - 06:52 AM

There seems to be some problems here.

First, focus is off, then it looks like monochrome image so a raw/debayering problem.

Yep. Thanks.

I think something "slipped"

I focus using "Live View" at max magnify; not the best way, but usually is acceptable.



#6 TOMDEY

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Posted 02 July 2020 - 07:59 AM

They mostly look like actual stars... the threshold where the stars leave off and the noise dominates is unclear. This is always a problem with dense star fields, clusters, especially globulars. It's quite easy to fool oneself into thinking our pictures are actually better than the data warrants. To your credit, you have not exacerbated the judgment by sharpening the image in software. That's something we more experienced guys try to get away with, and generally succeed in fooling ourselves quite successfully!    Tom

 

~click on~ >>>

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  • 80 dumbell nebula data existence comparison.jpg


#7 the Elf

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Posted 02 July 2020 - 08:42 AM

Compare.gif

 

Here is a toggle gif. As my FOV is much smaller I had to crop your image. I took the liberty to sharpen and darken it as well. Due to upload limits I also had to scale both down. Still you can clearly see that these are all stars and there are way more of them.

Here is my full res: http://www.elf-of-lo...l_2019_full.jpg

 

Your zoom lens shows strong astigmatism (bird wings). Get a decent prime lens like the Rokinon 135 or on a budget the vintage SMC Takumar from the 1960/70s. Any other used 80 or 135mm portrait lens from analogue times will be better than a zoom lens. For more details on that look here:

https://www.cloudyni...otography-r3236

 

clear skies!

the Elf


Edited by the Elf, 02 July 2020 - 08:49 AM.

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#8 Blackbelt76

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Posted 02 July 2020 - 09:21 AM

I am somewhat surprised my Nikon 80-200 2.8 doesn't do better. $900 for this lens 7 yrs ago.
I've seen coma in the left corner every time, but other than that it seems ok.
I think a major issue was my focus being off.
I usually do one test shot before committing to 1 or 2 hrs.

#9 endlessky

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Posted 02 July 2020 - 10:03 AM

I have been using my Nikon 70-300mm since I (re)started astrophotography in January of this year. I absolutely hate that lens, but it's all I have, and even if I have no pictures that I am proud of to show, at least I have learnt much in these few months and now I am ready for the upgrade.

 

The lens suffer from huge tilting: star nice and centered at 70mm, wanders 1/4 to the left of the frame as I zoom all the way to 300mm. Result: comet shaped stars in half the picture...

 

The "best" image I managed to take was a widefield shot of the Cygnus area (Crescent, Butterfly, North America/Pelican, Elephant Trunk Nebulae) with an old 50mm f/1.4 lens, that I had to stop all the way down to f/8 to have nice round stars in the whole frame. For comparison, the zoom lens starts at f/5.6, when it is at 300mm, and even if I stopped it to f/11 I wouldn't get round stars (because of tilting). Imaging at that focal ratio just isn't worth it.

 

Right now I am waiting for my 80mm f/6 triplet APO to arrive and then - I hope - it will be a whole different game.



#10 Blackbelt76

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Posted 02 July 2020 - 10:24 AM

I have been using my Nikon 70-300mm since I (re)started astrophotography in January of this year. I absolutely hate that lens, but it's all I have, and even if I have no pictures that I am proud of to show, at least I have learnt much in these few months and now I am ready for the upgrade.

 

The lens suffer from huge tilting: star nice and centered at 70mm, wanders 1/4 to the left of the frame as I zoom all the way to 300mm. Result: comet shaped stars in half the picture...

 

The "best" image I managed to take was a widefield shot of the Cygnus area (Crescent, Butterfly, North America/Pelican, Elephant Trunk Nebulae) with an old 50mm f/1.4 lens, that I had to stop all the way down to f/8 to have nice round stars in the whole frame. For comparison, the zoom lens starts at f/5.6, when it is at 300mm, and even if I stopped it to f/11 I wouldn't get round stars (because of tilting). Imaging at that focal ratio just isn't worth it.

 

Right now I am waiting for my 80mm f/6 triplet APO to arrive and then - I hope - it will be a whole different game.

I feel your pain.

70-300 is a tremendous amount of lens travel to accomplish that.

My 80-200 isn't nearly as bad and I have had some pretty good success with it as long as I take care to critical focus.

I'd love a 400mm 2.8 Nikon prime, but they are way out of my budget at $4,000


Edited by Blackbelt76, 02 July 2020 - 10:24 AM.


#11 endlessky

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Posted 02 July 2020 - 11:10 AM

The way I see it, unless you are buying lenses also because you enjoy daytime photography, from a purely astrophotography perspective, for the price you pay for a high end lens you can buy a fantastic telescope that would most than likely outperform the lens.

 

In the price of the lenses there are many things that are not needed for astrophotography: autofocus, diaphragms, active lens stabilizer, etc.

 

Why paying for all these extras, when for the same price you could get a top end telescope?


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#12 fewayne

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Posted 02 July 2020 - 11:40 AM

If you buy current auto-everything lenses new, I absolutely agree. But that compares apples to hydroponic imported bespoke GMO pomegranates -- old manual-focus primes are significantly cheaper, so The Elf's suggestion is a valid way to save money. Just saw a 300mm Pentax 6x7 lens with 1.4x teleconverter go for $207 on eBay, for example. Will that perform like a dedicated refractor of like focal length? No, but probably way better images than nothing-but-wishes!



#13 the Elf

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Posted 02 July 2020 - 01:08 PM

The strategy in a telescope is to keep the number of elements and lenses as low as possible so that the number of reflections is low and so that the few elements can be aligned properly to each other. As soon as a) the number of lenses and elements is higher or b) elements are being moved on some sort of spiral groves in the housing there is no way keeping them collimated at all. That is why a telescope consists of 2 or 3 lenses in one group at the front. Period. For photography there may be 2 lenses at the front and 2 at the end (Petzval, 4 lenses 2 groups) or 3 in the front, one corrector in the center (4 lenses 2 groups, quadruplet or flat field triplet) or 2 in the front and 1 correction. If you buy a separate corrector it comes with 3 elements.

Now take a look at the zoom lens, a Nikon 80-200:

https://www.lenstip...._80_200_bud.jpg

I count 16 elements in 11 groups, probably 2 moving groups apart from focus. Who on earth could ever collimate this? Most of the time these elements are not in a straight line and often tilted. Sometimes you can manually tilt the extended front barrel. Sure you can, you expect the lens to be moving at 70°C in your car parked in the sun as well as in deep winter. A telescope has no moving lenses at all. There is a focuser at the end, an extending barrel on rolls and steel tracks.

The Samyang/Rokinon 135 comes along with 11 elements in 7 groups, non moving apart from focus. Much easier.

The vintage Takumar comes with 5 lenses in 5 groups:

https://www.pentaxfo...00_4_optics.jpg

The downside of vintage lenses is the lack of low dispersion glasses and other special glasses we have today and often weaker AR coatings. But still, no collimation problem. Only in the high price range lenses compare with telescopes but a telescope with the same performance is cheaper! That happens for anything with a focal length above 400mm.

The worst object you can torture an optical device with is a point light source. They do not exist on earth. Only stars are de facto point light sources. Whatever problem a lens has got, a point light source will show it bright and clear. Welcome to the dark side!


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#14 endlessky

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Posted 02 July 2020 - 04:20 PM

Sometimes you can manually tilt the extended front barrel.

That's exactly what happens to my 70-300mm, too. When it's fully extended, I can manually tilt the whole barrel. Good lens for daytime photography (where everything blends together anyway, and - as you said - there is nothing round and sharp), but completely useless for astrophotography.

 

I am just hoping I am not putting too much hope into my soon to come APO telescope. I would hate to be disappointed by that, too. Is asking for round, nice, purple-less stars really asking too much?!



#15 Blackbelt76

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Posted 02 July 2020 - 09:09 PM

unlike the 70~300 my 80~200 focuses internally.
As mentioned in a couple posts up, it is still moving lens elements so collimating for star use

isnt too good although I’ve had some success with it.

Zooms that physically move outward I would not use for anything.

 

https://www.bhphotov...AiABEgL1GPD_BwE



#16 bobzeq25

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Posted 03 July 2020 - 12:33 AM

Stars are more demanding targets than most anything in terrestrial photography, so even excellent manufacturers making exxpensive lenses are unlikely to design for them.   Zooms are generally not best, too many elements trying to do too many things.  Primes (one focal length) are generally better.



#17 17.5Dob

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Posted 03 July 2020 - 12:44 AM

You are WAYYYY out of focus...Noise/hot pixels would be  tiny, sharp pinpoints...like focused stars "should" look like..

This was my very first  AP, using an EQ mount, (unguided) and an 80mm refractor...that cost $259 ...dump the zoom lens....

48409402816_37f3b979c4_c.jpg



 


Edited by 17.5Dob, 03 July 2020 - 12:48 AM.

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#18 the Elf

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Posted 03 July 2020 - 01:14 AM

 

...dump the zoom lens....

... keep it for daylight.

If you think about a telescope you need at least a FPL53/lanthan doublet or if you want no CA at all you go for an FPL53 triplet. Please note that a regular telescope is made for observing with eye pieces. For photography you need a field flattener or reducing field flattener. I have coached a bunch of people now who had serious trouble getting the spacing right, thus I recommend to look for an astrograph or flat field telescope like a Petzval, Qudruplet or whatever they call it, where the field is flat. This can be used like a lens: camera in, focus, done. This kind of telescope often cannot be used for observing because the backfocus is too short for a diagonal. If you want something for photography only an astrograph is perfect.

Here is a bunch of them, just to give you an idea:

https://www.teleskop...-Teleskope.html



#19 bobzeq25

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Posted 03 July 2020 - 01:16 AM

That's exactly what happens to my 70-300mm, too. When it's fully extended, I can manually tilt the whole barrel. Good lens for daytime photography (where everything blends together anyway, and - as you said - there is nothing round and sharp), but completely useless for astrophotography.

 

I am just hoping I am not putting too much hope into my soon to come APO telescope. I would hate to be disappointed by that, too. Is asking for round, nice, purple-less stars really asking too much?!

There are different qualities of APO telescopes.  Price is a pretty reliable guide.  The very best camera lenses can be even better than a modest "APO" scope.  At a corresponding high price.

 

Round stars require more than just a good scope.  Tracking needs to be excellent, and that's hardly trivial.  Refractors all have inherent field curvature, a field flattener is often needed, and spacing it properly can be an issue.  A few refractors have built in flatteners, quality varies among those also.

 

Bottom lines.  If you want to like your stars, viewing your images at a normal scale will help a lot.  "Pixel peeping" by zooming in is pretty well guaranteed to show something or other.  There are software tools to round stars, I pretty much use one (MorphologicalTransformation in PixInsight), routinely.


Edited by bobzeq25, 03 July 2020 - 01:20 AM.



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