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BYN Focus


ridgesoar

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I made progress on Frame and Focus, now I've moved onto imaging. (I tried to find a topic related to my question in the forums but don't now how to find it efficiently aside from reviewing every page of the forum. Is there a key word search function I'm not seeing?)

 

Imaging at prime focus on a 10" SCT with a Nikon D600. When I image it was my understanding that the exposure time is to be of sufficient duration that the red signal on the Histogram was to be near the vertical line separating the first third of the graph from the start of the second third. When I do this, at ISO 1600, it looks like the image is starting to get over exposed, beginnings of an overall white haze. Exposure time is on the order of 180 seconds to get the red signal at ~33% location from the left edge. Is there something I'm missing? My skies are fairly dark (able to easily see Polaris) and moonless night.

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Given that the hump is 1/3 from the left you're saying that I'm properly exposed regardless of the light pollution? Since I'm new to astrophotography, is there a way to remove the light pollution via Photoshop or should I use a light pollution filter (not sure if there's a way physically to install a filter at prime focus)?

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Light Pollution, while Unwanted, isn't actually "Noise" as it isn't Random.  So, it can't really be defeated by traditional Noise-handling procedures (Dithering, Stacking, Post-Processing Noise Reduction).

 

Light Pollution ("LP") can really only be subdued by either Shorter Exposures or a Light Pollution Filter.

 

 

Given that the hump is 1/3 from the left you're saying that I'm properly exposed

For most AP Image Processes, simply having a little bit of separation between "the Hump" and the Left Edge is sufficiently Exposed.  If "Optimum Exposure of 1/3" captures too much LP, consider shortening your Exposures but make sure there is a bit of separation from that Left Edge - then Stack and Calibrate and Stretch to address what the Longer Exposure might have produced.

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I don't believe I understand how to do focus prior to imaging. I have a non-GoTo mount. After aligning the scope I point it to a bright star and get it centered in the eyepiece. I then remove the diagonal and insert the camera at prime focus and proceed to use Frame and Focus with FWHM to achieve focus. Once complete, I reposition the scope to the object I want to image, spiral galaxy, however, I need to replace the camera with the diagonal in order to fine tune the positioning to get the object within the FOV and I need to change focus to do this. Once I have the object within the FOV I reinsert the camera but now I'm out of focus with no bright stars in the FOV. I'm confused, what's the process I should go through in order to image a faint object with no bright stars in the FOV.

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Because you do not have a GOTO mount, the "normal" LiveView-based focusing won't work. 

 

Here is one workflow that may work for you:

 

It will help to know approximately where your focuser needs to be when your camera is installed in your usual configuration. If you have a drawtube with a scale, it becomes easy. Otherwise if you have an SCT, it may be that you must remember that focus with the camera is X turns counterclockwise from where the centering eyepiece is in focus. That will get you close.

 

So now you are pointed at your target and close to focus. Use BYN's Frame & Focus to take a short snap image. Once it the snap image is displayed you can treat it just like a LiveView "movie". Double-click on the image to unlock the zoom box. Move the mouse pointer to the brightest star in the image and double-click again to lock the zoom box at that position. BYN should now display the FWHM of the star. Use that to adjust the focuser and then take another snap image; adjust the focuser, and repeat until you are happy with the focus (at the minimum FWHM). If you don't see stars with the short snap image then you will have to lengthen it until you do see stars.

 

I know that this process is not as fast as using LiveView, but it is very similar to what a CCD imager does to get focus, so it shouldn't be too cumbersome either.

 

I hope this helps.

 

Rick B

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John,

 

When I said that it would not work I did not mean that LiveView won't work. LiveView does work. It is just wrong for the stated workflow. Because you do not have a GOTO scope you have to remove the camera and put on an eyepiece and diagonal in order to move the scope. This step causes you to lose the focus for the camera. Then when you have the scope positioned on the imaging target there are no stars that are bright enough for LiveView. You discovered all this on your own.

 

All of this means that there are only a few ways that you can use LiveView focusing. I can think of 3. The first is if your imaging FOV happens to have a bright star. The second is if you can come up with an eyepiece/adapter combination that is parfocal with the camera. The third is to use a specialized device that can help with having an eyepiece that is parfocal with the camera. The device is called a flip mirror. In theory, the flip mirror would allow you to look through the eyepiece OR image through the camera by flipping a mirror down (to look through the off-axis eyepiece) or by flipping it up for imaging. The key to the success of using the flip mirror is the size of the mirror. A small mirror means a small field of view. You may need to see the full field that the camera will see in order to frame the image well.

 

The use of short snap images for focusing is the most common solution, doesn't require a hardware purchase, and does not require that you fiddle with the scope once you have the camera focused.

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John,

 

What you are seeing is normal for LiveView mode. Dim details and deep sky objects will not show up because the exposure duration of the LiveView frames is only a few milliseconds. It is, however very good for using bright stars for focusing.

 

For framing you need to use the Snap Image functionality to shoot longer exposures, on the order of say 10-30 seconds. Depending on the Save Snap setting these images may or may not be saved.

 

I hope this helps.

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A live view frame is only a few milliseconds of data, you won't see dim stars in live view and you certainly will not see DSO.  A few milliseconds is simply not enough to resolve anything but bright stars.

 

If you want to see anything but bright stars in Frame&Focus you will need to snap pictures; this is why this option is there :)

 

Regards,

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When in Frame&Focus Mode, also remember to set your ISO to it's Highest Value.

 

This will result in significant additional Noise, granted, but will amplify the Short-Exposure Signal so as to afford you some additional "Dimmer Stars" to aid in your Framing effort.  And THAT is the purpose of this Mode.

 

THEN, ensure that you restore your ISO Settings to "Usable Values" (usually 800-1600ISO for all DSLRs) when changing to Imaging Mode.

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