Is an 8" Edge HD worth it for me?

andrea tasselli:
Do the math, see which one is the better deal (hint: it ain't the LZOS).

I already have the Lzos in use with the 585.
I would buy the EHD additionally.

I stood infront the same decision a month ago. I have a Quattro 150P with the IMX585 and I recently ordered the 8" EdgeHD with an IMX571.

Yes, the field of view is the exact same with both of those setups, and the Quattro 150P is alot faster at F3.45, but I will get much more… and I mean MUCH MORE details with the 8" EdgeHD and the IMX571.
Also… I ordered the 0.7x reducer for the EdgeHD and plan to get the Hyperstar in the future aswell. So I'll have a 8" F10 (F20 with barlow) for planets, a 8" F7 for galaxies and smaller nebulas, and once I get the Hyperstar I'll have a 8" F1.9 for widefield milky way imaging.

I would've loved to get the bigger 9.25" EdgeHD as it has a bigger corrected imagecircle, but it would require a beefier mount and made it so much more expensive.

The EdgeHD is the most versatile telescope you can get at this moment, and I know nobody who regrets buying it.

The LZOS has a Dawes limit of 1.01 arc sec and the edge 8 has a Dawes limit of .57 arc sec. You’d theoretically be giving yourself the opportunity for twice the resolution. Seeing is going to be a big deal for imaging at that focal length. You’ll be limited to imaging only on nights with good seeing. I think it would be a nice upgrade especially for galaxy season.

Technically it's allmost four times the resoultion, as one pixel turns into 2x2 pixels.

But yes, good seeing and guiding is mandatory with those longer focal length scopes. If you're not living in a rural area then you won't ever see seeing conditions of less than 1" to begin with, as the urban area releases alot of heat leading to turbulent atmospheric conditions. I live in a small town/village with 8k population, and I get 2" of seeing usually.
Still, with the higher resolution of the image you can apply alot more software tools afterwards to sharpen/enhance the image.

Larger aperture, do it……

I recently bought an EdgeHD 8 and I love it. I took a little work to get it setup and operating smoothly, but I'm enjoying it. I use it with an IMX571 as well.

My region doesn't have the best seeing conditions, so I'm pushing it a little bit there. But with BXT, you can reduce that impact. Not perfect, but I'm happy with it.

If you're using an OAG, a larger frame guide cam (e.g. ASI174MM) is a must.

Rodolphe Goldsztejn:
Also, carefully plan for the backfocus requirements on the C8 EdgeHD. It's only 133 mm, which leaves little room for an OAG + filter wheel. With the reducer, it's even less, at 105 mm.
OAG at the 2000mm focal length is maybe not the best option, it becomes difficult to find a guidie star, and meridian flip doesn't help in this regard.
I use parallel guiding quite successfully, which solves both issues (OAG and backfocus requirement).
The C8 is a great instrument despite these points of attention.
My 2 cents.

The backfocus is no problem really, as 133 without or 105mm with the 0.7x reducer is alot more than the usual 55mm you get with most modern scopes. Most manufacturers that offer OAGs and filterwheels allerady take that into account, and the combination of those two is usually exactly 55mm. Atleast the big ones like ZWO, ToupTek, Askar or Pegasus all adhere to the 55mm backspacing standard.

But yeah. Using the C8 with an OAG is very ill advised imho, as the illuminated imagecircle simply isn't big enough when using the IMX571 sensor. You need to push the prism of the OAG so far out of center that there's basically nothing left to hit the sensor of the guidecamera. Especially when you use the reducer, which only has a 30mm illuminated image circle, compared to the 44mm for the C9.25, C11 or C14.

The common wisdom is that a guidescope needs to be atleast 1/4 the focal length of the mainscope, but I don't find this to be true in this day and age anymore, as there's plenty of people who guide their SCTs succesfully with ~250mm guidescopes. Mounts and the PHD2 software have gotten better to allow this.

As far as I understand the issue with mirrorlock and autofocusing on the SCT basically boils down to the last step of your autofocusing routine. It needs to be counterclockwise to put pressure on the focusing mechanism, which then should basically "lock" the mirror in place.

The mirrorflop is a sideways movement of the mirror, due to manufacturing tolerances of the central hole of the mirror and the tube it goes around. Atleast that's how I understand it and in this case the locking screws don't do much to prevent it.

Looking at all the images - and there's alot of them fortunately for SCTs - I don't see a real problem there, as it's easily fixed with simply doing autofocusing after meridian flip. Atleast that's the most common answer regarding this issue.

Go for it. Larger aperture + apsc sensor, the gain (tiny objects) in fov may not look much from the comparison above, but the details and resolution will be amazing.