Go Back   IceInSpace > Equipment > Astrophotography and Imaging Equipment and Discussions

Reply
 
Thread Tools Rate Thread
  #21  
Old 22-01-2009, 07:42 AM
bojan's Avatar
bojan
amateur

bojan is offline
 
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 5,973
Quote:
Originally Posted by Octane View Post
I don't know what algorithm is being used to subtract data.
That is a good point.. however, how do you know other tools are using algorithms that do not destroy original data? It would be nice if, for example, if DSS could be configured with more options...

I was experimenting a lot with my 400D and it seems it uses simple numeric subtraction. Of course, after de-Bayering, which is a huge compromise and an issue with all DSLR's (except some from Sigma, they use Foveon chip).
Some other cameras are known to use funny algorithms to remove noise, though (Nikon, for example, as demonstrated on Christian Buill's website).

(sigh) if we really want to have absolutely raw data, DSLR's should not be used.. only the camera with B/W CCD and standard filters collects data that are not distorted by un-documented algorithms, so they have scientific value.
But, as Frank, said, it is about fun.. we are all amateurs (like it or not), not professional scientits.

Last edited by bojan; 22-01-2009 at 08:16 AM.
Reply With Quote
  #22  
Old 22-01-2009, 12:00 PM
Terry B's Avatar
Terry B
Country living & viewing

Terry B is offline
 
Join Date: Mar 2006
Location: Armidale
Posts: 2,761
Just try it. Put the camera in the dark at room temp and take a dark frame a 5, 10, 15, 20, 30 mins etc. Look at the images. When the dark noise is becoming a significant anount of the image then you won't be able to extract reasonable data from the image no matter how little sky glow you have.
Reducing the temp will let you go longer but the room temp test will be a good baseline.
If you find a 10 min exposure has lots of pixels that are saturated then you need a shorter exposure or you lose too much data.
Reply With Quote
  #23  
Old 22-01-2009, 09:04 PM
batema's Avatar
batema (Mark)
Registered User

batema is offline
 
Join Date: Jul 2006
Location: Sunshine Coast
Posts: 1,809
Camera noise reduction question

I have just found out that my 400d has the noise reduction feature - god I'm slow - and my mate took Horse head 10 min with this feature not activated on his canon 1000d and then activated and the difference was amazing. If you are using this function do you then only stack the images once they have been taken or should you then also take some additional darks? My understanding (maybe wrong) is to increase the signal to noise ratio so if you stack 3 images with Noise reduction on does the signal become three and the noise become one nineth (inverse square law). Any advice much appreciated.

Thanks

Mark
Reply With Quote
  #24  
Old 23-01-2009, 05:56 AM
spearo's Avatar
spearo (Frank)
accepts all donations

spearo is offline
 
Join Date: Feb 2006
Location: Braidwood (outskirts)
Posts: 2,280
Mark,
you wont need darks if you use the ICNR.
frank
Reply With Quote
  #25  
Old 23-01-2009, 07:22 AM
bojan's Avatar
bojan
amateur

bojan is offline
 
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 5,973
Quote:
Originally Posted by spearo View Post
Mark,
you wont need darks if you use the ICNR.
frank
As Frank said...

The primary function of INCR is to compensate for dark current offset of the sensor, which is more or less constant feature for a given sensor, and specifically to get rid of hot pixels, which are pixels with excessive dark current.

To improve SNR, alignment and stacking (addition of pixel-by-pixel signal values) of number of light frames does the trick of reducing noise (random varioations of pixel values) and enhancing the signal (non-random varations).
Additional dark frames taken will not really help here.
Reply With Quote
  #26  
Old 23-01-2009, 04:20 PM
batema's Avatar
batema (Mark)
Registered User

batema is offline
 
Join Date: Jul 2006
Location: Sunshine Coast
Posts: 1,809
Thankyou Frank and Bojan
Mark
Reply With Quote
  #27  
Old 30-01-2009, 04:30 PM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
My 20c worth

Capture darks at varying ambient temperatures and exposures etc, create a library of darks, fllts, biases etc then you can use these over and over again and it doesn't matter whether you switch scopes the same darks can be used.

IMO
Reply With Quote
  #28  
Old 30-01-2009, 04:48 PM
Octane's Avatar
Octane (Humayun)
IIS Member #671

Octane is offline
 
Join Date: Dec 2005
Location: Canberra
Posts: 11,158
This idea is a recipe for a) disaster, and b) crappy final images.

Regards,
Humayun

Quote:
Originally Posted by TrevorW View Post
My 20c worth

Capture darks at varying ambient temperatures and exposures etc, create a library of darks, fllts, biases etc then you can use these over and over again and it doesn't matter whether you switch scopes the same darks can be used.

IMO
Reply With Quote
  #29  
Old 30-01-2009, 07:17 PM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
Quote:
Originally Posted by Octane View Post
This idea is a recipe for a) disaster, and b) crappy final images.

Regards,
Humayun
Everyone entitled to their opinion I've used this process on occassions and wouldn't say that my images have been crappy or a total disaster, albiet I'm still in the learning stage.



and to quote another imager from his NGC2070 post if he doesn't mind

"Thanks Rob

The cloudy area is natural from what I can tell, and it was certainly in the data as it appeared. I took several frames at 3x3 binning first and these didn't have it.

I've used darks from a library I've built over several days - at timings from 0.5sec to 3,600sec and at various temperatures. I matched the library darks with the lights individually. I hadn't taken any flats for this one - yet.

The dessicant has been recently baked, so that sould be OK. __________________
Cheers
Chris
"

Last edited by TrevorW; 30-01-2009 at 07:45 PM.
Reply With Quote
  #30  
Old 30-01-2009, 10:12 PM
Octane's Avatar
Octane (Humayun)
IIS Member #671

Octane is offline
 
Join Date: Dec 2005
Location: Canberra
Posts: 11,158
Again, my opinion, but, I believe any astrophotographer worth his salt would not employ the method of taking darks and flats (this, especially, is absolutely ridiculous) to build a library and then use them, /especially/ when it comes to DSLR imaging. Dedicated astrophotographic CCDs are a different story, thanks to their exacting build qualities.

A true dark matched to your light is the best way to go. Of course, it would be even better if a dark could be taken at the exact same time the light was exposing, but, I don't think we're there, yet (will we ever?).

Just the tip of the iceberg: how would you account for the accumulation and buildup of hot/dead/stuck pixels on your sensor over time, and under very specific conditions, such as temperature and humidity? Certain pixels may show up at ISO-400, but, not at ISO-100.

To me, this means you'd need to build a library of darks (I won't bother to mention flats because the idea of building a library of flat frames is preposterous) which would encompass anything from the absolute minimum time you'd expose your lights (typically 3-20 minutes), at every ISO, say, for at least one hour per ISO and shutter speed. The time taken to build this fallible system is a waste.

You're better served taking your dark frames after your light frames, and your flats (with associated darks) either before imaging, or directly afterwards.

At the end of the day, I guess it depends on how exacting you want to be and what you consider a good astrophotograph. Unfortunately, I think there is but a handful of people who take this methodical approach, yet, their results speak for themselves.

All my opinions. Any insults, implied anger, bad grammar and bad spelling, are entirely unintentionalal. Sorry.

Regards,
Humayun

Quote:
Originally Posted by TrevorW View Post
Everyone entitled to their opinion I've used this process on occassions and wouldn't say that my images have been crappy or a total disaster, albiet I'm still in the learning stage.



and to quote another imager from his NGC2070 post if he doesn't mind

"Thanks Rob

The cloudy area is natural from what I can tell, and it was certainly in the data as it appeared. I took several frames at 3x3 binning first and these didn't have it.

I've used darks from a library I've built over several days - at timings from 0.5sec to 3,600sec and at various temperatures. I matched the library darks with the lights individually. I hadn't taken any flats for this one - yet.

The dessicant has been recently baked, so that sould be OK. __________________
Cheers
Chris
"
Reply With Quote
  #31  
Old 01-02-2009, 07:51 PM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
No offence taken but I believe that in certain situations and if you use the same ISO setting for all your images note ambient temperature at the time etc a library of darks and flats can be build up over time which may be applied to lights as and when needed (by flats I assume your referring to an absolutely white image)

Moreso it is not 100% necessary to take your darks at exactly the same time you take your lights(subs) by more importantly at the same ambient temperature give or take a degree or two as expressed by another noted astrophotographer.

and I quote

"Trev, as for the darks: in 1x1 I do not use any darks at all. and for the 2x2 binned images I try to take the darks at about the same temperature +/- 1or 2 degrees. so, it is not essential to take the darks at the same time, but merely at the same ambient-temperature.

Starlight Xpress CCDs do not feature temperature-control, soI have a thermometer in my obs and I pay attention to it closely

Dietmar"

I also asked:

"Would I be correct in assuming that this can also apply to DSLR photgraphy in that

darks may be taken at at different time than the image frames as long as the ambient temp when taking the darks is within 1 or 2 degrees (or is their a greater temp differental that may be used) of when you took your image frames"

and the reponse

"basically I’d say yes.
All it is a bout is that the sensor is exposed to the same ambient conditions when taking the darks, as it was exposed when you took the lights, trev.
Cheerio,

~~~~~~~~~~~~~~~~~~
Dietmar Hager, F.R.A.S.
www.stargazer-observatory.com"


I was wondering are there others who may have accumulated darks and flats into a library of sorts for use ad hoc on those occassions where weather or something causes them to miss taken these are the same time.


Last edited by TrevorW; 01-02-2009 at 08:14 PM.
Reply With Quote
  #32  
Old 02-02-2009, 08:04 PM
tpeter
Registered User

tpeter is offline
 
Join Date: Feb 2009
Location: Budapest, Hungary
Posts: 1
First of all I'm new to the forum so: Hi everybody!

I'm about to buy a Canon DSLR and I would like to know what is the difference between an 1000d and a 40d. I know there are many, but I'm interested in the low-ISO-and-long-exposure performance only (e.g. 10min@ISO200 w/o darkframe filtering)

What do you think? I google'd a lot but couldn't find such a testphoto from Canon's entry-level DSLR. I would like to use this camera mainly for taking star trail photos.

Any kind of help would be appreciated
Thanks in advance!

-Peter
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 06:20 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
OzScopes Authorised Dealer
Advertisement
SkyWatcher Australia
Advertisement
Celestron Australia
Advertisement
Bintel
Advertisement
NexDome Observatories
Advertisement
Meade Australia
Advertisement
Lunatico Astronomical
Advertisement
Astronomy and Electronics Centre
Advertisement