Exposure time database ?

[philming]

Hi all !

I was wondering if there was some sort of a database somewhere, with indications of best exposure times for various deep sky objects, depending on the gear used ?

I'm going to start using an Equinox 80ED with a DSLR (prime focus), guided via a double dovetail bar with a 80/400 SW scope and a synguider, all of this on a CG5-GT, just for the record.

Thanx for your help !

[narrowbandpaul]

you can estimate these things for yourself. Maths isnt too hard.

paul

[Horus]

It would be good to have a quick Ref chart for us new to AP.

[narrowbandpaul]

these things are so specific to the setup and site locations and time of observation that such a one stop shop for exp times is completely impossible.

the rule of thumb is as long as possible avoiding

1)saturation

2)trailed stars

the longer the better. Noise will increase, but proper image calibration (darks and flats) removes this to a good extent.

[philming]

Well I'd dearly love to do the math myself ! ;-)

But what should one start with ? What are the items of the equation ?

I guess the magnitude of the object is the first thing to consider of course but what then ?

I should probably see pictures taken by others on an object to object basis and see what they used if i can find the information ?

[Catanonia]

It would be very hard to do due to location, kit and all the variables.

There is no real math to do other than experience.

Look at posts in this section of the forum on targets and most people give details of equipment used and exposure lengths.

Use that to build your own 'knowledge' and combine with your own experiences.

Typically though, for the Messier Objects you should be looking for at least 2 hours data assuming you have relatively dark skies.

[ollypenrice]

I think the database is out there on peoples' websites etc. You can estimate by correcting for focal ratio. Exposure time on extended objects goes as the square of the f ratio so f5 takes a quarter as long as f10, which is why I always say f ratio rules in imaging.

There are other factors; In Ha you might like to use 30 min subs, but... polar alignment, guiding, aircraft, passing cloud... so you might settle for 15 mins. You may well be limited by sky fog if not at a dark site. I never am, here, but am limited by other things first. How sensitive is your camera? A non antiblooming camera will be faster than one with an antiblooming gate.

Then there are compromises like going deeper versus bloating stars.

So in the end I thnk you find your own salvation. Certainly making each sub as long as possible without over exposing or hitting other limits is the way to go in the fight against noise. In terms of how many subs, too many is never enough, to paraphrase Jim Steinman. In a roundabout way I once got 11 hours on M42 and it was better than 5.5 hours, yet the object is bright.

Olly

[Dara]

I think (and this is a novice speaking so prepared to be entirely wrong ) that how much exposure is required can be subjective depending on which aspect of the target is the most desired, e.g. a central area can be bright, and outer areas fainter and more 'wispy'. Exposing for detail in the central area may mean the outer areas are not visible, and vice versa that the central area is over-saturated and not have the detail.

[philming]

That's why you can also easily combine different layers on photoshop...

[swag72]

As a newcomer to astrophotography, I have been concentrating on getting 5 minute subs. That generally puts the histogram slightly to the left and doesn't blow out the highlights, which is what I am wanting.