TIps needed for M45 - Pleiades Cluster

[MylesGibson]

I am planning on imaging the Pleiades cluster this coming winter. It is still a few months away so I have time to prepare. I have a friend who wants a photo of it for a personal reason and I want to be able to produce a good picture. But I do have modest equipment so I was hoping people could give me some tips.

My equipment is as follows:

• Scope - Starwave 102ED-R with 0.8x flattener (f/5, FPL53 glass)
• Mount - EQ3-2 with dual axis motor drive (with good polar alignment I can usually get 2-3 minute exposures without trailing. It's all I can afford for now so don't need the "HEQ5 minimum requirement" type comments)
• Camera - Canon EOS 1000D (unmodified)
• Software - Astrophotography Tool

I have no guiding as it stands, but as mentioned, I can get up to 3 minutes with good polar alignment and good weather conditions.

I would like to be able to get some of the nebulosity in this cluster, but I also don't want to over-do the stars. Even in some of my other pictures, there are small halos around brighter stars, even with FPL53 glass, so I don't want exposures that are too long for fear of having bad looking stars (or am I worrying about nothing here?). I can make them a bit better in processing, but still haven't nailed that technique properly. I've seen people get some really nice images of this cluster using my scope (I searched on astrobin) and would love to get something close to the images of M45 on there, although I understand and accept they will not be as good.

If only taking 2 min exposures, will I require longer overall on the target? Maybe multiple nights? If anyone has any tips on overall time to aim for, techniques or anything that does not include buying extra equipment (simply can't afford it) then I would love to hear it.

[RayD]

I think the biggest mistake that people make with this target is to overdo the exposure length in order to drag as much detail out of the dust as possible.  Clearly you've identified this issue and understand the problem it causes witht the main (seven sisters) stars, as they are pretty bright.  My advice is shorter subs, and lots of them.  Alternatively you can take some long (for the dust) and some short (for the stars) and process them separately in order to blend the pair.

BTW, 2 mins is still actually quite long for the stars on M45, particularly with a CMOS sensor.  From memory when I imaged it last I think I only used 30s subs for the stars. However, 2 minutes, or even slightly longer (3 mins) will be fine if you want to do some work on dealing with the stars in post.

I'm sure there will be many others who will have their own stories of 15 min expusures with no problems, but my experience of M45 is that, whilst an easy target to image, it's one of the harder ones to image well.

Edited July 24, 2020 by RayD

[MylesGibson]

Thank you @RayD for the advice! When you did 30 second subs, were you able to get any of the dust at all? And if so, how many subs? 

I haven't got the hang of blending yet, but I may look through some tutorials and give that a go. I did think 2 mins was quite long, but I do want to get the nebulosity too. It just seems a bit of a conundrum in my head! 

[RolandKol]

7 minutes ago, MylesGibson said:

Thank you @RayD for the advice! When you did 30 second subs, were you able to get any of the dust at all? And if so, how many subs? 

I haven't got the hang of blending yet, but I may look through some tutorials and give that a go. I did think 2 mins was quite long, but I do want to get the nebulosity too. It just seems a bit of a conundrum in my head! 

I managed to image Whirlpool Galaxy using short subs from London....
7sis should be more than OK, but there are other challenges...

 

 

Edited July 24, 2020 by RolandKol

[RayD]

27 minutes ago, MylesGibson said:

Thank you @RayD for the advice! When you did 30 second subs, were you able to get any of the dust at all? And if so, how many subs? 

I haven't got the hang of blending yet, but I may look through some tutorials and give that a go. I did think 2 mins was quite long, but I do want to get the nebulosity too. It just seems a bit of a conundrum in my head! 

The 30s I shot was for for stars only.  If you are looking to image only one integration time, then you will need to go slightly higher I would think, but you will just need to get lots of subs.  I would try the 2 mins to see how blown out the seven sisters look, but don't be afraid to reduce this and up the count.

Edited July 24, 2020 by RayD

[RayD]

Indeed you do end up with lots and lots of subs to process, which in itself is an issue, but it does at least let you get enough data to get a really good image out of it at the end.

There are, of course, ways to deal with it in processing, but that can also be a little challenging.

[Seelive]

You could try using DSS Entropy Weighted Average (High Dynamic Range) stacking.  I've used it successfully to image M42 so that I got the full faint detail of the nebula but was still able to see the Trapezium stars.  I used 20 exposures each of 180 sec,  60 sec, 20 sec and 7 sec.  However, for the Pleiades, I just stacked 20 x 180 sec to get the nebulosity (Canon 200D, 400mm telephoto at F5.6, ISO1600, EQM-35 mount).  You can see the resulting image here https://britastro.org/sites/default/files/styles/xl_image/public/member_images/M45 400mm F5.6 ISO1600 20x180s HR.jpg?itok=mIFNHrCA

 

Edited July 24, 2020 by Seelive

[MylesGibson]

On 24/07/2020 at 16:41, Seelive said:

You could try using DSS Entropy Weighted Average (High Dynamic Range) stacking.  I've used it successfully to image M42 so that I got the full faint detail of the nebula but was still able to see the Trapezium stars.  I used 20 exposures each of 180 sec,  60 sec, 20 sec and 7 sec.  However, for the Pleiades, I just stacked 20 x 180 sec to get the nebulosity (Canon 200D, 400mm telephoto at F5.6, ISO1600, EQM-35 mount).  You can see the resulting image here https://britastro.org/sites/default/files/styles/xl_image/public/member_images/M45 400mm F5.6 ISO1600 20x180s HR.jpg?itok=mIFNHrCA

 

That's a fantastic result at the end there! How do you use the high dynamic range stacking? And what difference does it make to the pictures? Did you have to do any processing to stop the stars going bloated? Or did this type of stacking help with that?

[Seelive]

Assuming that you use DSS for stacking your images, put the different exposure lights and associated darks in separate groups, then just select the Entropy Weighted Average (High Dynamic Range) option in the stacking tab.

This is a crop of M42 stacked using the Average option on the 20 x 180s exposures.  The core and the brighter stars are all saturated.

This is stacked using the Entropy Weighted Average option with the 20 x 180s exposures plus 20 exposure each of, 60s, 20s and 7s.  The core and brighter stars are no longer saturated and the trapezium stars are now visible.

Of course the nebulosity of M42 is significantly brighter than that of the Pleiades so the method may not be as effective on the Pleiades (I certainly haven't tried it).  The 7s exposure was chosen so that the trapezium stars would just saturate the camera sensor and the 180s exposure was set to keep the level of sky background caused by my local light pollution at a reasonably level.  The mid exposures were therefore chosen with a x3 factor.

[MylesGibson]

@Seelive that was very helpful indeed, thank you for that. I'll certainly have a play with these settings when I get the chance to photograph it!

[Seelive]

Best of luck.  To start with, try to get as many 'long' exposures as you can, the short exposures take no time at all.  They don't need to be taken all on the same night and you can stack different combinations to get the best the final result.  Just remember to set the DSS threshold level to a value so that it detects sufficient stars (>12?) on the shortest exposures so that they are included in the stack (you might get hundreds on the longest exposure but just a few on the shortest).  Perhaps one other tip, if some of your long individual exposures have not got perfectly 'round' star images, try Kappa-Sigma stacking them with sigma set to around 2.  As long as the star 'un-roundness' (I'm sure that's not a word) is not the same in every image (i.e. star trailing), it can help circularise them.  Once I initially rejected several dozens of images taken during high winds because the stars were slightly elongated.  Looking closely, they were generally 'elongated' in different directions and it turned out that stacking every image gave a better result (less noise but still circular stars) than just selecting the 'good' images.

[MylesGibson]

@Seelive I haven't really played around with all the different stacking settings. I didn't know about the Kappa-Sigma settings. I've seen it, but was never too sure what those other settings do! That is again, a very useful tip!

[Seelive]

As I understand it, the Sigma part is effectively how 'strong' the effect is, the Kappa is how many times it iterates. The smaller the Sigma and the larger the Kappa the greater the effect it has on the image. If you're not familar with the term sigma, search for Gaussian distribution on the net to see what it is (but probably avoid the maths!)

[MylesGibson]

@Seelive I'll look into that! Although, as an engineer I'll probably look into the maths! 

[MylesGibson]

Thanks for the help people. Had my first go at this over the weekend and am happy with it as a start! Will have another go some other time and play around with exposure times to see if I can find a good balance! 

[Seelive]

That came out quite nice.

[MarkAR]

Great result. Really picked up the reflections well.

[MylesGibson]

Thanks guys. It was slightly  humid and I don't have a dew heater so I did end up with a small amount of dew on the lens. That, coupled with exposure time possibly too long has given me a few bloated and overexposed stars.... Post-processing is definitely something to work on, but I think I'll go for shorter exposure times on this next go, and take more of them!