Hardware and software advice needed for upgrading to guided imaging

[ianpwilliams]

Another couple of questions:

I'm hopeful that I'll be able to keep the tripod and toolbox in the shed, keep the rest in the flat, and then each session I would carry the scope (cameras attached), with the mount and batteries in a ruck sack. If that is the case then would I still need to use the dehumidifier bags in the shed?

Also, when it comes to camera darks, they would take ages because the lights would hopefully be long. So would there be any issue leaving the camera in the shed on a chair overnight? I thought I could leave it with the remote timer attached and fresh batteries, taking lots of darks, and then both batteries would run out overnight at some point. If I did do this though then maybe I would need the dehumidifier bags after all for the sake of the camera?

[cjdawson]

I'd not try for M31 at the moment.  Try M42 instead!   It's a much bigger target.  Also you can use the trapezium to guide on, so again much easier all round.

Humidity promotes corrosion, so I'd still use them anyway.  As for the camera, there's no problem with taking the camera off the scope to take darks.  Just make sure to put the lens cover on, and put it into a box with a sealed lid - ideally a dark box.   However, the whole point of dark frames is that they should be taken at the same temperature as the light frames.   I'd say that it's probably not the best idea to put the camera somewhere warmer for the dark frame time.    In addition, is your shed secure enough to leave an expensive camera?  Thieves might not be looking for a camera, but they'll certain grab and go if they see something shiny like that.

[ianpwilliams]

I was king of hoping to go for a galaxy rather than a star cluster, and I already have a nice unguided image of M42. That said, this may well end up being a dry run anyway, so maybe I should go with M42 instead.

For the darks, it would be more convenient to leave the camera in the shed as it's lockable, and it's never been broken into before either. And I kind of assumed that it would be a similar temperature as outside. I can't leave it in a box because it's a shared garden. If not the shed then I guess I would have to do them with the camera on the scope, but that would be very time consuming. I could put the camera on or in the toolbox to take darks while I packed up, but that still wouldn't save that much time.

[ianpwilliams]

How many darks is enough to make it worthwhile?

And what kind of sub length should I go for to begin with? Maybe 1 minute? Or more?

[Pompey Monkey]

How many darks is enough to make it worthwhile?

And what kind of sub length should I go for to begin with? Maybe 1 minute? Or more?

Take exposures as long as your guiding will support or until the histogram peak gets about 1/3 way across the graph, whichever is shorter.

As for the Andromeda Galaxy, it's on the wrong side of the sky and will be getting lower down by the minute. It's usually best to go for a target in the East because by the time you've set-up, aligned and got the guiding going, anything in the west will probably be descending into the murk.

I second what CJ suggested - go for M42, or even the Pleiades for a first guided target. You'll be glad that you did by the end of the night  

[ianpwilliams]

Yes, I believe that I need to take test shots with longer sub length each time, until the luminance peak in the histogram peak drops to zero around 25%-30%. I'm guessing that my sub length will be limited by the histogram rather than the guiding, as I do have some light pollution. I managed 60 seconds unguided so anything 3 mins plus would be great. I'll try 1 min and up it by 30 seconds until I either get star trails from limited guiding or too much histogram.

Andromeda is to the West on the weekend, but it's quite high throughout the evening. Orion isn't an option because it's really low to the West. Pleiades is nice and high, but not as an exciting a prospect.

My only worry is that taking into account the fact that 9/10 nights are cloudy here, combined with the way the targets move throughout the year, that it might be a long time before I get another crack at Andromeda. Although it looks like there might be two opportunities this weekend, so maybe I could try Pleiades on the first night and Andromeda on the second if the first night goes well.

[Pompey Monkey]

 Orion isn't an option because it's really low to the West. Pleiades is nice and high, but not as an exciting a prospect.

That's odd. Orion's rising in the East when it get's dark here

[ianpwilliams]

Yes you're right, although that may be too low in the East for me as the three storey house would be in the way, even at the bottom of the garden.

[ianpwilliams]

How about the number of darks? If I was able to take, say, an hour's worth of 3-min subs (which would be 20), how many darks would I be looking to take? I'd be surprised if I had an hour to spare for taking darks when taking into account taking flats, packing up, having work the next day etc.

[Xiga]

If you dither between subs you can get by without using darks at all.

[ianpwilliams]

Yeah I remember someone talking about dithering earlier in the thread, and it does seem like the obvious way to go. But it also looks fairly complicated, so I have to delay that until I know I can do normal guided imaging.

I just don't know if I need 5 or 50 darks to make it worthwhile. My guess is that any number of darks will help, so if I leave them til last then I'll just do as many as I can.

[Tinker1947]

How about the number of darks? If I was able to take, say, an hour's worth of 3-min subs (which would be 20), how many darks would I be looking to take? I'd be surprised if I had an hour to spare for taking darks when taking into account taking flats, packing up, having work the next day etc.

I would leave the delay between images to 20 seconds help to keep the DSLR sensor cool, this will add to the " hours worth of 3-min subs", as for Darks do them while you pack away the gear.

[ianpwilliams]

OK I'll try 20 second intervals. And I'll try Orion if I can, although it's so low that it may not be an option, in which it'll be Pleiades. Looks like clears skies tonight anyway...

[ianpwilliams]

Certain things I'm wondering about when it comes to tonight are:

Plate solving - it is supposed to be automatic, but it's not clear what you do if it doesn't work. Just use star-alignment I suppose?

Focusing with the Bhatinov Mask using images rather than using live view - how do you know how much to turn the focuser, and in which direction?

PHD2 calibration, which is supposed to take 5-10 minutes - does it do anything while it's calibrating? Will I be able to know that it's doing something during those 5-10 minutes?

[philherbert]

The things I'm least confident about when it comes to tonight are:

Plate solving - it is supposed to be automatic, but it's not clear what you do if it doesn't work

Focusing with the Bhatinov Mask using images rather than using live view - how do you know how much to turn the focuser, and in which direction?

PHD2 calibration, which is supposed to take 5-10 minutes - does it do anything while it's calibrating? Will I be able to know that it's doing something during those 5-10 minutes?

Hi Ian,

I'm a few weeks ahead of you in my efforts in the same area.

Plate solving - as long as you take the coordinates from the mount, solving works pretty well for me - 40 seconds at most. You can test this on an image in advance to be comfortable it stands a chance of working.

Focusing - simply put you don't. But turn the focusser a quarter in one direction, take another image, and see if it looks closer to focus. If not you should be able to estimate whether it was in the correct direction or not. I'd recommend not using the Bhatinov mask until you are relatively close to focus by eye - it's best for fine tuning.

PHD2 calibration - you will see if moving the guide star while it is calibrating (although you won't see the scope move). It makes a number of moves N, then S, then W and E (I think in that order). If you've got your step sizes reasonably set up, it works fine.

Good luck with the clear skies tonight. Wrap up warm!

Phil

[ianpwilliams]

Thanks Phil, very useful info there. I'm fairly confident that I will get no data tonight and more than likely it will end up as a dry run with there being so many new processes involved in guided imaging compared to unguided. I just hope that it can be productive enough that the next session can produce some results. One thing I don't have to worry about is the cold, because I only recently discovered down jackets. Those things are unbelievable!

[ianpwilliams]

OK I'm mid-session, and things are actually going well, although I'm not sure how exactly! And it took 90 minutes to get to the imaging stage! Anyway, I tried a 1 min sub (Orion), then 2, then 3, then 4, then 5, and still no star trails! The histogram was peaking a bit high, so I lowered the ISO as I upped the sub length. Is that cheating?

Now if all goes to plan my 12 x 5-min subs should finish about 9pm, allowing for 20 sec intervals. Then I'll have to do flats, which will take me through to 9:30. Which only really leaves 30 mins (or less ideally), because I have work early in the morning. So would it even be worth taking maybe 4 darks? Or should I just not bother? Or I could go to bed late I suppose...

[ianpwilliams]

Well I've got ten 5-min subs, and some flats and darks. Haven't got time to stack and process tonight, but just one single 5-min sub of Orion on the camera screen looks as good as my final unguided processed stack, so I'm very hopeful! And I can't tell you how good it feels to look at a 5-min sub and see perfectly round stars with not even a hint of star trails!

[cjdawson]

Talk about teasing!  post the sub up so we can all see!  Then process it tomorrow and let's set how you do.

[ianpwilliams]

Sorry, I had to go to bed, and now I've got to go to work! It's teasing for me too, I want to get to work on them! I'll post a sub up later. Both tonight and tomorrow are possible for imaging sessions as well though (if I can recharge all the batteries in time), so the processing may have to wait while there's data to be had!

[ianpwilliams]

When it comes to the rule about the luminance peak dropping at around a third, is that having the ISO at 800 for example, or is it an any ISO? Because lowering the ISO while increasing the sub length keeps the luminance peak from dropping too high, but then if the guiding allows for very long exposures then the ISO can end up quite low.

[ianpwilliams]

Here is my previous processed unguided Orion image, and one of my 5-min guided subs from last night.

Aside from the obvious light pollution in the 5-min sub, I think the single sub has more detail that the processed unguided stack, and it has rounder, nicer looking stars. So I'm optimistic when it comes to processing the guided stack!

One issue I had wass that Orion was getting close to my local friendly neighbourhood tree near the end of the ten subs, and I think there may be branches at the edge in the last one or two subs. So I'll have to decide whether to stack all ten and accept that I'll have to crop, or stack one or two less and not have to crop.

Anyway, what do you think of the sub?

[ianpwilliams]

Also, once I finally managed to calibrate PHD2 last night (it wouldn't work at first, and I couldn't see the option to increase the step size, so I increased the RA and DEC size in EQMOD, chose a different star, and it seemed to work) I was able to guide up to 5 mins (possibly longer, I didn't try), with 100ISO, with no star trails and a good histogram.

Assuming that I manage to calibrate again next time, should I straight away go for 5-min subs at 100ISO? I guess my conditions would be similar each time after all. Or when PHD2 successfully calibrates, are there different levels of success each time? So if we say that 5-min subs at 100ISO might be good for my conditions when it comes to the histogram, maybe the PHD2 calibration might not be as good and might not allow for 5-min subs?

[cjdawson]

Once you are autoguiding the limit really more down to sky conditions rather than the mount.  Your sub looks great, I see no trailing at all, and whilst is looks washed out - that's actually fine because the night sky isn't black.    Maybe for M42, how about taking a set of 30 second exposures to bring out the trapezium, then take a set of longer exposures.   This will then give you the processing possibility of combining the image to show everything.

I'm not sure that lowering the ISO does anything benefitial for astro photos. You'd have to increase the exposure time to compensate, however, I believe that all you will be effectively doing is turning down the gain in the camera.  I'd not bother going above ISO800 however.  Having a stack of images will have the effect of cancelling out the noise.  It'll be interesting to see what others say about this.

I do hope that you are taking darks and flats.  From the sub that you posted, I think you'll get some benefit from them.

[ianpwilliams]

Yes I took some darks and flats, so hopefully they will help.

The reason I was lowering the ISO is because like you say, your sub length is limited by your Sky conditions, and they say that in the histogram of your sub the luminance peak should drop to zero at about 30%-40%. But they don't say if that is relative to a particular ISO. If I had taken a 5-minute sub at 800ISO then the luminance peak would have been off the charts and the image would have just been white. But by turning the ISO down I was able to get a 5-minute sub with a reasonable luminance peak and where I could see the image.

What I don't know is whether it's OK to turn the ISO down to enable longer subs which still have good luminance peaks, or if that is essentially cheating.