bit of a question

[brrttpaul]

imaging tonight with a DSLR . I always start off at ISO 800 now as it seems the best signal to noise ratio, so last night I tried 2 min subs and they were round enough for me, the problem i get is washed out images on anything over a min, ok i know the moon is out, but without lowering the ISO ( and in particular CCD cameras )how do you manage 15 min subs or twenty without it being washed out? I was hoping to increase my subs to 2 or 3 mins but all I will have is washed out subs

[wuthton]

One word.... Skyglow. The less light pollution you have the longer you can go. 

Also the worst source of light pollution is out tonight, currently due south. 

[brrttpaul]

totaly understand that, but given that the majority of people live with skyglow  why bother buying expensive CCDs etc if all they can manage is a few mins

[RikM]

CCDs generally have larger pixels and greater well depth than DSLRs. This means you can expose longer without saturating the sensor. Also people imaging from very dark sites need to use longer sub exposures to get the sky background above the read noise. Using narrow band filters also allow long exposures.

[wuthton]

33 minutes ago, brrttpaul said:

totaly understand that, but given that the majority of people live with skyglow  why bother buying expensive CCDs etc if all they can manage is a few mins

I never do any broadband imaging from home as my LP is about as bad as it gets outside of a city centre but armed with a mono CCD and narowband filters I can achieve some very satisfying results with 30 minute subs.

[JR1987]

I image from outer London in my back garden and I manage 5 minute subs at ISO400. at 800 I cant go over 2 1/2 minutes really.

But I do use a decent LP filter though, without that I bet I wouldnt get over a minute.

 

[wxsatuser]

If your limited to short exposures just take a lot more of them to equal what you would get with longer exposures.

It's total exposure that counts.

[brrttpaul]

Thanks guys, I tried the pinwheel last night @90 sec subs and that was washed out with the moon LP etc. I am thinking of selling the CCD ( I just cant get on with it) and maybe investing in an LP filter. For me it has to be fun and the CCD took some of the fun away from it. Maybe I can drop the ISO down to 6 or 500 and see how that goes

[Starlight 1]

Before I set any times I run PHD for 5 mins and see how good the sky look and work for that .

[JR1987]

11 hours ago, wxsatuser said:

If your limited to short exposures just take a lot more of them to equal what you would get with longer exposures.

It's total exposure that counts.

Is that definitely true though? I thought if your stacking lets say, 2 minute subs of a Deep sky object.. youll pick up all the main areas no problems.. but the fainter areas usually on the outer areas of perhaps a nebula or galaxy may only get picked up every few frames or so, therefore being recognised as noise in the stacking process because they are not in the majority of the subs.

[toxic]

just an example 2 pictures un processed from canon dslr 1st is from the 400D at 50 second exposure and no filter used and the second 1 is the 1100D at 900 second exposure with a baader uhc-s filter well worth getting the filter if you ask me.

[ollypenrice]

12 hours ago, wxsatuser said:

If your limited to short exposures just take a lot more of them to equal what you would get with longer exposures.

It's total exposure that counts.

Well... there are some assumptions here. The number of doses of read noise, for instance, and the ability to get faint signal above the read noise. However, the OP should follow your advice and take lots of subs with a good dither. Dither is vital for DSLRs and should be large scale.

Olly

[wxsatuser]

1 hour ago, ollypenrice said:

Well... there are some assumptions here. The number of doses of read noise, for instance, and the ability to get faint signal above the read noise. However, the OP should follow your advice and take lots of subs with a good dither. Dither is vital for DSLRs and should be large scale.

Olly

No free dinner in this game.

[dph1nm]

In fact one should point out that if  you have a bright 'washed-out' sky there is no point taking longer subs, as you will certainly already be well above the read noise of your camera, and so observing as efficiently as you can.

NigelM

[dph1nm]

22 hours ago, JR1987 said:

Is that definitely true though? I thought if your stacking lets say, 2 minute subs of a Deep sky object.. youll pick up all the main areas no problems.. but the fainter areas usually on the outer areas of perhaps a nebula or galaxy may only get picked up every few frames or so, therefore being recognised as noise in the stacking process because they are not in the majority of the subs.

No - the only recognition of noise which goes on is if you clip in some way (sigma, kappa, median, whatever), but this only removes extreme outliers from the noise distribution. If your fainter areas are buried in the noise they will not be outliers and so all their detected light will contribute to the final stack.

NigelM

[brrttpaul]

That filter certainly works, yes I have been trying to look at dithering. So far the way i see it is I will need a guide scope/ camera, and PHD (I use BYN that has a dither button), i havnt long put a post up of the all in one cameras? i like the look of the skywatcher one the more i read about it (just the small screen is a put off)

[JR1987]

2 hours ago, dph1nm said:

No - the only recognition of noise which goes on is if you clip in some way (sigma, kappa, median, whatever), but this only removes extreme outliers from the noise distribution. If your fainter areas are buried in the noise they will not be outliers and so all their detected light will contribute to the final stack.

NigelM

Ok cool.. So now id ask why would anyone bother doing anything over 2 minutes? Surely there must be some advantage to doing 15 to 30 minute subs.. or people wouldnt do them due to the risk of error being hugely increased with subs that long..

[brrttpaul]

thats what i was trying to get to, the only thing i can come up with is less photos rather than have say 60 one min subs you can have 10x6min subs

[wuthton]

1 hour ago, JR1987 said:

Ok cool.. So now id ask why would anyone bother doing anything over 2 minutes? Surely there must be some advantage to doing 15 to 30 minute subs.. or people wouldnt do them due to the risk of error being hugely increased with subs that long..

There's about a bazillion factors that dictate your optimal sub length such as f-ratio, pixel size, well depth, OSC/Mono, guiding accuracy and the big one... LP. 

Anyone doing 30min subs will be using a mono CCD with narrowband filters. 

[RikM]

I use 3-7 min subs with a DSLR, 5-10 min subs with a mono CCD and LRGB filters or 15-30 min with Ha or OIII filters.