barlow back focus
Posted 23 June 2022 - 08:46 PM
I have this barlow. I am interested in using it with my 8" SCT. The lens part unscrews and can be used in an imaging train.
Is there an exact distance I should place this from my sensor? I can't find any docs on it.
On my Edgehd 8 would I put the beginning of the barlow at 133mm from the rear and then an exact distance between that and the sensor?
Posted 23 June 2022 - 09:14 PM
The spacing between the scope and the barlow is not particularly critical, although if you want to maintain an exact f/10 focal ratio on your EdgeHD then you should place some thought into where you place the barlow. But, this is complicated because you don't really know the particulars of the barlow's design (like where its entrance pupil sits).
As for the spacing between the barlow and the sensor, that depends upon the barlow. So, without any documentation all you can do it to make a best guess or try to measure the resulting magnification ratio in your images (taking into consideration that your EdgeHD may not be outputting at exactly f/10). Thus, if you were sure that the EdgeHD was focused so as to deliver an f/10 focal ratio and that your barlow is specified as 2X then your final focal ratio at a given (correct?) spacing between the barlow and sensor should be f/20 (2 x 10). And, of course, you can calculate focal length by just multiplying the f-ratio by the aperture of the scope, so in this last case you'd be trying to achieve a focal length of 20 x 200mm = 4000mm.
Best case, and probably way overkill, focus you EdgeHD without the barlow at the specified backfocus for the EdgeHD. Then experiment with the placement of the barlow (without refocusing) so that you get the exact magnification that is specified by the barlow. There will be some spacing between the scope and barlow where the image will be in or close to focus (without changing the focus on the EdgeHD) and then you can adjust the spacing between the barlow and the sensor to where you get the barlow's specificed magnification ratio. This may require an iterative process since the change in one spacing may affect the required spacing on the other end or side of the barlow.
However, the above technique is probably completely unnecessary if you have a simple barlow with some knowledge of the spacing required between the barlow lens and the sensor to achieve its stated magnification ratio.
So, it's probably way too much work for most users and will not likely result in better results but it should serve as an aid in actually getting close to what you are after (in terms of setup and magnification ratio). You can measure your resulting magnification or f-ratio or focal length using a plate solving tool or you can just compare the image size between using and not using the barlow.
In any case, I suspect that the spacing you want between the barlow and the sensor is something around 50mm (between the lens and sensor). Note as you change that spacing you will most likely also change the magnification ratio of the barlow. Most often the larger the spacing (between the barlow and sensor) the higher the magnification.
Knowing the above (the approximate spacing required between the barlow lens and the sensor) you can use simple geometry to determine where to place the barlow in relation to the back of the EdgeHD or scope. Since the recommended backfocus on an 8" EdgeHD is 133mm and the spacing between the barlow lens and the sensor is 50mm (assumed) then you can calculate where to place the barlow by just dividing that 50mm by the magnification factor of the barlow and then subtracting that from the 133mm required for the backfocus on the 8" EdgeHD. So, the spacing between the EdgeHD and the barlow lens should be 133mm - 50mm/2 = 108mm.
The above should place the primary mirror position (focus point) exactly where it would be when not using the barlow since the barlow will effectively extend the final focus distance by its magnification ratio. Just member to measure these distances from the center of the lens in the barlow, not from the ends of the barlow housing. This also only works if you have a simple two-element barlow, it won't work (exactly) for more complex designs like a Tele Vue Powermate or any similar, multi-element focal extender.
Edited by james7ca, 23 June 2022 - 10:07 PM.
- johnsonjp34 likes this
Posted 24 June 2022 - 07:11 AM
Thanks James for the thorough explanation. Looking forward to trying this out with the planets coming up sooner!
Posted 24 June 2022 - 11:04 AM