How many Darks?

[Kevin17]

Hi 

 

How many lights vs how many darks / bias do i need.

 

i normally take for instance, 15 x 180 sec subs of something as I like to get at least an hour per target, the max i seem to be able to go with my uncooled camera on a cold night is about 180-240 seconds per sub, then 15 x darks at the same exposure and about 50 x 0.5 sec bias for stacking. i found these ratios on a website when i was starting out.

 

is this the right amount, am i taking too many / not enough?

 

Thanks 🙂 

[Kevin17]

PS. Im currently sat in garden imaging whirlpool galaxy...

 

[RaDec]

Hi Kevin - I normally take the same number of darks, flats and dark flats and  as lights which seems to work. You don’t mention flats - do you not take them?

However, I think if you have vast volumes of lights, you don’t have to match the numbers exactly. 

Also, what capture and processing software do you use, out of interest?

Keith

[Kevin17]

Hi,

 

Thank you for the reply, no i dont currently bother with light and dark flats as i dont have a way of taking  light flats that works reliably and my pictures seem come out okay without - my camera doesn't suffer from APM glow.

 

i take:

 

-Lights.

-Darks (with the lens cap on, same rotation / focus / temperature as lights  / camera settings and duration)

-Bias (with the lens cap on, same rotation / focus / temperature as lights / camera settings 0.05 second exposure)

 

I stack and process using Affinity (same as photoshop) as it only costs £40and has a stacking tool built in as well as all the functionality as photoshop.

Edited February 27, 2023 by Kevin17

[Kevin17]

i capture using the ASI software suite that comes with the camera it seems to be very easy to use - although i downloaded an update this morning so i have learning to do again...

[Adam Y]

So far I've been doing around 25 of each of the calibration frames.  Maybe 50 if I have hundreds of lights (I'm limited to very short exposures with my alt-az mount and long focal length).

 

Kevin you bring up that you take darks and the bias frames.  Correct me if I'm wrong (most likely) but aren't bias frames just for calibrating the flats?  That being the case, I don't know that you'd need the biases.  Also, I don't think rotation or focus need to be considered for the darks.  

 

Looking forward to seeing that Whirlpool, Kevin!!!  Tonight's project is M81, but I doubt I will get anything.  I've reached the limits of my equipment for some of the fainter stuff, but I'm going to try anyway since it all helps in learning/remembering how to do this stuff in the dark!

[mechanoid]

If you don't use flats, you don't need the bias frames at all.  Both the light and dark frames contain the same bias, so when the dark is subtracted from the light, the bias is cancelled out.

 

The more darks you can take, the better. Subtracting a dark fame always adds noise to the image, so the more dark frames, the less the added noise.  However, with an uncooled camera there is probably a practical limit to the number that can be taken close to the light frames temperature.  My ATIK One cooled CCD camera has such a low dark noise I don't actually use dark frames at all.

 

I find flats essential with my set up (Skywatcher Quattro f4.0 Newtonian) as the vignetting significantly darkens the field corners.  I use a photoluminescent panel, but I made a perfectly servicable diffuser out of the cut out bottom of a plastic tub, with several Tesco plastic bags stretched across it at various angles.  Point the scope vertically upwards at a diffuse sky (grey works very well !) and stack a dozen or so frames.  It would be worth checking the intensity over the full frame.  If it drops by more than a few % you may need flats.  Some image processing software such as StarTools will correct for Vignetting without needing flats.

 

The darks can be taken "on the bench" since they don't depend on either rotation or focus. The problem will be keeping the right temperature.  One thing, plastic lens caps are too transparent.  Use aluminium foil or a metal cap when taking darks.  That usually goes for the telescope lens cap as well, as any external light will get through to some extent.

[TerryMcK]

Flats really are essential to get rid of dust bunnies and minimise vignetting. You could buy a flatfield panel such as these ones from 365Astro https://www.365astronomy.com/index.php?route=product/search&search=aurora to build up your astro equipment.

 

I have the 100mm one and the 160mm one. They are both really good and once you have them you can use them on all future kit. I use the 100mm one on my ZS73 (I acquired that scope first) and Redcat 51. The 160mm one I use on the bigger aperture scopes although you can use the larger one on any.

 

Some people use a tablet like an iPad displaying a white screen and cover the scope with a white sheet to diffuse the light a little more.

 

On Darks, as David has already mentioned, you will find it difficult to maintain the right temperature as you have an uncooled camera. I found shooting darks a problem in my DSLR days but if you can keep them within about 5 degrees of the lights you may be ok. An uncooled sensor does heat up during long time exposures.

 

15 to 20 darks should be enough and if you keep a record of the temperature (it's in the FITS header) you can re-use them over and over with lights of the similar temperature. That way you are not shooting darks every time you go out.

[RaDec]

Hi Kevin - did you get a result with your M51 data?

 

[Adam Y]

I was wondering the same!

[Kevin17]

Hey Adam and RaDec, i actually gave up on M51...

 

I don't know what happened but my data wasn't very good.