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BYE with CLS in camera filter

Don El


First, I am using camera optics, so all my photos are wide angle. My pink-orange sky is so light polluted, that exposures above 20 seconds are impossible bright. That 20 second exposure is when I'm aimed at or near the zenith, otherwise the exposures have to be shorter. I have been experimenting with a CLS filter from Astronomiks. I wasn't expecting 3 stops of light reduction. I cannot see any stars with the camera viewfinder and only a couple of stars can be found with BYE. I have not been successful using the snap image function even though I can count many stars on the image. The best I can do gives me soft round balls for the stars when I increase image size from the standard view. From image to image the histogram of the star fluctuates more then can be accounted for by focus change alone. These images do not stack. I can focus my camera with the filter in place when I snap extended objects. I get reasonable tight stars with the camera without the filter and those images do stack. How can I improve my focusing with the filter?

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[ How can I improve my focusing with the filter?

Thanks for your help so far, but let me restate my problem. Frist, I'm sorry the question and the information are stated so poorly. I don't speak good stacking/astro/photography very well yet.

I can't focus with the filter in place by indirect focusing methods. I shoot a short exposure in the area of interest in this case, Alkid, in the dipper handle. In live view I cannot see this star. In the short (20sec) exposure I can. Then I go back to live view and move the  rectangle to the vicinity of where I saw the star in live view, then shoot another exposure. When I finally get the star centered in the box I can then start measuring and adjusting focus from frame to frame with FWHM until the value is minimized but I still am not near enough to focus for effect. So far the filter is useless to me.

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Thanks for clarifying.

Even through a telescope it is difficult to see even the brightest stars in LiveView. I would not even try to see stars in LiveView with a lens in place, unless it is a very long focal length lens (at least 500 mm).

I do not understand why you say that focusing with short (20 second) exposures is not working for you.  When you say "I still am not near enough to focus for effect", I am assuming that you mean that when you finally start to take long exposure images, after getting the FWHM as low as you can, that your images are still out of focus.  Is that true?

What FWHM values are you typically getting when you start and when you finish your focusing run?

What focal length and f/stop are you shooting at?  If you have a zoom lens, you could try to focus when you are zoomed all the way in and with the f/stop at its smallest possible value.  This would make the stars their largest and brightest.

This process should be the same whether or not you are using a light pollution filter. Perhaps it would be simpler to practice focusing via short exposures without the filter, until you get the hang of it. You could also try using the moon as your focusing target, if it is visible.  It is also bright enough to be able to focus via LiveView.

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Yes my long exposures, 2:30minutes, yield very soft focus. I am using a Canon 70D, with a 70-200mm f/4 lens @f/4 and an ISO value of 1600. I am using a very old and very stiff tripod with a iOptron Sky Tracker. The focus is after shooting several 20 sec exposures to get the FWHM as low as possible. The value of this FWHM is about 5.

If I shoot without the filter I can focus in Live View with BYE get a reasonably low FWHM (sorry I can't remember the values.) Then if I shoot 30 second exposures they are stackable and I begin to see some detail. I'm using Alkaid to focus on and shoot a field containing M-51. So far, I have only stacked about 6 minutes of non-filtered exposures. I can see the two cores, a faint indication of the spiral arms and a cloud of nebulosity surrounding the two cores. If I try to shoot longer then 30 seconds, my sky becomes so bright (orange-pink) that I totally loose all the stars in the field. I haven't stacked more exposures because I want to use the filter. I havn't used higher focal lengths because, it is difficult for me to aim a camera at a point near my zenith with any accuracy using a ball-head. This however is a problem that is solvable and I expect to aim with much better accuracy by week's end.

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Through the Wide-field View of a Camera Lens, so little light from any specific star is collected that you really NEED to select the Brightest Star(s) available as your Focusing Target.  Alkaid is Mag 1.8 while Arcturus and Vega are both Mag 0.0 - 5 times brighter.

You can also increase the apparent brightness of LiveView by setting the Camera's Exposure Simulation" setting to ON, and then the combo of ISO and Exposure set in Frame&Focus will effect the LiveView Display without needing to expend time on "short duration Snapshots".


Using an iOptron SkyTracker does add additional complication to your targeting of specific Stars or Regions, but once you are able to get a "decent focus" using the Brightest Star Available it is also easier to use the ViewFinder or LiveView to reposition the Camera to your Imaging Target.

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Thank you all for your insights. You are right the filter lowers the light by only 1 1/2 stops. This measured while shooting white clouds with and without filter. I'm a bit nervous about focusing on a bright star and then slewing my camera over half the sky and hoping to maintain focus. To date I have been picking a reasonably bright star near my target...  But if I must I must. My camera is and has been set to view the Live View screen as seen with the camera settings. I will raise the ISO for focusing.

I think the aiming problems have been solved. I can now set, and read RA from the mount and I am using an electronic angle gauge to set Dec. One other thing. I thought that the clip-in filter was a bit sloppy in the 70D. There seemed to be some movement and the 2 leaf springs on the clip were not engaging. I rotated the clip just a couple of degrees to engage the springs. It's much firmer now

Again, thanks for all your help. I'll post to this thread if I have any success.

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I finally got a chance to shoot last night. The marine layer has been wrecking havoc every night, but last night was good to almost 11:00PM. Increasing the ISO and the focal length certainly paid off. So I can aim AND focus. Now I need to learn all about all those frames besides the light frames that I need to shoot. That's a topic for a different thread. Thanks for all your your help.

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When you say that "I cannot see any stars with the camera viewfinder and only a couple of stars can be found with BYE." Do you mean in LiveView?  That would not surprise me.  Only the brightest stars are visible in LiveView, even without the filter.  Still, if you can see 1 star you can measure its FWHM and focus on it.


Then you say, "I have not been successful using the snap image function even though I can count many stars on the image."  Again, if you have one or more stars in an image, you should be able to measure the FWHM and adjust focus.  Just make sure that the camera settings (ISO and exposure) do not change while you are adjusting focus.


Your comment, "From image to image the histogram of the star fluctuates more then can be accounted for by focus change alone." confuses me.  When you are only interested in focusing, the histogram is not relevant.


You say "These images do not stack." I would not waste time trying to stack images that you know are not correctly focused.


Your statement "I can focus my camera with the filter in place when I snap extended objects." appears to contradict your statement that you have not been successful using the snap image function.  What do you mean by "extended objects"?


Even though your narrative is confusing, I get that you are having difficulty when focusing the lens.  Here are my suggestions.


To begin with, I would for focusing purposes, aim the camera at one of the brightest stars in the sky, like Arcturus or Vega. 


Focus needs to be very close to infinity, but many stock lenses go past the infinity position. So, I would start with the lens in that position that is beyond infinity as far as the lens will go). Take short snap exposures (in Frame & Focus) at high ISO. Lengthen the exposure until you see the bright star and keep that exposure (and ISO) for the duration of your focusing run. Position BYE's zoom box around the star and notice the FWHM value. Then start looping whereby you adjust focus, take an image, measure FWHM, compare to the previous FWHM and repeat. What you are trying to do is make small focus adjustments so as to minimize the FWHM of the selected star.  If you are moving the focus ring counter clockwise and the FWHM of the star is getting smaller, then it starts to get larger, you have gone too far and you need to reverse the direction that you are turning the focus ring.  It does take some practice, but it is doable. 




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Here are a couple of screen shots of histograms for a stock canon t3i with and without the CLS filter. This was a shot of a white piece of paper in sunlight.


The difference in shutter speed can be translated into fstop.


If your camera is full spectrum modified, and can post similar for that camera as well.




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