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iksose7

Pleiades Star Cluster @ 200mm

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Hi all, heres my attempt at M45 this year. 

Exposure details: 

60x210 seconds, f2.8, ISO 800, calibration frames, 200mm

With just under 3 hours 30 minutes this has fallen far from how i wanted it to be. I planned on getting 10 hours! But with cloud forecast all week i'll settle with this till later in the year. 

I shot using the lens wide open and using the 3rd point focusing method, hoping to have enough light grasp to pick up some of that faint dust in the area. I picked up 7 hours over 2 nights and thats when problems started.

My first imaging session was the first time i used my new dew heater, i focused and then turned the heater on which shifted the focus i think, leaving me with over 3 hours of un-stackable data! This image is made up of data from the second session, you can see that the stars appear to trail and i wanted to ask if anyone using this focusing method has ever been left with stars like those in the image? They appear to streak a little bit and look as if they are rotating around a center point. I have spoken to one other person who has experienced this. 

Because i have recently started using Backyard EOS, im not aware how to set up the live view so you have the 4 intersecting lines (if its even possible) so when focusing i roughly guessed where the intersecting lines would be. If i didnt have the star in the right place, could it have caused the stars to appear the way they do?

Anyway, heres the image.

gallery_26473_2818_44225.jpg

Clear skies!

Edited by iksose7
  • Like 6

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Nice work Callum, you have picked up quite a bit of the low intensity nebulosity there.

I'd try to get longer exposures instead of more of the same if you can, that way you can get more of the low intensity nebulosity and blend it with this image. Taking many hours of the same exposure length will only increase the S/N ratio, it won't make it brighter (though it will allow you to stretch it further with less noise, but if you haven't captured the signal, you can't increase its intensity).

Regarding focus, I always check it every other frame. A lens' focal point changes slightly with temperature, the dew heater is only heating the front element, not the entire lens so it will change during an imaging session. I have heard of the intersection at 1/3 point focus but never use it. I always find some star somewhere in the field and focus on that (easy with this target), many times per session. I don't know what I would do if I couldn't find a star...I'd have to slew to somewhere there is one, refocus then slew back. No Bahtoniv mask, no FWHM, I just eyeball it at 10x magnification on the LCD. Works for me though your mileage may vary...

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Thanks Stuart!

Yes, longer exposures are needed desperately! The first time i tested out my NEQ6, after a long time tweaking polar alignment i was managing 5 minute subs. But now thats it freezing at night, i dont have the patience to get PA spot on so i settle for anything over 3 minutes. Next payday will go towards a guiding set up!

I am still in the middle of my Backyard EOS trial and i absolutely love the FHWM function for focusing. Combined with my micro-focuser you can get focus spot on. But like you said, the dew heater may change that over the night. Will have to start checking focus every half hour or so. I dont usually use the 3rd points technique but did for this as i wanted to use the lens wide open to avoid star spikes. I prefer shooting at f4 and probably will do from now on, its a much simpler process!

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The rotation looks like polar alignment more than focusing problems

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Trailing aside nice shot caught plenty of nebulosity.I use a bahtinov mask and a program called bahtinov grabber(free) to focus.However I am with Francis on this one it looks more like PA  

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I love the blue nebulosity in your picture of Pleiades, I can never make it out when I'm observing it though. :sad: Would a filter help?

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I love the blue nebulosity in your picture of Pleiades, I can never make it out when I'm observing it though. :sad: Would a filter help?

Thanks Daz!

To be honest i'm not an observer other than through binos so couldnt tell you! I'm guessing the more aperture you have the more of the blue nebulosity you would see?

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F2.8 is a very fast focal ratio - I use it all the time with a DSLR ;-)  

You certainly won't be needing longer exposures at that fast focal ratio - not unless you have extremely black sky.

You already have over 3 hours of data and you are showing plenty of faint wispy bits - I think the key is in further stretching the data you already have and taking even more similar exposures.

I also think it is worth taking a few shorter exposures (which don't saturate the bright stars so much) to combine with the existing data.  You can create an HDR image or perform some kind of layering to combine them.

Mark

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Nice capture. I agree that the main problem seems polar alignment, not focus.

I love the blue nebulosity in your picture of Pleiades, I can never make it out when I'm observing it though. :sad: Would a filter help?

Reflection nebulae like these are not helped by UHC filters, although wide-band LPR filters can help a bit. I have seen the nebulosity around the Pleiades, but only from very dark sites (in the Alps in particular). The nebulae do not need much aperture (my 80mm was fine) but they do need very dark and transparent skies, and you need good optics with little glare or internal reflections.

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PA for me, too. We see rotation. Are individual subs like this? Or does it build up in the stack?

Psychobilly managed to get perfect stars wide open using the thirds method. He sent me some test shots which were great.

A big issue is focusing physically and at F2.8 I'd say fingers were out. Telescope Service do these devices which I'd consider a must. 

IMG_1193-M.jpg

Olly

By the way, there are faint reds in there as well which might be worth hunting for in the data.

Edited by ollypenrice

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Telescope Service do these devices which I'd consider a must. 

IMG_1193-M.jpg

I'm intrigued Olly.

What are these devices and how do they help?

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No, all my subs were like that Olly. Upon closer inspection of the single subs, it shows that the stars do trail (from 10 o'clock to 4 o'clock). But i dont think that explains the larger odd shaped stars. On a single unstretched sub its is apparent that the bigger stars are diamond shaped and do indeed appear to rotate around a centre point (as if i were imaging Polaris).

Imho, i'd say its something to do with focus. But i dont know too much! Not to say i've been put off by Pshycobillys technique, i'm sure i have messed up somewhere along the way and will definitely use this technique again, just be more careful and make sure stars are spot on next time!

Oh and i already have the TS Microfocuser ;) hated it at first but couldn't do without now!

What do you mean by hunting for faint reds?? A colour boost?

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If you have problems at 10 and 4 (directly opposite each other on the clock face) then you could have astigmatism in the system. Likely if you get good focus at 10 and 4 then 7 and 1 will show the same problem at right angles to 10/4...

Edited by fwm891

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Looking at the full size image on Flickr, I think the weird star shapes near the corners are probably caused by lens aberrations.  Unfortunately you would need to stop down the lens to cure this, which means less light getting to the sensor.  To confirm or refute this, try taking some pictures of a point source of light in the middle and corners of your image frame (it doesn't need to be a star).  You can then determine which F-ratio gives good enough results.

Mark

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Hi Francis,

The 10 and 4 o'clock is the direction of star trailing, not the location of star trails. The trailing appears all over the image, not a lot but noticeable when zoomed in. The strange shaped (diamond shaped) stars trail at right angles to the ones i just mentioned which makes me think its not a PA issue but rather a focus, although clearly PA was off a bit.

I dont think i am explaining it very well so i drew a quick pic. 

This is exaggerated obviously. The lack streaks being the normal star trailing as expected if PA is off a bit, traveling from 10 to 4 o'clock. The Blue streaks would be the diamond stars which dont run at the same angle as the normal stars and appear to rotate round an imaginary center point

gallery_26473_2703_20519.jpg

Not to say astigmatism is not the problem! I will be having a look into that after work :)

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Thanks twin!

Yes mark, i usually shoot at f4 and find it pretty much perfect but didnt want star spikes on the cluster! Think i'll have to make some aperture masks at some point. 

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Yes mark, i usually shoot at f4 and find it pretty much perfect but didnt want star spikes on the cluster! Think i'll have to make some aperture masks at some point. 

That makes sense. The diaphragm blades would give you some very serious looking diffraction spikes on the Pleiades!

Mark

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