Help would be much appreciated...

[ryandsimmons]

a 30 second exposure on most DSO's is pretty short, especially at F10 have you thought about getting a reducer?

I will do as soon as I have paid off the current stuff.  And going by the other night I need a dew controller first as well.

Never ending costs...

[Stargazer33]

I will do as soon as I have paid off the current stuff.  And going by the other night I need a dew controller first as well.

Never ending costs...

Welcome to astronomy and astro-imaging in particular!

[wxsatuser]

Great image Mike it certainly makes life easier having fast optics not that it cant be done with slower F ratios it just becomes a battle with the elements and more frustrating.

Alan

Thanks Alan.

Recently it's been about an hour then get clouded out, so the twin shooter helps on that score.

It has'nt been cheap and as others have said, AP can really hammer the ol wallet.

I'm lucky that I can spend a bit on my hobbies and not worry to much but I do appreciate that it's

a bit more difficult for others with normal commitments.

[ryandsimmons]

Cheers for the suggestion of the veil nebula!  Did a test run tonight and it is definitely a future target!  I did 3 targets, all around 20 plights, plus one set of darks / bias / flats.

That gave me this for the veil.  Not impressive in itself, but it really has potential if I do 150 subs tomorrow I think!  (I also did the Hercules Cluster, which just becomes a blob, I think due to minor focusing, and I did Triangulum Galaxy, which just didn't come out visible enough to make it a viable target.)

Ignore the colour and overstretching, I have gone to extremes to see how much data is in there...

veil by Ryan Simmons, on Flickr

[jtlandreneau]

Sorry, still trying to get my head around this.  In particular I am trying to figure out why the Iris Nebula is such an issue, when I can get the Dumbell so easily?  Especially as the Dumbbell is Mag 7.3 which is slightly less than the 7.1 of Iris.

Is is simply that Iris has a bright core, then low luminosity around it whereas the Dumbbell is pretty even?

Apart from the Dumbbell is there anything people would recomend I stand a shot at with F10?  Looking at star maps I can see the Elephant Trunk and North America Nebula (both of which I think are out due to size?) then the ponly slight chance may be Pacman Nebula?

I was really hoping for a Galaxy or a Nebula, but all the galaxies are quite low at the moment (Whirlpool creeps over houses to the west now, and Pinwheel only just comes over houses to the east.  Are my only viable targets for tonight really the Dumbbell, and the various clusters?

I recently tried Iris and had the same problem. I thought with ~30 minutes of total exposure I would start to see at least some detail, but the experts showed me their images at *4 hours* and they were just starting to show the dark nebula

Have you tried Lagoon (M8)? or Trifid (M20)? The ring (M57) is also fun to shoot. Lagoon was the first nebula I snagged a picture of after many pictures of M13 and I wasn't disappointed with only 36 minutes of exposure. 40 minutes on M20 got me clear dust lanes and the beautiful red and blue glow.

Good Luck! Clear Skies!

[dph1nm]

Not so.

You were imaging at f6. The OP was imaging at f10.

Your setup will be capturing well over twice the amount of data in the same time as the OPs. Imaging at f10 is sloooooow.

Not necessarily so! Roughly the same number of photons from the galaxy will be captured by the two setups. If you display the two images at the same scale then they should look the same, unless there is significantly more read noise in the f10 image (which there might be, but there is no way of telling).

NigelM

[dph1nm]

Well no, and with respect - had you taken 9 x 10min subs instead of the 160 x 30sec subs it would have resulted in a nice image, with the signal well above the background noise and which didn't require the bejeezus stretching out of it to see the result. What you have there is a very nice 30sec exposure, but a not-so-nice 10min exposure will win every time.

This is only true if your shorter sub has a significant read-noise contribution. Otherwise sub-length has no effect on the final image.

NigelM

[Pete Presland]

if this is the case and there does seem to be an argument for it, why do maunfacturers make most imaging scopes so fast?

[Zakalwe]

Not necessarily so! Roughly the same number of photons from the galaxy will be captured by the two setups. If you display the two images at the same scale then they should look the same, unless there is significantly more read noise in the f10 image (which there might be, but there is no way of telling).

NigelM

Not sure about that. The amount of detail captured is a function of focal ratio.

This is only true if your shorter sub has a significant read-noise contribution. Otherwise sub-length has no effect on the final image.

NigelM

Longer subs will have captured more faint detail. 1 x 10 minute sub is not the same as 10 x 1 minute subs as far as faint detail is concerned.

[dph1nm]

Longer subs will have captured more faint detail. 1 x 10 minute sub is not the same as 10 x 1 minute subs as far as faint detail is concerned.

Not true. Apart from read noise considerations (and these may well have to be considered, I am not saying you can always ignore them), there is no difference in 1x10 or 10x1 or even 1000x0.01 - you will still collect the same number of photons and the s/n will be the same - there is no 'loss of faint detail' (I am assuming one doesn't use an ISO so low that you have quantisation issues of course).

NigelM

[dph1nm]

if this is the case and there does seem to be an argument for it, why do maunfacturers make most imaging scopes so fast?

I think it is more for practical reasons. Fast scopes are smaller, so easier to handle -  you get a wider field of view (in my option the real reason for short f-ratio imaging) - and their imaging scale tends to be better matched to the pixel (and overall) size of available detectors (which are mostly designed for consumer cameras). It is not easy (or cheap) to get ~1" pixels to cover the FOV of a 14" inch f10 Meade LX200.

NigelM

[Zakalwe]

Not true. Apart from read noise considerations (and these may well have to be considered, I am not saying you can always ignore them), there is no difference in 1x10 or 10x1 or even 1000x0.01 - you will still collect the same number of photons and the s/n will be the same - there is no 'loss of faint detail' (I am assuming one doesn't use an ISO so low that you have quantisation issues of course).

NigelM

Hi Nigel,

I'm not an expert in this so I'm, open to correction.

In theory, a stack of images should gather the same number of photons, but I don't think that that happens in practice. If a part of the image is faint (and let's stretch this a bit), so that only one photon per minute arrives from it, then, again in theory, a stack of 10 x 1 minute will have gathered 10 photons. In reality though, those photons will be removed in stacking- after all the stacking process will not know if the photon is real or as a result of noise.

Stacking will reduce the noise in the image (as a square root of the number of frames). It won't increase the signal though.  To increase the signal, you have to increase the exposure time (up to the sky-limit).

[ollypenrice]

I have no time at all for the short subs argument. I've simply done too many practical tests to believe a word of it. I am absolutely wedded to the 30 minute sub for luminance, colour and NB. For the same total exposure time they wipe the floor with short ones. 

30 seconds at F10 is not going to get many objects but what about the small bright planetaries like the Blue Snowball, which play to the scope's FL. Or the RIng?

F10 or F6 for an object that will fit on the chip either way? I don't think it matters much. You get the same number of object photons but the reducer puts them onto fewer pixels, filling them faster at a cost in resolution. Shown at the same size I don't think there will be much in it. This is the F ratio myth again. If you reduce the F ratio by adding aperture there is no myth, it is faster. If you reduce the F ratio by reducing the focal length, though, enter the myth. A focal reducer is only faster if you want all that it puts on the chip. If you want to crop then don't bother with one.

Olly

[IanL]

Hi Nigel,

I'm not an expert in this so I'm, open to correction.

In theory, a stack of images should gather the same number of photons, but I don't think that that happens in practice. If a part of the image is faint (and let's stretch this a bit), so that only one photon per minute arrives from it, then, again in theory, a stack of 10 x 1 minute will have gathered 10 photons. In reality though, those photons will be removed in stacking- after all the stacking process will not know if the photon is real or as a result of noise.

Stacking will reduce the noise in the image (as a square root of the number of frames). It won't increase the signal though.  To increase the signal, you have to increase the exposure time (up to the sky-limit).

Stacking increases the signal to noise ratio but not how you have explained it.  Stacking increases the signal (by virtue of there being more total exposure time in the stack = more samples of photons). Stacking also increases the noise in the stack, but the noise in increases as the square root of the increase in signal.  In other words, the signal goes up much faster than the noise, but both increase. Assuming all the frames are of the same length, then the square root of the number of frames relationship is a valid way of expressing this, but it is not really the number of frames we are talking about, but rather the total number of samples (i.e. photons) which should be in direct proportion the the total exposure length of all the frames.

It's a purely statistical process, so let's start with something analogous my maths teacher used to try to drill in to our heads at school; tossing a coin.  We want to determine the probability of getting heads, and so we do a number of coin tosses and count the number of heads vs. the number of tails. In the long run, we're going to come up with a ratio of 50:50 (i.e. probability of heads = 0.5).  "In the long run" is maths teacher speak for "it's possible that you could get 100 heads in a row, but it isn't probable".

So the question is, if I sit down and toss the coin 100 times in a row and count the heads, should I expect a different outcome if I sit down and toss the coin 10 times today, 10 times tomorrow and so on for 10 days?  Clearly not, the point that my teacher was trying to beat in to us was that the coin has no memory of the result of the previous tosses, it's just a random process and all that matters is the probability of each "independent event" (which is a statistical term meaning exactly what it says; the outcome each coin toss does not depend on the results of previous coin tosses, getting heads has a probability of 0.5 each time).

Back to imaging now.  The arrival of a photon on a pixel in our camera within a given time period is an independent event.  So let's say we want to know the probability of a photon arriving from the patch of sky that the pixel is looking at within a 1 second period.  Perhaps the probability is 0.5, just like the coin - so in half the seconds, one photon arrives, and in the other half no photon arrives (i.e. in a 10 second exposure, 5 photons arrive).  Okay so what if the probability for a different pixel looking at another patch of the sky was a bit higher, maybe 0.8?  Then we'd expect a total of 8 photons to arrive in a 10 second exposure.  Guess what?  The patch of sky the second pixel is looking at is brighter than the first pixel's patch of sky.

The key point is that each photon has no knowledge of the arrival of previous photons.  Each is an independent event just like the coin toss.  So if you were measuring the probability of a photon arriving from a patch of sky in each second, would it matter if you took 10 x 1 second exposures, or if you took 1 x 10 second exposure?  In a perfect world, it would make no difference whatsoever. You should expect the same result, i.e. lots of short exposures should be just as good as one long exposure of the same total length.  One way or the other you sampled the same number of photons, just like you tossed the same number of coins in the analogy above.

In reality though, the world (and your camera) are not perfect.  You have to contend with various imperfections, the most important of which for this discussion is read noise.  Each time you read an image out of the camera a random number of electrons will be added to the measurement of each pixel by the chip/camera electronics. The upshot is that there will be more noise in a series of short exposures than a single long exposure of the same total duration. The shorter your exposures, the greater the number of them you need to reach the same SNR as the single longer exposures, and 30 second exposures are pretty short for faint targets and you will need a lot more of them than you would for longer exposures.

Hence my original advice that you need both more exposures and the individual ones need to be longer.  Five to ten minutes is a rule of thumb - if you have extreme light pollution you may have to go for much shorter exposures, and if you have super-dark skies, you can certainly go for longer ones.

The second process that occurs in stacking is outlier rejection, but that is implemented as a second, quite separate process from the integration process above.  In outlier rejection you just look at corresponding pixels across all of your frames and reject any that are statistical outliers (i.e. much darker or brighter than the same pixels in the other frames).  These might be aircraft lights, satellites, cosmic ray impacts or (if you dither your images) hot and cold pixels.  There are lots of simple to complex rejection algorithms, and which to use depends on your software, the contents of the frames and how many of them you have.  The most sophisticated algorithms only need about 30 frames to work well, and most need fewer than this, so you shouldn't deliberately shoot lots of short frames in order to help pixel rejection.  (I've posted examples in other threads where I have only five or six long exposures and these stack fine using the right rejection algorithm).