I was wondering what the correct distance from the sensor in the STF8300M to the lens of a barlow should be for prime focus? Having some issues with sharp focus, so looking for some insight. Images are close, but still, even with maximum focusing effort, particularly planets have a soft appearance. Image train: LX200GPS SCT/ Moonlight focuser /Televue Big Barlow 2x /nosepiece /AO8 /SC2 /FW8 /STF8300M. Per Televues' site, the Big Barlow is 3x at 100 mm from the Barlow top surface. The imaging setup looks like it measures out in backfocus at 104mm, so beyond what Televue puts up. Thoughts? Is there a prime focus distance issue here?
In a simple Barlow lens, the magnification depends on the (negative) power of the lens and the spacing to the image plane (eyepiece or camera). If the distance is increased the magnification will also increase. It's possible that a multi-element lens might have a "working range" beyond which the quality declines. However, if you're only a few mm off it should work okay. If your images are soft then either (a) you've got too much focal length and are oversampling the seeing, or (b) something in your image train isn't making good images, or (c) the seeing is bad tonight. I use a simple formula to calculate resolution: Pixel Size (arc-seconds) = 206 * Pixel Size (microns ) / Focal Length (mm) For example, if you have a 8" f/10 that's about 2000 mm focal length. A 5.4 micron pixel gives you about 0.56 arc-seconds per pixel. The situation will be a bit different for deep sky and planetary imaging. Deep sky: Not being on a mountaintop, the best long-exposure seeing I get is around 2.5 arc-seconds FWHM (the best I ever got was 2.1 arc seconds... those conditions lasted under an hour). A "fair" night is 3 to 3.5 arc-seconds. If you're on a mountain then you can get under 2 arc-seconds... if on Mauna Kea you can get under 1 arc-seconds. You want about 3 pixels across the FWHM. I run at 0.7 arc-seconds per pixel, which gives me good sampling on the best nights. You camera without a barlow probably does a good job for deep sky. Planetary: With very short exposures you can "freeze" the seeing, which allows you to run with more resolution. You take a lot of very short exposures and toss out the bad ones prior to stacking. Let's assume for the sake of argument you can get 0.5 arcsecond FWHM in these conditions. In that case you might aim for 0.15 arc-second resolution. Your camera with the barlow - assuming you actually get 3X - will be about 0.19 arc-seconds for an 8" f/10. It's in the ballpark. Planetary imaging requires good seeing and excellent optical quality. The reality is that optical quality in commercial schmidt-cassegrain telescopes can be variable. Over the years I've owned five different SCT's... two were good, two were fair, and one was absolute garbage.
doug - really great info. that is super helpful, especially to a newbie. Thank you! Suppose that's the case when they are punched out in a factory - maybe a tad less quality control. I'm fortunate. My scope collimates easily and stays that way and large targets (like the moon) or terrestrial distance shots are pretty sharp. Just trying to get the sharpest images possible - I'm jealous of those that have planetary images from smaller scope, especially to the level of detail. Scope envy perhaps. LOL. Again, thank you.
