M31 first attempt with auto guider

[skyquest1028]

Almost 5 hours in. I'm debating on whether or not to keep going or move on. I guess you can never put to much time in or am I wrong. Should I increase my sub time from 5 minutes to 10? I was able to get a 20 minute iso 100 sub. The stars weren't perfectly round but the color was great. My birthday is coming up so I should be able to score a comma corrector. What do you think.

[nmoushon]

It looks good. Why would you use 20min subs at iso 100 though? defeats the purpose of doing longer subs.

[skyquest1028]

It looks good. Why would you use 20min subs at iso 100 though? defeats the purpose of doing longer subs.

I tried at 400 iso but I guess there was to much city light so the image came out all white. Originally I was just curious if my setup could do it. I'm not sure if I should stick to one sub length or vary it with each session. Maybe something like 5 minute subs, 10 minute subs, and some 2 minute subs. I'm trying to avoid taking so many darks.

[nmoushon]

I tried at 400 iso but I guess there was to much city light so the image came out all white. Originally I was just curious if my setup could do it. I'm not sure if I should stick to one sub length or vary it with each session. Maybe something like 5 minute subs, 10 minute subs, and some 2 minute subs. I'm trying to avoid taking so many darks.

I would suggest taking shorter subs and boosting the iso to at least 400. Taking shots at iso 100 is wasting time. I have to deal with plenty of LP to so i understand where your coming from. Theres people that could explain it better than I but increasing the exposure time does not compensate for decreasing the iso. They are not equal. So your iso100 20min sub is more like a iso400 5min sub. Not quite equal but something similar to that. Someone could do the math that could show you exactly what the comparison would be.

If your trying to avoid taking lots of darks theres a couple ways around it. 1. Would be to drop the iso which decreases noise build up over time. But then you have to take more lights to compensate for the loss of photons you recieve from lowering the iso. Which you could have used to take darks. Theres a fine line here that has plenty of people on both sides of it. I personally try and stick to iso 800. 2. If your not already taking flats really helps a lot. They are a lot faster than darks to take but a lot harder to get right. 3. Buy a CCD camera that is cooled or find a way to cool your DSLR. I have a dslr and want to switch to CCD but the money required is a lot so I'm currently looking into the latter.

[ultranova]

Hi, Just my personal preference but you might be better off using iso 400 or 800 and cutting your exposure time ,

as 400 iso is 4 time more sensitive than 100 theoretically you should be able to use 4 times less exposure time ,

and 800 is 8 times more sensitive.

So you might be better off doing a 2 minutes exposure with iso 800 rather than a 16 minute at 100 if that makes sense

This will allow you to get more exposures in and darks for a given amount of time, plus the shorter exposure times will

also help with guide problems.

As for the darks unfortunately to get the best picture with an SLR they are unavoidable normally only do as many darks as lights.

Hope this helps Paul

[kruska]

long exposures depends on polar alignment and your mount quality

I presume that your f is around 800-1000mm considering image size

Exposures- times are important but poor guiding may create worse frames on average mount.

of course long exposures are better than short even comparing 5&10 min subs and it is important to achieve quality light frames after all (because of image quality)

keep on photographin'

We are waiting to see next images

[Gina]

I find ISO 1600 is alright with an uncooled 1100D running at an EXIF T of 20C. ISO 800 should be really good and anything less is a waste of time. With the colder nights coming on the sensor temperature should be less and produce less noise. I find the sensor runs at about 14C above ambient for long exposures.

[skyquest1028]

Thanks for the comments. Just as an FYI I use ISO 800 on all the 5 minute subs. I used ISO 100 on the 20 minute sub because of the LP. I'm using an orion XT8 scope and ATLAS EQG. I think I'll spend another night trying to capture more data.

[skyquest1028]

I would suggest taking shorter subs and boosting the iso to at least 400. Taking shots at iso 100 is wasting time. I have to deal with plenty of LP to so i understand where your coming from. Theres people that could explain it better than I but increasing the exposure time does not compensate for decreasing the iso. They are not equal. So your iso100 20min sub is more like a iso400 5min sub. Not quite equal but something similar to that. Someone could do the math that could show you exactly what the comparison would be.

If your trying to avoid taking lots of darks theres a couple ways around it. 1. Would be to drop the iso which decreases noise build up over time. But then you have to take more lights to compensate for the loss of photons you recieve from lowering the iso. Which you could have used to take darks. Theres a fine line here that has plenty of people on both sides of it. I personally try and stick to iso 800. 2. If your not already taking flats really helps a lot. They are a lot faster than darks to take but a lot harder to get right. 3. Buy a CCD camera that is cooled or find a way to cool your DSLR. I have a dslr and want to switch to CCD but the money required is a lot so I'm currently looking into the latter.

I just got my canon XS so a CCD camera will be down the road. I tried 1600 but it was too noisy. 800 iso seems to be the sweet spot. I have a library of darks now, so what I plan to do is to take just a couple of darks and then average them together with my library to help even it out. I take flats by opening notepad and sticking it on the end of the tube. I use APT to automatically set the flats. It seems to work well although I probably need to put more time in making them absolutely correct. I think the problem I'm seeing now when using flats is that it's adding more noise into the image. I'm trying to get my Flat dark settings correct.

[Gina]

Time to pop out and see if the clouds have cleared by any chance

[skyquest1028]

long exposures depends on polar alignment and your mount quality

I presume that your f is around 800-1000mm considering image size

Exposures- times are important but poor guiding may create worse frames on average mount.

of course long exposures are better than short even comparing 5&10 min subs and it is important to achieve quality light frames after all (because of image quality)

keep on photographin'

We are waiting to see next images

Pretty close on the f. I'm actually at 1200mm (I think) with the Orion xt8. I spent quite a bit of time understanding polar alignment and using the drift method. That is of course, before I purchased my auto guider. I can't tell you how much time I save just by lining up the scope with the marks on the ground and adjusting the clock for Polaris. Now I can do 5 and 10 minute subs with no problems. 20 minutes seems to be the line. Maybe at some point I'll focus on getting the polar alignment close to perfect and then aim for 20 minute subs.

[Gina]

Time to pop out and see if the clouds have cleared by any chance

No such luck!! It's raining again too!

[andyo]

Hi,pretty much what has already been said.I nearly always use iso 800.The more subs you take the better the signal to noise ratio after stacking ,the longer the exposure time the more detail you capture.Reducing the iso wont solve the light pollution problem without losing detail it just means you have to increase the exposure time to capture the same detail to compensate for the lower iso setting at best you may get less noise.Best bet would be to use light pollution filters when imaging and software when processing to solve the problem.

[andyo]

Hi,pretty much what has already been said.I nearly always use iso 800.The more subs you take the better the signal to noise ratio after stacking ,the longer the exposure time the more detail you capture.Reducing the iso wont solve the light pollution problem. it just means you have to increase the exposure time to capture the same detail to compensate for the lower iso setting at best you may get less noise.Best bet would be to use light pollution filters when imaging and software when processing to solve the problem.

Sorry cant edit my post above but have altered it in the quote to what it should have been

[skyquest1028]

Sorry cant edit my post above but have altered it in the quote to what it should have been

I always thought that a longer sub meant less noise to a certain point. Does it really capture more detail? Should I focus on 10 minute subs rather than 5 minute subs. If you use a light polution filter do you have to increase your sub length to get more detail?

[Daniel-K]

I use a lp filter just the standar 2inch one from skywatcher there ok but the cls clip in is the way to go you don't need to take longer subs with them but they sure allow you to take much long subs with the pesky orange glow I was getting an orange glow after 200 secs with the sw filter were as my dried imaging next to me was doing 400 with no glow!

[Merlin66]

I'm under the impression, having seen some of Craig Stark's work and comments on various camera sites, that the digital "ISO" number is only achieved by internal manipulation of the raw captured data within the camera i.e. muliplying the RAW signal by X, X being the gain....

If this is correct then the same "signal" is there in the RAW whatever the the ISO setting. If you "boosted" a 200ISO image by x4 (to make a 800ISO) image would you not see the same background glow???

(I've ended up with a compromise of 200ISO for spectroscopy)

[lukebl]

Agree with all the above. Crucial points:

1: Get a Light Pollution filter. It really will make ALL the difference

2: Shoot using RAW, not JPEG. As said above, it makes the ISO irrelevant (as I understand it) and you get higher quality images.

5-10 minute subs should be fine.

[andyo]

I always thought that a longer sub meant less noise to a certain point. Does it really capture more detail? Should I focus on 10 minute subs rather than 5 minute subs. If you use a light polution filter do you have to increase your sub length to get more detail?

longer subs dont mean less noise,more subs will give a better signal to noise ratio when stacked .Yes longer subs will capture more detail, basically the brighter the object being imaged the shorter the subs would need to be (you dont want to overexpose).Experiment with the exposure times to see what is best.

[Quatermass]

The image itself looks really nice I tend to stick to ISO of 800 sometimes mixing in a few 1600s if needed, M31 is a target you can come back to many times and is a wonderful galaxy.

[skyquest1028]

The image itself looks really nice I tend to stick to ISO of 800 sometimes mixing in a few 1600s if needed, M31 is a target you can come back to many times and is a wonderful galaxy.

Thanks! I will stick to 800 and play around with the exposure time. Maybe aim for 10 minute subs.

[skyquest1028]

Agree with all the above. Crucial points:

1: Get a Light Pollution filter. It really will make ALL the difference

2: Shoot using RAW, not JPEG. As said above, it makes the ISO irrelevant (as I understand it) and you get higher quality images.

5-10 minute subs should be fine.

Well then, I have two accessories on my list. A light LP filter and Coma Corrector. Birthday and Christmas should take care of that.