Do your 300 or 180 sec subs look like this ?

[leemanley]

Have been playing around with some data from a few weeks ago, conditions were great, calm, cool and moonless. 
I got to thinking though how come my 300's look fairly washed out, so I'd like to ask if this is normal ? I can't say that I have seen another example of a sub like this to compare to, so if anyone has anything similar, would they be prepared to share on here ?

I used my Canon 550d, iso 800, tungsten mode, on the 200PDS on both examples, as you can see the 180 isn't suffering that badly  
 but the 300 really starts to ramp it up !

I live in a pretty dark area, the nearest town is 4 miles low down to the south, so I don't suffer too badly from LP.

It would be interesting to see some others for comparison.

Thanks Lee

[leemanley]

ahh yeah, the dreaded flightpath, I'm just outside Knutsford.

[Dave_D]

ahh yeah, the dreaded flightpath, I'm just outside Knutsford.

ouch... right in line with the runway...

[Tinker1947]

is there a reason for using Tungsten,  as a posed to Daylight setting.

[Knight of Clear Skies]

Yes, this is normal, the longer the sub the more light it collects. Even at a dark site the sky isn't black, faint stars and skyglow contribute some light. It's a case of tuning your ISO setting to the exposure length and sky conditions. The advice I keep hearing is to check the histogram, the peak should be no more than a third of the way across. I believe this helps prevent over-exposing some areas of the image and losing data.

[tingting44]

you want to exposure to saturate the histogram about 1/3'rd. that will give you your exposure time you need....at a guess im guessing that 2nd image is around the half way mark... a decent LP filter is all ways good to up your exposure time

here is a RAW file of M39 - 240sec sub from my very light polluted garden

[tingting44]

and here is a single 600sec sub from my dark site of M31

[lukebl]

That doesn't look too bad at all. Mine normally look pale like that till you tweak the levels.  I tweaked it a bit for you. You need to take a lot more, along with darks and flats, and stack them to get rid of the noise.

[tingting44]

is that a single sub Luke? you did not say what exposure it was ?

do not worry i see you copied ops original image lol

[Christopher Davenport]

Mine always go like this, especially the 300sec and up.

Three ways to combat it.

LPS filter, lower ISO, or processing.

I mostly do the later, lp changes for me through the night.

[ollypenrice]

You have to cut back the black point in order to reset the background level but, at a dark site, long subs are the thing.

Oly

[Knight of Clear Skies]

You have to cut back the black point in order to reset the background level but, at a dark site, long subs are the thing.

Oly

Is it just a case of reducing the amount of read noise in the stack Olly or is there more to it than that?

[wxsatuser]

I normally go for 25% of the histogram but up to 40% should be ok.

I always use an LP filter, an Astronomik clip with lenses and a Hutech with a scope.

Here is a RAW file.........Canon 60Da 200mm L f4.5 iso1600 for 300secs

Second image is same file stretched.

RAW

Stretched

[Pete Presland]

unfortunately light pollution is something we all have to put up, unless you can travel to a dark site. i use a 200PDS and my images look almost identical to yours.

[wxsatuser]

To expand this a bit here is a RAW with an Astronomik clip.

Second image same file but channels aligned

Third image a bit of stretching.

Canon 60Da Sigma 105macro f4. iso 1600 300secs

[leemanley]

I've been working solid for the last 2 days so haven't been able to catch up on this thread of which I really appreciate all your replies, I had a look at the histo on the 300 sec sub and yes, it is sitting just right of centre, the 180, about a 1/5 of the way from the left, this tells me in similar conditions I should be aiming for around 240 secs for a bit of balance, I have stacked an hour and half of 180 subs and I must admit it is quite hard work trying to pull out any good stuff (its supposed to be M52 and the Bubble) I knew I was pushing it a bit but thought I might of got a hint of it, but had no luck.
I know the limitations of my unmodded cam, something I am in the process of addressing at the mo, but maybe I'll go for an LP filter for now

is there a reason for using Tungsten,  as a posed to Daylight setting.

I read somewhere a while a go that it was the best setting, have tried daytime and it came out quite orangey !

You have to cut back the black point in order to reset the background level but, at a dark site, long subs are the thing.

Oly

I need to study this Olly, its on the growing to do list 

[wxsatuser]

If you images are orangey with daylight you must have moderate LP at least.

Without an LP filter here my subs would be well orange.

The only one I have as a comparison is this 13 sec RAW sub done without an LP filter.

The orange glow shows in this short time and would be quite obvious on long subs.

[dph1nm]

One always has to be careful about interpreting DSLR images displayed on the web. They will have been scaled up in some non-linear fashion from the original RAW. I have a Canon 1000D - a single sub of a minute or so can appear bright orange on the camera screen or in Canon;s DPP software. But there is a little button in DPP marked 'linear' which turns off any scaling - my orange images immediately go jet black! That is what the RAW image really looks like.

NigelM

[Tinker1947]

Is your DSLR Modded.......

[Cozzy]

Leemanley,

just for info, the Bubble you are after is out of shot (bottom middle) of your image I think.

Tim

[wxsatuser]

Thanks Nigel.

Yes the real RAW files will be almost black with the histogram pushed hard up to the left.

I suppose it would be hard to compare all black files so only showing these as a comparison to the OP.

Hopefully not to much confusion on that score.

[leemanley]

Leemanley,

just for info, the Bubble you are after is out of shot (bottom middle) of your image I think.

Tim

Your right Tim !

[leemanley]

Managed to get out last night, spent from 11 o'clock faffing about with alignment issues to finally start shooting at around 1.45 am !
Taking on board the histogram info ( which I always knew about but never put into practice for some reason !) I made a start on gathering some data on an old favourite that had come into view above the trees, that being M45.
I took a shot at iso 800, for 3 mins, checked the histo, it was tight up to the left, so ramped it up to 5 mins, below is the result



I left the scope running and went to bed.

I have got 32 of these currently cooking in DSS, watch this space for the outcome.....................

[ollypenrice]

You now need the Levels and Curves routine. Try Rob's explanation; http://www.middlehillobservatory.co.uk/articles-primers/Levels%20and%20curves.htm

Olly

[groberts]

One always has to be careful about interpreting DSLR images displayed on the web. They will have been scaled up in some non-linear fashion from the original RAW. I have a Canon 1000D - a single sub of a minute or so can appear bright orange on the camera screen or in Canon;s DPP software. But there is a little button in DPP marked 'linear' which turns off any scaling - my orange images immediately go jet black! That is what the RAW image really looks like.

 

Interesting, but are you saying you should put your subs through this process or just that it is an example of what the underlying RAW picture is like and, subject to other points noted herein, it is resolved through stacking and post-processing?