Longest exposures without auto-guiding?

We need to know the sensor to answer. The accuracy requirements for a 294c is much lower than a 183c based on pixel size. Also, you should consider the reducer for your scope. It’s pretty cheap and is actually quite good. With the reducer, careful alignment (you’re already likely pretty good), large pixels (an SLR or similar) and possibly a careful tune on your mount, you should be able to go 1 minute without guiding. But if you have a 183 or other small pixel camer, 30 seconds is likely about it. Especially at 560mm.

Just to give context to what you are asking of the mount, I have a small pixel camera on a reduced 102ed (571mm) and I can’t get good stars on my smaller mount with guiding. I need guiding below 1 arc second and my EQM-35 can’t do that even with a guide scope.

yes, from my experience, that's about the best you can expect. Of course it'll depend on the mount quality and the FOV. 

@NeilM Are you staying away from auto-guiding for a specific reason? or just curious what you can do without it until you buy that gear?

Exposures without guiding are somewhat shrouded in myth, and realistically there is no way to give an objective answer without you measuring the periodic error of your particular mount. There is great variation between samples and so a general consensus of a particular model of mount is only that - a general idea.

For example here is the periodic error of my AZ-EQ6:




This graph shows the periodic error of my mount with the RA axis in blue and the DEC axis in red, as measured by PHD2 (guiding turned off at around the 1/5th mark). From the moment the RA starts to drift to the end spans about 10 minutes. Drift in the declination axis is due to polar alignment, the up and down motion of the RA axis is due to unsmoothness (is that a real word?) of the RA worm gear. All mounts exhibit a periodic error in the RA axis, but how much depends on how much you paid for it and just pure luck. Mine is i think a little bit worse than average, but its not unusual.

As you can see there is really no point in this graph where an exposure of say 1 minute would have good round stars. 30 seconds may be ok for most of the time but there are sharp points where those would be terrible too. Here the conclusion should be that unguided exposures at any significant focal length are impossible (and this is generally the case for all non-premium mounts).

How long you can go depends entirely on how bad your periodic error is. You can measure this by the way. The easiest way is to hook up a guider and let it measure it for you like i have done here with PHD2 guiding assistant, but i am assuming you dont have one or you would be guiding already. The second method is to take a batch of short (say 30sec) exposures for around 10 minutes and then measure star sizes of these and see how many are acceptable and how many are not. Then you get an idea on just how many exposures you can expect to throw away in an average stretch of time. You can repeat the experiment with longer or shorter exposures to get a better idea, but usually even 30sec exposures will have a large fraction thrown out because they are trailed.

If I may make a suggestion, the first question you should ask is how long does my exposure need to be. Then examine if you can manage that exposure time. When one is equipment limited, one should choose the shortest exposure that doesn't affect the quality of your final image. There is an "optimal" subexposure where you do not gain more by longer subs. If you search online for Robin Glovers talks you will find this all in great detail.

So, if there were no noise from your camera sensor, you could use any subexposure you wanted. In that case 600x1s exposures would give an identical result to 1x 600 s exposure, except the latter would require a much better mount, tracking, polar alignment, a shelter from the wind, etc. 

Since we do have noise from the camera, you must consider the read noise. The idea is that your sky background produces noise, so all you need is a long enough subexposure so that the read noise is much less than the sky background noise. A rule of thumb is only 5% of the noise in the sub should come from the camera read noise. This is a good choice, because you  really need to increase subexposures a lot if you try to improve on the 5%. 

By this criterion, the optimal subexposure time, Ts =10*Nr^2/Esky . 

Nr is the read noise in electrons, get it from your camera spec for the gain you are using. Esky is your sky background per second in electrons. Simply take a sub of some length of time (doesn't matter how long) measure the sky background in adu (Nebulosity in free mode can give you this value, just put the cursor in background areas of you sub). You will also need to measure the background in a dark frame and subtract this value. So lets say it was a 10 second exposure, divide by 10, for a 20 second exposure, divide by 20. In other words if you used 10 s for this background measurement, Esky = (LIGHTbackground - DARKbackground)/10 s gives you the Esky per second. Any exposure is fine, just use the same exposure for the dark and light background measurement. Convert the adu value to electrons using your gain from the camera specification in e/adu. And plug everything in the equation.

You now have your optimal subexposure time in seconds. You can run subs longer, but if you do the math you find you have to make massive increases in exposure in your subexposure time to really improve the noise. Note, if you add a LP or NB filter, or change your gain, you have to redo this procedure. The procedure looks complicated, but it actually takes only a few minutes to collect this data, and a few more minutes to do the measurements and calculation.

So if you have a low noise sensor and high light pollution, optimal exposures can be less than 10 seconds, while in a dark sky site with a high read noise, the optimal may be an hour or more exposure. 

There is a bit of a myth that longer exposures are necessarily better. To some extent this was driven by high noise CCD cameras in the early days assuming dark skies, if you have light pollution and a low noise camera your subs may be shorter than you might think.
CS
Rick

Neil, It can get overwhelming to try and get everything working well for astrophotography. You are definitely off to a good start.

Your 80mm is a good scope and this is the perfect time of year for wide nebula targets. IMO, it will be easiest to use a small guide scope and inexpensive guide camera. You can find a good deal on used ones over at Cloudy Nights. Something like a mini guide scope should attach right on your Vixen shoe (where you may have a finder). That will be far easier than an OAG. Then, you can stay on target by using PHD2 or other guide software even with periodic error and polar alignment error (stiill, get polar aligned the best you can).

Good luck,

Randall

One final comment on this thread.  Last night I did my best to improve my polar alignment even further so that I could take 60s subs without auto-guiding.  This was right at the limit  - because any longer and I was getting elongated stars.  Admittedly my target (M101) was closer to Polaris which made it a little easier.  I also tried a higher ISO of 1600 but I wasn't happy with the noise so I think I will stick to ISO 800 in the future with my DSLR.  Many thanks again.

Neil

https://www.astrobin.com/4pwe8u/B/

Wow, thanks Rick.  I do remember reading that the Nikon D750 sensor is somewhat ISO invariant.  That said I did notice a big difference between ISO 800 which I have used in all astrophoto's so far, and 1600 which I used for the first time last night.  I am definitely a long way from being saturated.  Specifically I noticed 'green banding' in the image which I have never seen before.  I've read that banding is common with some Canon DSLR's but I hadn't seen it with mine before.

I always use fresh darks, biases and flats.  Last night I took 20 each of d's, b's and f's.

There must have been something else going on.  I checked that both the high ISO and long exposure NR is switched off in the camera, so is wasn't being influenced by that.  The other factor is that M101 is almost certainly the lowest surface brightness image I have tried to capture so far, and with the partial moonlight and neighbor's lights the light pollution was quite high last night.  So my starting signal to noise ratio was lower than it has previously been.  And finally, last night was the warmest night so far in my short astrophotography journey so that might have played a factor with the noise too.  What I saw may have had nothing to do with the ISO.

It's still early days for me so I will keep experimenting!  I've read that dithering can help with the banding?  I can try that at some point too.
Neil

Hi Neil
Regarding noise on different nights, unless you subs are very short and your sky very dark, a reminder much of the the noise in your image will be from the sky background and from the signal itself, with less impact of the read noise (very little impact of read noise if you sub is long enough). The lowest S/N will be for faint signals close to the background, so will be dominated by background noise. if you background increases 4X (say due to the moon), then S/N in the faint parts will half! Likely this is one effect you are seeing. 

Second, with an uncooled camera dark noise is going to increase dramatically, the sensor counts thermally excited electrons as photons received, so this random noise can very large if it is very hot. Darks are the key, but note if your image was taken at 30C, your darks need to be taken at 30C as well, to be a good match.

Banding comes from fixed pattern noise in your camera. Darks should do a reasonable job to eliminate this, again if you can get the same conditions in your dark. Dithering helps fixed pattern noise (not thermal noise). So if you have a band with different sensitivity that is fixed in your sensor, if the light frame moves around enough, then this fixed noise moves around from light to light. Thus from frame to frame it becomes random. If you are not guiding then you will get some drift from frame to frame. This is actually dithering, though it is not totally random. So do let the image drift a bit from frame to frame. When I am not guiding I also add in a deliberate shift in my light with respect to the target, in addition to some drift, but only every 15 minutes or so, so that the band will be sampled over more pixels. Note if the band in your sensor is wide, then the movement in dither needs to be wider than the band. With enough light frames and some manual dithers I rarely see pattern noise.

Note in processing, in your stacking, if you have more than about a dozen lights do an outlier rejection/correction in your average, not just a simple average, there should be an option in your stacking softwared.

Finally, for one hour you got a very decent image. M101 though big is quite faint, not an easy galaxy to do. To achieve a really good image it will need more total exposure. So I would not judge much from 1 hr on such a challenging galaxy.

Good progress, keep at it, it is a long learning curve, and one that never really ends.
Rick

Back to your request for real experience—

At 2350mm, I can’t shoot longer than 20 seconds, if that, without guiding on a CEM40, which is a respected mid level mount. But I’m on a tripod and dolly, so flex might be limiting things a bit.