So how bad is the EdgeHD 9.25 reducer anyway?

I recent juggled with the decision to go with the 9.25 Edge or the plain old 9.25 SCT.

There were two factors it boiled down to for me:

• I don’t have an observatory, so shooting at native focal length just wasn’t going to happen for me. The large scope is a sail and any breeze will impact your subframes. The main benefit of the edge is to get a flat field at the native focal length. I even struggle a little bit shooting at about 1450 and f6.3 with the Starizona .63x reducer. The whole setup is much more sensistive to environmental factors than if you were shooting at around 800mm FL or less. If you’re going to reduce it anyway, there’s some debate out there about the plain SCT with a Starizona corrector actually being sharper than the Edge with the Celestron Reducer. I haven’t been able to test this side by side, but I’ve been pleased with the Starizona corrector with my non-edge 9.25. Ultimately, my thought was that if it’s unlikely that I’ll use the native focal length, I might as well save myself $1,200 and go for the non-edge scope. (I actually saved even more than that when I bought mine.)

• Speaking of reducers, there’s not really other options for reducing the edge, other than the Celestron .7x Reducer and the Hyperstar. Whereas the plain SCT can utilize the hyperstar, Starizona Night Owl .4x, and the .63x (starizona or celestron). I guess the Night Owl is being redesigned and it’s hard to find one used, but nonetheless, I use a 533MC, and it would be a great fit for me. It’s nice to have the another reducer option that would cover such a wide range of focal lengths. with the plain SCT and those reducers, you’d be shooting at about 525mm, 870mm, 1450mm, 2350mm, and barlowed (presumably at 2x) for planetary around 4700mm. The edge loses out on the 870mm length, and is a little slower at 1600mm. Plus, depending on your skies, being at a slightly lower FL around 1450mm may give you slightly better sampling than 1600mm.

In the end, as I mentioned, I decided on the plain SCT to save some money, since I wouldn’t be able to take full advantage of the native FL of the Edge (not without considerable expense at least (probably need a new mount and observatory too! 😅)

John,

I am also a friend of “no spikes”. My experience is based in the Edge 11, which is a similar design, but the C9.25 has a somewhat slower main mirror (f2.3 vs f2) and less curved secondary, which results is slightly better optical specs. For the Edge11, I discussed some issues in the following post:

https://app.astrobin.com/forum/topic/49075/reducer/celestron-edge-reducer-some-experiences-and-experiments#post-79255

basically, the native Edge has a much cleaner optical performance than the reduced Edge. Problems are worst in the blue when you see blue flares well within the area covered by a MFT detector, so it should be worse for an APS-C. I pixel peeped a lot of images on astrobin, and it seems a very common problem. You can mitigate the problem by using Baader’s fringe killer., but loose quite a fraction of the blue light (which is starved anyway owing to pollution in suburbia). BXT can also help fixing it (though not always). but a fix is a fix, ie a repaired image rather than a good image.

So I ended up using most of the time my Edge 11 at native f10, using the reducer only when I really need the FOV (and even then I would consider mosaicing instead + binning)

CS Matthias

I am not a fan of reducers. Save yourself the trouble and just get a bigger sensor. Reducers sound good in theory but in practice they often have trouble with aberration and/or stray light. I have never had good luck with any of the reducers that I’ve tried—ranging from Celestron to Astro-Physics. I’m even ready to get rid of the field flattener on my ASA600 because of its problems with stray light.

John

I have an EdgeHD 9.25” with the reducer and an asi2600mm. The stars in the corners are not great, but BlurX helps and I tend to just crop them out. I don’t have any issues with the center of the image. This is the master in Luminance on the Cocoon Nebula with just an STF, no other processing. F/10 is very slow so I definitely did not want to image at that speed. You can check my uploads for processed images. I am in the process of upgrading to 3nm filters.

📷 image.png

andrea tasselli:
https://www.stellaroptical.com/santel-mk9.htm

NE-FL-Astro

John Stone:

andrea tasselli:
https://www.stellaroptical.com/santel-mk9.htm

Thanks.   This looks one heck of a planetary scope for sure!   But it's focal length/ratio much longer/slower what I'm looking for for DSO.

However this one would do the trick:  https://www.stellaroptical.com/newt-mn106.htm

I don't know the size of the corrected image circle on it or the back-focus available but in theory it checks all my boxes.

However I think that webpage was last updated in 2005.   Are these scopes still available?

John Stone · Sep 2, 2025 at 08:05 PM

When you image at native F10 what do you do about the extreme oversampling of the seeing?  Bin?

But then your already long exposures need to become longer to overcome the additional 2x read-noise you get when binning x2 CMOS cameras, right? 

The normal 1.5e- error on the IMX533/571/455/451 becomes 3e- if you're in HCG (double that for LCG) and that's just for Bin2.

But then you might run into dynamic range issues due to the longer exposures, right?

I'm curious about your thinking on the limits of binning vs exposure time.

It depends on your seeing conditions. I just go with the extreme over sampling on my ASA600 because the seeing is sometimes sub-arcsecond and BXT does better with slightly over-sampled data. If the seeing is poor, it’s easy to bin in processing. Regardless, most of the time, reducers are more about field of view than sampling. Under 1.5” conditions, the optimal seeing for a 10” scope at F/10 is about 7.2 microns, which might be ~6.9 microns for a 9.25”. That’s about perfect for 2×2 binning with a 3.786 micron pixel, which will double the SNR. With a reducer at say F/7, the optimum sampling becomes around 5 microns, which is slightly under-sampled for 2×2 binning and slightly over-sampled for no binning. Read noise is independent of exposure length so the only concern is to make sure that it’s small compared to photon noise. As long as you aren’t doing lucky imaging with super-short exposures, the read noise for most “common” long exposures (say 3-10 minutes) isn’t going to be a problem. And…all of these considerations pale in comparison to the problems you’ll face if you find stray light problems or serious halos around bright stars introduced by the reducer—and that happens all the time. Yes, some reducers work well (and if they are perfectly made, they should) but I have fielded so many questions behind the scenes from folks who have problems with their reducers that I’ve lost count. The biggest problem is that if a reducer doesn’t work well, you can’t fix it. The simple fact is that reducers are VERY hard to make correctly. If it’s not right, it either has to go back to the manufacturer, you have to sell it, or you put it on the shelf and go back to plan A, which is running the scope without a reducer. That’s why I’m telling you to skip all the intermediate steps and just start with a bigger sensor and skip the reducer in the first place. Maybe it will work out if you ignore my advice but remember the words of Dirty Harry, “Do I feel lucky? Well, do you…?” I’ve personally played the odds and every reducer that I’ve ever bought has ended up on the shelf.

John

I’m using the 2400mc with the edge 9.25” at f/10. This camera has larger pixels and work well at bin1. The other approach I would go is bin2 with the 6200mm as John recommends.

I have the reducer but don’t use it. I think the full frame / large pixels-binnning/ f-10 combo works very well with most target I would frame with an SCT