Budget is ~$10k, looking to set up a remote (bortle 1) relatively fast imaging system with medium/long focal length for up close nebula shots and galaxy hunting.
I’m thinking one of the following:
Celestron C14 Edge HD (slowest, highest focal length)
Planewave CDK 12.5 (slower, high focal length)
Askar SCA 310 (Fast, medium focal length)
There are clear benefits/drawbacks to all of these, just curious what any users think.
To my mind this is really and apples or oranges choice. Of the two long FL instruments and since you’re going remote (deep sky imaging only) I would go with the CDK. It’s going to give you better overall optical performance.
The Askar isn’t anywhere in the focal length class of the other two.
I think relevance of long fl is waining with the demise of large-pixel CCD sensors. Check local seeing against pixel scale of desirable system first. Oversampling factor of 3+ wont result in better images, however slow light gathering power will be clear loss. I’ve chosen parabolic mirror with newtonian-type reducer in prime focus, without secondary. Works extremely well. Also, things like Delta Rho from Planewave or OS Veloce are worth considering IMO.
Sergey Rakov · Mar 3, 2026, 04:45 PM
I think relevance of long fl is waining with the demise of large-pixel CCD sensors. Check local seeing against pixel scale of desirable system first. Oversampling factor of 3+ wont result in better images, however slow light gathering power will be clear loss. I’ve chosen parabolic mirror with newtonian-type reducer in prime focus, without secondary. Works extremely well. Also, things like Delta Rho from Planewave or OS Veloce are worth considering IMO.
That’s interesting and I’ve often though about trying it but I worry about three things:
A large central obstruction
Odd diffraction patterns from cabling
Heat from the camera disturbing the wavefront
How have you mitigated those effects?
I would also wonder how you deal with focus and filter changes?
It’s quite possible that I’m just displaying my ignorance here, but I’m not sure I understand the appeal of ultra long focal length telescopes.
With a 14” Celestron Edge OTA (focal length 3920 mm) and a typical 3.5um pixel camera, you’re going to have 0.2 arcsec per pixel at F11. You’ll be lucky if seeing is 0.8 arcsec. So on a typical day it seems like you’d get pretty much the same results with an Edge 9.25 (focal length 2350, 0.33 arcsec/pixel) after cropping.
The 14 inch Edge costs about $3K more, and given that it weighs 45 lbs vs 23 for the 9.25, you’ll probably end up spending an extra $3K or more for a suitable mount, and maybe higher monthly fees at the hosting site. If you end up unhappy or leave the hobby, the 9.25 will be a lot easier to sell.
đź“· IMG_4540.jpeg
I will try to address your concerns
Central obstruction from camera and corrector is actually less then secondary, required for 17” f3,5 will produce. The larger issue is with filter wheel. I previously used ZWO mini and consequently imx533 small sensor camera - since I mostly were interested in galaxies it was viable compromise. Later I’ve obtained custom double-carousel wheel for six 36mm filters (actually there is 8 slots, but 2 need to be empty) and managed to install larger imx571 sensor without expanding CO at all. Theoretically, even 6×50mm wheel of this type won’t present significant CO for mirrors close to 20”.
Cables are routed along the spider and don’t cause any additional diffraction spikes. Somewhat offset CO also doesn’t seem to affect spots in meaningful way.
Focuser is 3” custom, works by moving corrector assembly. EAF is used as drive part.
As to thermal interference (besides camera I have heated baffle for corrector lens) - I had my doubts, but in practice turning heated/cooling on never produced adverse effects on FWHM. Possibly with comparatively short (1,2m) FL it’s not an issue. With RASA it seems fine as well
đź“· IMG_4541.jpeg