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jjosefsen

First attempt ever (M13) - lots of questions inside!

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Hi there,

Had my first attempt at astrophotography last week, my target was M13 - the Great Cluster in Hercules.

I have been observing for about 7 months now, and I originally wanted to try EEA, but the cost of getting an astrocam put me off it.

Instead i started reading about short exposure astrophotography with DSLR's a while back, and since my wife has a rather good one (Nikon D7100) I figured I would save the money on the astrocam and get the other things I needed (Motors, Polar scope, T-Ring, bahtinov mask, etc.) instead.

Turns out imaging is very time consuming and very complicated, but when I saw those first unprocessed exposures come in.. Wow! Quite exhilarating!

I was originially hust fidding around with the gear, the bahtinov mask, the camera and was first time trying the Backyard Nikon software, so I didn't attempt a proper polar alignment, just sort pf eyeballed it..!

 

Please feel free to comment or critique the image, im here to learn!

Im currently reading the book "Astrophotgraphy on the go - Using short exposures with light mounts" and it is giving me lots of insights and valuable information.

So flats are going to be a priority going forward, so is building a Bias and Dark library so I don't have to spend time doing those at night! Im also going to do alot more darks and aiming for a total integration time of 120minutes.

 

Data:

26x45s lights @ ISO400 (only 9 were good enough to stack! :hmh: )

10x45s darks @ ISO400

100x1/8000s bias @ISO400

No lights, still trying to figure out my process here..!

Software used: Backyard Nikon (great tool!!), DSS, Photoshop CC 2017 (just stretched and brightened and cropped)

Gear: Skywatcher Explorer 150P on EQ3-2 mount with Enhanced dual axis motor drives and SW polar scope, Nikon D7100 unmodified and used at prime focus

 

Questions:

  • What could be the reason for the very low number of lights I was able to stack (9 of 26) ??
    • My initial thought is the bad polar alignment and maybe because I didn't really balance the scope after switching from EP to camera, which is much heavier??
    • My worst fear is that the mount is not "fluid" enough in its motions when tracking, and I don't really feel comfortable stripping down the mount to be honest!
    • I have attached one of the really bad subs, it does have really elongated stars!
  • I don't have a goto system, the main problem is the focus changes when switching between EP and camera, any tips on how to find targets when imaging?!
    • I have both a SW 9x50 RACI finder scope and a Rigel Quickfinder on the scope, one solution is to stick to stuff I can see in the finder scope I guess..
  • How important is it to get focus on a star near the target?
    • I was considdering marking the prime focus on the focuser drawtube with a pen, but that might not even be good enough?
  • Flats! I think im going to have to do them before or after imaging, as I can't leave the camera on the scope (wife's camera!)
    • Can I use the T-shirt method at night, by shining a flashlight at the cloth covered opening on the scope while snapping the frames?
  • The image attached - The stars seem quite "bloated" is it overexposure, bad tracking, bad focus?
  • My main goal right now is to be able to do "good" consistent 60 - 75 seconds exposures, while i learn editing, is this goal attainable with my current setup?

 

I really hope some of you can answer some of my questions or confirm or deny some of my suspicions!

 

Thank you in advance and clear skies!

Stretched and lightened.png

bad sub.jpg

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What was the reasoning you threw away so many light frames? DSS has an option to stack the best 0-100% of frames, it's not very good. I would just look at every image individually, if you see a satellite/plane/meteor or there is significant star trailing then throw it out. Then put DSS to stack 100% of your frames.

Focus using a bright star like vega, or Arcturus. Once focused just find your object in the finder scope. Atleast that's how I do it, it might be harder for you depending on your finder scope. But for me it's really hard to find m13 in an EQ mount as it's almost strait up. Your focus seems good though.

Other than that a good first image.

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36 minutes ago, Galen Gilmore said:

What was the reasoning you threw away so many light frames? DSS has an option to stack the best 0-100% of frames, it's not very good

Well it was DSS which "unchecked" these frames pr. default" and they had a significantly lower "score" in DSS thatn the others. A few of them did not even get a score after registering.

I assumed this meant that DSS found them too bad to even stack.

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Good first image!

As for your setup: EQ3-2 with 150 P. The scope is really too heavy for the mount, especially if you add the weight of a camera and finderscope. You can keep the weight down by removing anything you don't need, such as the finders. But expect the longest exposuretime to be no more than the 45 minutes you have now. And also expect to reject a large number of light frames due to tracking errors. Just shoot as many light frames as you can during a night. If you image one target during the night with 60 light frames, of which you can keep 50%, you can still end up with a very good image. On the other hand if you image two targets with only 30 frames each of which 50% to keep, you will probably end up with two half decent images.

The EQ3-2 is a very fine mount for a lightweight setup, and you should definitely try using it with your dslr and a 50 - 200 mm lens (not necessarily a zoom lens). With 200 mm you will be able to capture larger dso's like the Andromeda galaxy (M 31), without much trouble.

You may get better performance from a (budget) mount if you strip and clean it. But with the EQ3-2 you have to be extra carefull. This mount has no ball bearings in the RA or DEC axis, and the thick grease is needed to make the mount run 'smooth' when under load. As long as your mount works ok, I would recommend not stripping it ('If it ain't broke, don't fix it.') But you can improve tracking by trying to eliminate the backlash in the RA and DEC axes. This doesn't require stripping the mount.

As you probably have noticed, the stars are pear-shaped towards the corners (especially on the left hand side in your image). This is coma and can be corrected by using a coma-corrector (obviously). You can further improve the stars by accurate focusing. A Bahtinov mask is a very cheap investment and worth every penny.

I wouldn't bother creating a dark library when using a DSLR. Dark libraries are only good when you have a temperature controlled camera. You may even try calibrating without darks. Most stacking software has a feature called 'cosmetic correction'. This removes hot pixels during the calibration process, and can replace dark frames. Even more so if you also use aggressive pixel rejection during image integration. Flats, on the other hand, are necessary. Another way to improve image quality is to dither (move the mount about 15 pixels in a 'random' direction between exposures). 

 

Good luck. And above all: have fun.

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After all that you said that you was doing I was really shocked of the image you produced..its really quite nice..not perfect but wasnt expecting that....

Firstly polar alignment is really important to get right..or as best as you can..balance is also really important for tracking and less wear and stress on the gears and mount..just getting those 2 points would give you a far better image..

M13 is a great target....lets see some more..

 

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Use your BYNIkon and your Bahtinov mask  and LiveView to focus on a star near your target.

Then using your finder to point to where the target should be.

Take a 30 sec jpeg at max ISO and platesolve it using PlateSolve 2.

This will tell you the centre RA/Dec of your image.

Nudge in the right direction and repeat until target is centred.

Michael

 

 

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That's a pretty good image for starting out in AP!

You've already found out that AP is quite complicated - so best to make it as simple as possible. Forget about darks, bias, flats (for now), targets and concentrate on these :

1) Balance - the image below shows star trailing from one of your 'bad subs'. The star trailing isn't smooth and has jitter in it. This is probably a sign the RA motor is struggling to move the mount and could be due to balance. This youtube video explains balancing a scope : https://www.youtube.com/watch?v=hGduG2jB9ec

2) Polar alignment - this becomes more important with increasing exposure time. With your polar scope (ensure it is set for the correct time of year) it shouldn't be too difficult to achieve good polar alignment for imaging with reasonably short exposures.

3) Focus - focus on a bright star using the Bahtinov mask. A trial image can be made to check it.

4) With the above achieved, just point the scope to a region of the sky with plenty of stars (Milky Way if possible) and take exposures of, for example, 45 secs with the RA motor running. Stars should be round except towards the edges of the image where there will be some coma elongation pointing away from the image centre.

5) Once you can achieve the above, which might take a little practice, point the scope towards a bright target that is easily seen using the finderscope and image this target. If all is well move on to the next step.

6) When you are happy with being able to image bright targets it is time to take flats. a t-shirt across the OTA is fine for diffusing the light, but don't point a torch at the OTA as the illumination won't be even. Just point the OTA at an evenly illuminated area (I use the ceiling) that isn't too close (greater than 2meters is fine). Others point to the sky, using something like a t-shirt again.

When you are comfortable with imaging as above, you should be able to produce very acceptable images.

Finally, here is a thread about imaging with an EQ3 and you will find great images and help there : https://stargazerslounge.com/topic/279039-the-eq3-dso-challenge/?page=1

Hope this helps!

 

judder.jpg

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Next time you try, do one thing for sure: align your camera along RA and Dec. This is easy: eyeball it square, first off, then just take a picture of about 5 seconds and slew the mount slowly in RA during the exposure. You'll get a picture of long star trails. Rotate the camera and try again till the trails are horizontal. This will mean that you can tell instantly which axis any tracking problems are coming from.

Also assemble the setup with the camera in this position. (Pieter Vandevelde knows what he's doing...) 

Imaging trip S-France August 2015

It gives the best dynamic balance and the minimum need for counterweight extension.

Polar alignment does matter, especially unguided. The DARV method is probably best for your purposes. https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

Flats: twilight and T shirt is a good idea but is the bottom of your scope open to light? If it is this won't work. Even with a T shirt you'll need an even light source if doing them in the dark. When stacking your flats be sure to use a master bias frame as a dark for your flats. If you don't they are likely to over-correct.

Personally I'd also use a master bias as a dark, as Tony Hallas suggests.

You made a good start!

Olly

 

 

 

 

 

Edited by ollypenrice
Typo
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Thanks a lot for all your advice, lots of experimentation on the menu going forward.

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Great image of a great object, it really gives the beginners like us a lot of satisfaction. Well done. 

I have the same set up but only just got my motors at xmas. Since then the learning curve carried on being steep and the most important thing I have worked on is polar alignment. I didn't realise you had to rotate on the RA axis for different times of the year. I use this app to give a good idea, here's a picture for today's view through the polar scope. 

IMG_3763.thumb.PNG.730329895ec94facd7dff52f9eafce41.PNG

Also I struggle with flats so I'm not getting too hung up on that just yet  

@ollypenrice, thanks for that picture, must try this as well. 

+1 for the eq3 DSO challenge thread, great advise on here  

 

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I think everything's been said, you've got off to a good start.

It may be worth buying a stainless steel 1.75" tripod which is much stiffer than the stock aluminium one. I use one with my EQ3 mount.

Also, spend a while learning how to set up your mount balancing it and setting the worm wheels for minimum friction. It's worth getting some lovely slippy, clean, white teflon grease for the worm and wheel.

I have a 150PL and it's even heavier than your 150P and I can image with it, but these scopes are at the 'edge of the envelope' for the EQ3, so even with a well adjusted mount expect to lose some subs. You're some way from thinking of your second scope, but if you do the 130P-DS is lighter and shorter focal lengths and works really well on an EQ3 mount for imaging. Check out the 130P-DS thread for amazing images made with one.

This thread on imaging with the EQ3 has a lot of useful advice, and look at Nige's long exposure guided images in recent posts.

 

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On 07/30/2017 at 22:40, Stub Mandrel said:

I have a 150PL and it's even heavier than your 150P and I can image with it, but these scopes are at the 'edge of the envelope' for the EQ3, so even with a well adjusted mount expect to lose some subs.

 

How long are you typically exposing with your setup?


I've fiddled with the mount and balancing now, and it seems really smooth, so think it really will be better.


Also tried positioning the camera as suggested by @ollypenrice

Seems to have helped a lot with balance..


Unfortunately too much wind to do tracking test tonight.

Thanks everyone for the tips..

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7 hours ago, jjosefsen said:

How long are you typically exposing with your setup?

With the 130P-DS 1-2 minutes, best when not pointed too high up. I have the 'new style' polar scope which makes a huge difference.

I managed 5 minute guided subs recently and should be able to go longer (light pollution permitting) if I could see the stars!

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