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Where am I going wrong with imaging? (Alt Az / Deep Sky))


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I'm new to the astrophotography hobby. I have experience with astronomy. I am struggling to make decent deep sky images (other than M42). The images don't seem to have much definition or brightness despite a decent overall exposure time. See the below images. I have seen on this forum that people are able to take awesome images of the below objects with my same setup. Is anyone able to tell me if I am missing something, please? Do I need even more exposure time?

I use a Celestron 6SE with unmodified Canon 600D. It has a goto alt az, no EQ. I use a bahtinov mask to focus. Both images were taken with the native focal length of 1500mm, no filters or eyepieces.

The image of the Triangulum Galaxy is 180 x 15sec ISO800 images. The Crab Neb is 250 x 15sec ISO800 images. Both images had their appropriate flats, darks and biases (30 of each). I use SIRIL to stack the images, which I have had good success with M42 before (see below).

Any advice would be appreciated!

20210104 - Triangulum Galaxy2.jpg

result2.jpeg

20210101 - Orion Nebula.jpg

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M42 is much brighter than other two objects.

Fact that you are imaging at 1500mm FL and using short exposures is the reason you are having issues with fainter objects.

You are working at 0.59"/px - which is way too high resolution even for premium AP mount and most sky conditions.

Without going into too much detail - with short exposures read noise becomes important factor - when you spread out light so thin on your pixels - then it really becomes issue. You need very strong signal to offset read noise issue.

M42 has it - others don't, so total of 1h or less is simply not going to work on those targets.

You can do few things to improve situation:

1. pay attention to focus

2. Bin your subs after calibration and debayering - at least 4x4

In fact - if you still have your original stacked image (still linear without any processing) - bin it 4x4 and try processing it again. This will improve things somewhat, but don't expect miracles.

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Have you read "Making EveryPhoton Count" by Steve Richards, available from FLO? It's the essential guide to deep space imaging.

A C6 SE is not the most suitable tool for deep space imaging.

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The second image also is definitely not in focus so that isn't going to help with the quality of the image.  

It is hard work to start imaging with a long focal length telescope.  Your set up needs to be that much more refined to be able to manage this.   Getting a wedge might also help as you won't need to track in two axes at once, once properly set up.  

But really to start with you want a short focus length refractor. 

Nevertheless it is a long journey and for your starting images they are very good. 

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Consider this example of M33 and this one as well.

Both using the same scope you have, and taken with a DSLR.
They both use a focal reducer which has the effect of concentrating the captured light onto fewer pixels in the camera. This makes the object appear brighter but at the expense of having a smaller image.

They also both use longer exposure times. This has the effect of allowing more faint detail to be captured, rather than being submerged in the inevitable "noise" as vlaiv says.

I realise that with an Alt-Az mount you are limited in your exposure times before the "field rotation" effect starts to smear the stars into ellipses. However another benefit of a focal reducer is that the smaller images allow longer exposures before this happens.

So my suggestions would be: get a 0.63 focal reducer. With that, see how long you can expose for before getting squishy stars. Take many more exposures to stack. I would suggest a minimum total exposure time of an hour, but the more you have the better.

Edited by pete_l
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5 hours ago, vlaiv said:

M42 is much brighter than other two objects.

Fact that you are imaging at 1500mm FL and using short exposures is the reason you are having issues with fainter objects.

You are working at 0.59"/px - which is way too high resolution even for premium AP mount and most sky conditions.

Without going into too much detail - with short exposures read noise becomes important factor - when you spread out light so thin on your pixels - then it really becomes issue. You need very strong signal to offset read noise issue.

M42 has it - others don't, so total of 1h or less is simply not going to work on those targets.

You can do few things to improve situation:

1. pay attention to focus

2. Bin your subs after calibration and debayering - at least 4x4

In fact - if you still have your original stacked image (still linear without any processing) - bin it 4x4 and try processing it again. This will improve things somewhat, but don't expect miracles.

Thanks, Vlaiv. I'll keep an eye on focus and give a binning a go now. Cheers.

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5 hours ago, Cosmic Geoff said:

Have you read "Making EveryPhoton Count" by Steve Richards, available from FLO? It's the essential guide to deep space imaging.

A C6 SE is not the most suitable tool for deep space imaging.

Great, thanks. I'll get that book. I know the 6SE isn't ideal, but its what I have :)

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5 hours ago, Whirlwind said:

The second image also is definitely not in focus so that isn't going to help with the quality of the image.  

It is hard work to start imaging with a long focal length telescope.  Your set up needs to be that much more refined to be able to manage this.   Getting a wedge might also help as you won't need to track in two axes at once, once properly set up.  

But really to start with you want a short focus length refractor. 

Nevertheless it is a long journey and for your starting images they are very good. 

Thanks, Whirlwind. I had wondered if that image was out of focus. I was sure I used the bahtinov mask correctly, but maybe I knocked the camera after that or something. I notice the stars in my images are circular, rather than pin pricks 🤔

Yes, a wedge would certainly help and I'll probably get one, one day.

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3 hours ago, pete_l said:

Consider this example of M33 and this one as well.

Both using the same scope you have, and taken with a DSLR.
They both use a focal reducer which has the effect of concentrating the captured light onto fewer pixels in the camera. This makes the object appear brighter but at the expense of having a smaller image.

They also both use longer exposure times. This has the effect of allowing more faint detail to be captured, rather than being submerged in the inevitable "noise" as vlaiv says.

I realise that with an Alt-Az mount you are limited in your exposure times before the "field rotation" effect starts to smear the stars into ellipses. However another benefit of a focal reducer is that the smaller images allow longer exposures before this happens.

So my suggestions would be: get a 0.63 focal reducer. With that, see how long you can expose for before getting squishy stars. Take many more exposures to stack. I would suggest a minimum total exposure time of an hour, but the more you have the better.

Thanks, Pete. Good points. I do have a 0.63 focal reducer, but thought that would degrade the image, as I would be farther away. However, I see that you mean about collecting more light. I'll defo try a reducer next time I'm imaging. If the reducer allows me to increase the exposure, that would be awesome too; although, I'd still struggle with the wind ruining subs!

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Thanks for all the help, really useful stuff.

It sounds like I'm expecting too much from 30-60mins of exposure time. I've found it hard to get more total exposure time due to the losing half the subs to wobbly images, which I assume is the breeze, as I am not with the telescope the vast majority of the time.

I take it there aren't any other super easy astrophoto deep-sky objects (that aren't clusters or stars) like the Orion Nebula out there?

It appears I'm not doing the focusing very well either. Do I need to focus more than once per object, assuming I don't touch the setup at all?

Finally, does anyone have a simple way of knowing how much ISO to use?

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6 minutes ago, SpaceDave said:

Finally, does anyone have a simple way of knowing how much ISO to use?

Use ISO400, 800 or 1600 - there won't be much difference (I guess - go with higher for shorter exposures - that should reduce read noise somewhat)

Wobbly images happen not because of wind but because of mount - even EQ mounts have periodic error in their drive trains and don't follow earth's rotation perfectly - that is why people guide.

You are using Az mount that needs to track in both axis at once - two sources of error versus one with eq mounts. Mount itself is not really high quality in the first place.

Orion nebula is pretty much as bright as it gets, so it is hard to find one as bright as that.

Try maybe M81/M82?

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5 minutes ago, vlaiv said:

Use ISO400, 800 or 1600 - there won't be much difference (I guess - go with higher for shorter exposures - that should reduce read noise somewhat)

Wobbly images happen not because of wind but because of mount - even EQ mounts have periodic error in their drive trains and don't follow earth's rotation perfectly - that is why people guide.

You are using Az mount that needs to track in both axis at once - two sources of error versus one with eq mounts. Mount itself is not really high quality in the first place.

Orion nebula is pretty much as bright as it gets, so it is hard to find one as bright as that.

Try maybe M81/M82?

Oh, thats interesting. It's the drive train! So I guess its common to lose a lot of images to that error for folks who use an autoguiding mount.

Thanks for the ISO info, I'll opt for 1600 then.

I'll put M81/M82 on my hit list!

 

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9 minutes ago, SpaceDave said:

It's the drive train! So I guess its common to lose a lot of images to that error for folks who use an autoguiding mount.

No, that is why you auto guide - so you don't loose subs.

Once you start auto guiding on EQ mount, if your guiding is good - you don't loose any subs - unless something drastic happens that can't be guided out - like cable snag or you bump your telescope accidentally  or whatever.

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4 minutes ago, vlaiv said:

No, that is why you auto guide - so you don't loose subs.

Once you start auto guiding on EQ mount, if your guiding is good - you don't loose any subs - unless something drastic happens that can't be guided out - like cable snag or you bump your telescope accidentally  or whatever.

Ah, I see, that would be nice :)

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The good thing is the 6SE mount is more stable as it is the mount also used for the 8SE which is heavier.

Personally unless you are aiming for a galaxy I would loose the 6SE and use a camera lens with your DSLR on your mount. What camera lenses do you already own? This would have the potential to increase you exposure time to 30-45 seconds depending on where you aim in the sky. Can use a flash hot shoe mounted red dot finder. Many DSO are huge and can be captured with smaller lenses.

A thread of similar members imaging

https://stargazerslounge.com/topic/228101-the-no-eq-dso-challenge

 

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Posted (edited)
15 hours ago, happy-kat said:

The good thing is the 6SE mount is more stable as it is the mount also used for the 8SE which is heavier.

Personally unless you are aiming for a galaxy I would loose the 6SE and use a camera lens with your DSLR on your mount. What camera lenses do you already own? This would have the potential to increase you exposure time to 30-45 seconds depending on where you aim in the sky. Can use a flash hot shoe mounted red dot finder. Many DSO are huge and can be captured with smaller lenses.

A thread of similar members imaging

https://stargazerslounge.com/topic/228101-the-no-eq-dso-challenge

 

Thanks Happy-Kat. Thats an interesting thread! I'll look through it.

I don't own a lens :tongue2: It is something I could purchase in the future though!

Edited by SpaceDave
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