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I've been struggling to find an answer to whether a 2.5" focuser is likely to cause vignetting on a full-frame (36x24mm) camera sensor.
This would be for an f/7 130mm scope, probably used in conjunction with a 3" 0.79x focal reducer (stated to have an illuminated image circle of 45mm).
Whilst the focal reducer should fully illuminate the sensor (having an M68 connector on the scope side and M48 on the camera side), it is not clear whether any "mechanical" vignetting might occur with a 2.5" focuser drawtube.
I imagine this would depend on the backfocus of the scope - i.e. how far the focal plane is behind the end of the focuser. If it's a long way back, I can visualize the light cone potentially being restricted when it enters the drawtube inside the OTA. Hopefully, the scope isn't designed this way, but it's hard to tell.
If there is no mechanical vignetting, it does raise the question of what the benefits of larger focusers are. Greater load capacity certainly; improved stability? maybe. But does it actually give a wider unvignetted image circle?
I'd like to apply some science to this question, rather than buying a bigger focuser "just in case"!
Thanks for any answers!
Greetings SGL members.
It has been some time since I have posted any new film images. It is true that I have been having a fling with a mistress named DSLR, but my heart is in these images done in the way I learned thirty years ago.
My last imaging session according to my Log book was October 11, 2012. I had just finished a roll of Superia that night and I almost never returned. I began experimenting with digital. It was fun and productive but I missed my old film images. I was delayed further about one year ago. Without warning I suffered a severe heart attack. It was quite the scene. I was a goner for sure if not for the help of some cracker jack EMT's that kept CPR going. I was gone for over 15 minutes. The ER got me back and the chopper flight to an emergency heart catheterization lab placed a stent and after several months of rehab I am back and good as new!
I had one great session in late June. For several back-to-back nights the weather cooperated enough to image with a new lens and I wanted to share with you all.
The images below are taken with the Pentax 67 and the 400mm F/4 SMC Takumar lens. I used apertures of f/4.8 or f/5.6 for 40-50 minute exposures on Fuji Neopan Acros 100 black and white negative film.
You can read about my project plan and inspiration here:
You can view the images and read about them here:
Here is a legend to the several images taken in June.
Pictures at Eleven by Nightfly Photographic, on Flickr
Below are some of the results. These are preliminary images that were scanned rather hastily, but these results look promising. I need to go back and remaster each image, something that will take me awhile. A good task for winter when I am shut indoors.
Messier 24 Region by Nightfly Photographic, on Flickr
Region of Dark Lanes in Ophiuchus by Nightfly Photographic, on Flickr
The Southern Scutum Star Cloud by Nightfly Photographic, on Flickr
The Dark Nebulae Around Theta Ophiuchi by Nightfly Photographic, on Flickr
Apologies for the smaller sized image here:
The Great Star Cloud in Scutum by Nightfly Photographic, on Flickr
Messier 22 Region in Sagittarius by Nightfly Photographic, on Flickr
The Great Sagittarius Star Cloud by Nightfly Photographic, on Flickr
Thanks for looking. I hope you enjoyed them.