Looking for big aperture astrograph

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.

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.

John Tucker · Mar 3, 2026 at 05:48 PM

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.

Well personally I wouldn’t use a long FC cat like that, I’d employ binning to bring it up to a more sensible imaging scale and speed the system up dramatically.

A C14 imaging at bin3×3 will be 900% faster than a C925 unbinned with the same camera (on extended sources)

Of course your overall image size will be severely reduced, but if your targets are small PNs and galaxies that shouldn’t be an issue

Sergey Rakov · Mar 3, 2026, 05:56 PM

📷 IMG_4540.jpegI 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

📷 IMG_4541.jpeg

Thanks for giving the close look, always interesting to see what can be done that’s a little different.

Rick Krejci · Mar 3, 2026 at 08:34 PM

Craig Towell · Mar 3, 2026, 06:38 PM

If I was in your shoes I’d get the CDK, pair it with a full frame camera like a 6200 and run it binned. X2 in good seeing for luminance and x3 or even x4 in poor seeing or for colour.

With modern CMOS cameras with low read noise, I wouldn’t bin at all unless you just want smaller file sizes assuming you’re sky background is well above the read noise floor. Binning at image time is identical to resampling in post unlike the CCDs of old.

I’d rather re-sample in post and choose the resampling amount that gives you star and object detail which may not be what binning gives you. Depending on seeing, that may be 0.8x, 0.5× 0.4x…whatever you think looks best. And you may have better seeing than expected, but binning is irreversible. Deconvolution like BlurX also works better with oversampled raws and then downsampling as well.

Yeah that is kind of what I meant but I wasn’t clear… with cmos I would also just capture in bin1×1 and then resample in post

I don’t see any cloud proofing.

Does your idea include cloud proofing? (Other than the clouds can freely drift through the cage….)

Out of three I would go with the CDK.

The FoV might be on the smaller side with that focal length of the EdgeHD 14, but it depends on the size of your objects you want to image. On top of that I would expect the CDK to be the more convenient scope (open tube, less dew, wind and ventilation concerns). The Edge has still more aperture, though.

📷 edge14_cdk12-5.jpg

(Target is the Owl Nebula)

The Askar looks like a nice scope with a way bigger FoV which should be nice for wider fields. But for smaller targets you will be cropping a lot.

📷 scope-comparison.jpg

(Target is the Owl Nebula)

For me a No-No is the backfocus of the scope itself. Depending on your setup you can’t even install a mono system with an OAG. (38-66mm)
📷 sca310_backfocus.jpg

Edit: I checked the optical efficiency of the CDK vs the SCA310 and the CDK12.5 collects 1.44x more photons than the SCA310.
I assumed/looked up the following parameters:
SCA310:
Mirrors with 95% reflectivity
Corrector with 3 lenses with 99.5% transmission each
62% obstruction

Source: https://www.teleskop-express.de/en/askar-sharpstar-173/flatfield-cassegrain-telescopes-319/sharpstar-sca310-310-mm-f-3-8-aspherical-cassegrain-astrograph-19390

CDK12.5:
Mirrors with 96% reflectivity
Corrector with 3 lenses with 99.5% transmission each
42% obstruction

Source: https://www.planewave.eu/produkte/teleskope/cdk125-astrograph-f/8-mit-quarzglas-optik

📷 grafik.png

📷 grafik.pngSource: https://lambermont.dyndns.org/astro/code/compare-telescopes.html

I’ve owned and interferometrically tested both C14 Edge systems and a 20” CDK system so I can say a little about both of them for imaging.

1) Edge 14

PROS: These systems can be diffraction limited. The overall system wavefront is smooth and the field is very well corrected. Good optical alignment is very easy to achieve and if you don’t mess with it, the system will hold alignment fairly well. These are the least expensive telescopes for the aperture so they can represent excellent value. The plate scale is actually very good for galaxy imaging and there are a wide range of accessories available for these scopes. Vignetting is “reasonable”—even up to a 36 mm x 36 mm sensor. Since the secondary is supported by the corrector plate, there are no diffraction spikes.

CONS: The optical quality appears to be quite variable. You may get a good one and you may not. You’ll only know when you set it up to try it. The hardware is inexpensive black oxide screws made of soft metal so screws may be easy to strip. The mechanics are marginal. Focusing with the primary mirror introduces image shift so you may need an external focuser. The factory dovetail is way too flimsy for serious imaging so it should be replaced with something more solid.

2) Planewave CDK Scopes

PROS: These systems have a diffraction limited design over a very wide field and can produce excellent imaging. They are easy to optically align and they have excellent mechanics with high quality hardware. Factory support used to be outstanding; however, things have recently changed to a support ticket system. Hopefully, the factory is keeping up and support quality is unchanged.

CONS: The optical quality appears to be quite variable. I’ve seen a number of Planewave telescopes that have had to be replaced because of poor optical quality. Even the good optics that I’ve seen tend to suffer from excessive mid-spatial frequency wavefront errors that can affect overall image quality. This is a complicated way of saying that the optical figure is not very smooth. To be fair, I have not tested very many PW telescopes; however, the mid-spatial frequency wavefront errors are typically produced by the way that PW makes their optics. Because their fabrication methods don’t allow figure control all the way to the edge of the glass, the primary mirrors are equipped with an aperture mask. Sometimes that mask does not completely hide the errors at the edge of the mirror and that results in problems with excessive SA, which appears to be a recurring problem. The good news is that PW has been VERY good about fixing problems with the optics when they occur. Beyond that, price is the only other “downside.” PW telescopes are more expensive than Celestron telescopes but they also have higher mechanical quality with fused silica optics and a carbon fiber truss.

If you get good optics, either of these telescopes can produce outstanding results. I ran an Edge 14 in NM for about 5 years and a number of images from that scope are still on display in my image gallery if you want to check it out. I also have test reports for my CDK20 in my AB gallery and if you search Cloudy Nights, I posted two detailed test reports for interferometrically testing two Edge 14 telescopes. As I recall, one of them achieved a Stehl of 0.92 so it was quite good.

John

Photon_Collector · Mar 3, 2026, 03:54 PM

• Celestron C14 Edge HD (slowest, highest focal length)

• Planewave CDK 12.5 (slower, high focal length)

• Askar SCA 310 (Fast, medium focal length)

C14 - Closed system, easy to collimate, remains dust free inside for decades. Check out presentation by Bob Majewski on TAIC for optics analysis.

CDK 12.5. Open system, cools faster, can collect dust or spider webs depending on location. Collimation of these scopes is an art that needs to be learned.

SCA 310 - I think you meant Sharpstar, not Askar. I think these optics as advertised are too good to be true. Collimation is the hardest, and sometimes doesn’t work at all. I’d go with a RASA or a Celestron SCT with Hyperstar accessory. Hyperstar can be added or removed, so two scopes in one. F2 systems also require patience and time to collimate well.

Bortle 1 will give nice results only if the local seeing is good, like approaching 1 arcsec. Otherwise long focal lengths will not be utilized to their fullest.

Good luck!