C8 Edge HD for galaxies and DSO's vs 120 mm apo

Hi Ian.

I don't know how valid my vote is on this matter, since the largest refracter I own is a William Optics GT81, so I don't know what kind of result you would get from the Skywatcher. I do though own the C8 edge HD with the 0.7 reducer, and I've been very happy with what it can do. To me it the ideal scope for galaxies, since it's easy to set up due to the size, but at the same time you can get some really nice details on those galaxies.
The C8 was my entry into the world of AP, so to me it was very important, that it was a rather user friendly telescope. The only mistake I made, was I started out wanting to guide with an OAG. That did not work very well for me due to the small field of view. For a long time I then guided with a Z61, but have now stubbornly moved back to OAG, which now seem to work. (A bit more tricky to find a guide star than on the GT81).

Also if you want to image planets and the moon, the C8 is also very capable.
I believe that the C8 is a really good quality telescope, that can deliver very impressive results. So I can highly recommend it. 

An example of the C8 with a 60Da/ 071MC pro:
https://www.astrobin.com/bl5kuh/?nc=user

Note that you might be oversampling a little bit with those cameras on the C8.
Kind regards and clear skies
Dan

Hi Ian,

Before our small island will be totally isolated, yesterday, I have ordered a C8 XLT for both lunar and galaxies,
believing/hoping that is the right choice. I still have a 0.63x reducer in that I do not worry about the image edges.
Imaging the moon with a small sensor like that in the ASI290MM should be painless, especially with the Edge.

Since the Edge HD 800 is natively fully corrected you won't encounter any issues with elongated stars.
If I had the budget I'd go for the Edge HD 800, but it is out of stock anyway. Hope to share C8 images soon.

In my view the C8/Edge advantages are:

• Compact, light weight
• Easy to collimate, holds collimation for quite a while
• Mirror does not or hardly contaminate
• Fastar/Hyperstar compatible for imaging at f2 (needs approval by domestic minister of finance and/or sell your ASI2600MC)

The disadvantages:

• Temperature adjustment takes time (the Edge has vents to accelerate equilibrium)
• Focusing by moving main mirror MAY cause tilt (the Edge has a mirror lock).
• 0.7x Reducer for the Edge is expensive (you need to sell your 70mm APO)
• No 10:1 dual-speed fine focusing like APOs

Should you go for the Edge please look at multi-star guiding in PHD's latest development version.
For increased rigidity I'd go for the Losmandy-style dovetail since your mount can handle it.

Anyway, have fun!
Robert

Dan, Robert, PP55, and Michael

I appreciate all the comments - very helpful.  Its especially useful to see the results posted.  The used C8 edge HD that I have my eye on already comes with a dedicated reducer, dew shield, etc.  I have been the using ASIair pro for the past 6 months with the small apo and it uses a variant of PHD guiding, but its not multi-star.

Best,
Ian

Hi Ian,

Yes, the edge 8 is an excellent scope for imaging.  I have used it for several images last year (my other scope also being a 70mm refractor).

The increased focal length does create challenges.. but this is going to be the case no matter what scope type you use.  I recommend using an OAG.  For auto focus I have been using the celestron focus motor and its been working well.

I use the ASIAIR pro as well, and I don't have problems with guiding. Just keep in mind, that you need a rather long focal length on you guide scope (preferable 1/3 or better of the image train).
A note to the collimating. The first long period I never collimated the C8. It holds the collimation very very well. I've switched to Bobs knobs now, since I'm planning to use the C8 for a bit of planetary as well. That switch will reduce how long it's holding the collimation.

If you are able to buy the whole package with reducer and dew shield from your friend for a reasonable price, I think it's worth considering.

Hi Dan,

Yes, was thinking about the guide scope... I could use the 70mm apo to guide - its native fl is 420 mm, which would be almost there with the use of the reducer on the C8 edge.

Good point about collimating, this is something I always do with my 15" dob, I meant to ask. 
Best,
I

James:
Hi Ian,

Yes, the edge 8 is an excellent scope for imaging.  I have used it for several images last year (my other scope also being a 70mm refractor).

The increased focal length does create challenges.. but this is going to be the case no matter what scope type you use.  I recommend using an OAG.  For auto focus I have been using the celestron focus motor and its been working well.

Thanks James,  the OAG would save a lot of mass!

Yeah I'd also like to cast the vote that you should forget the dream of using a guide scope on an SCT like the EdgeHD, SE, etc., without having distorted stars (and therefore, signal).

This is not because of the focal length; when I first started guiding, I got a 70mm guide scope to piggy back and it was a phenomenal piece of equipment, and the guide star was more than adequately sampled, even when imaging at prime focus (2032mm). The problem is that the guide scope will fail to account for the biggest sources of error in the system: differential flexure and mirror movements ("drift" and "flop" ) . I guided very well with that setup but never achieved round enough (I'm picky) stars because of these problems, and they change throughout the night because of the angular differences as the mount tracks. In some instances, I had football-shaped stars with excellent guiding. This is of course because the guider is keeping the star centered according to what the guide camera sees while piggybacked on the telescope, however it can't correct for movement of that system with respect to the imaging scope, nor can it correct for the movement of the mirror in the SCT. I was confident there wasn't TOO much differential flexure (it seemed very solid), however to this day I don't know which problem dominated. Maybe they both did equally. Neither was fixable, however. I had to return the lovely guide scope and move right into off-axis guiding. I use 5-minute subs now, and this would be impossible with the old setup. I keep almost all of them (seeing hurts me more than guiding).

Off-axis guiding is challenging. You have a high focal length at the guide camera (few, and dim, guide stars in the field of view, as well as distortion of the stars, being at the edge of the light cone). However it picks up every single problem that the imaging sensor sees and I keep most of my 5-minute subs with razor-sharp, circular stars.

In my opinion, the only proper way to even think about off-axis guiding is to use stellarium (free), and program in your specific optics. As mentioned before me above, you can see the focal length and FOV there, but you can also program in the OAG and use the program to plan exactly what angle and center coordinates you'll need for a specific target to guarantee you have a BRIGHT guide star in the field of view of the guide camera. With extreme brevity, the steps are determine ALL equipment in the optical train that you're using (OAG, spacers, rotator, etc.) and get that finalized, get the pick-off prism as far into the train as you can without casting a shadow in your images (this is hard to get optimized- too far out, and your guide stars are noodles...), find your field of view and get that into stellarium, determine the FOV of the guide camera and program that in, determine the distance between the guider and imaging FOVs and program that in (by placing a planet in the guider, taking an image through the imaging camera, plate solving the latter, and moving to the center coordinates in stellarium- then set the distance such that the guider FOV ends up over the planet in stellarium), and finally calibrate and mark the 0deg angle on the scope using plate solving and stellarium again. Yes, this takes some nights of setup.

However, I can now choose a target, go to stellarium and plan out the angle and composition for a target that gives me a bright guide star in the pick-off prism, and move the rotator to the correct angle, all during the day. At night, it's as simple as polar aligning, then using my sequence software's "center" feature to center to the coordinates determined in stellarium. My target will be centered, and the guider will see the planned, bright guide star immediately. No fuss.

It's a steep learning curve, but truly the only way to guide at high focal length properly, and actually incredibly easy to use once set up. I plan to have a detailed step-by-step for the above process on my website within a week.

Regarding the Hyperstar, I was considering that at a certain point, but could understand on various posts, that the C8 may be capable of the Hyperstar, but it's not practical. You need the 9.25 or even the C11 for it to make sense. The camera, wires etc. simply blocks to big a percentage of the mirror.

I don't know where you got your info but shooting HyperStar (HS) on an EdgeHD 8 is very practical (its why Starizona sells an EdgeHD 8 version [v4] HyperStar). The key is you get the correct camera configuration - aka "soda can"  (the Atik 460EX. ASI1600, QHY/ASI294, etc.). Obstruction is not an issue if you keep that in mind.

As others have suggested, if you go HS then using a guidescope/camera setup is fine since your FOV will be so wide (390mm). The HS is 25 times faster than native! The EdgeHD 8 w/HS is very mount friendly.

If you go native (2032mm @f/10) or with a focal reducer (FR) (1422mm @ f/7) you'll have to go with an OAG as the weaknesses of the guidescope will occur (footballs, differential flexure, etc.) At these FLs a good mount is VERY important - an EdgeHD 8 w/FR setup will begin expose your mount's weaknesses, at native it will be very shaky, anything from 11" and above will crush your mount if its mediocre. You will need an excellent (aka $$$$) mount/pier to carry the weight/FL.

Don't let the OAG intimidate you...if you have the initiative to do AP, an OAG is just another hurdle that will make you stronger once you get over it. The hardest part will be backfocus. Using an FR will speed things up and make guiding a bit more friendly. Another decision point is what guidecam and spacing you'll have based on OAG you go with. Sometimes you'll luck out.

While there are several (at times seemingly major) challenges, and over all more costly than a single good frac and mount of similar FL, the ability to shoot in three configurations on a singe OTA is a benefit. That said, a big frac with a big pixeled camera will get you very sharp images (and a very empty wallet) but you'd need three fracs to do what an Edge can do. What is saved in OTA cost is made up in cameras costs and learning how to manage three configurations on one 'scope can also appear daunting...but it can be done.

It all depends on what you want to shoot and how much $$ you wanna spend. Like everything else of value, stubborness and willingness to press through challenges is also required.

CS & GB!

I have imaged with several SCTs and other long focal length scopes.  I second the opinion that guide scopes don't work well for these, although it is mainly a matter of focal length.  Differential flexure of one type of another will likely affect many of the exposures.   Some have used guidescopes with their SCTs, but one person I worked closely with admitted he was using exposures of 5 minutes or less, accepted some oblong stars, and was willing to throw away some of his exposures.  This is a debated topic, and you can see how it works for you, and I have to say my experience lies in the 2000-3000mm focal length area.  Shorter FL may work better with a guide scope and I will be the first to admit that using a guide scope for guiding is much easier in many ways than other solutions.  My solution was to use an On axis guider (preferred if you can fit it into the optical path), an off axis guider, or a camera/filter wheel with an integral guiding chip.  These methods provided a great yield of subs with round stars.  But the smaller FOV of these solutions require another component - a rotator.  I love to image galaxies but many lie away from the plane of the Milky Way with few guide stars.  Or almost no guide stars.   Because I like to image different objects over the night, I needed a controllable way to rotate to select a guide star anywhere in the area of my target at different rotations for each, so a rotator was essential.
Good luck on  your imaging, and don't hesitate to add your experiences to the topic for all of us to learn from!

Frank

Hi Ian,
That’s a tough decision, although I am not sure they fit in the both category. I categorize my OTAs based on their FL, and the esprit 120 FL is half of what the E8HD would give your while reduced. I have both, but haven’t used the esprit 120mm yet as it has been mostly cloudy since I got it recently, but I have done a lot of work with the C8. You can check what I was able to achieve with it in my gallery (propeller to owl are with the edgehd, as well as the crab. All reduced except M101)

Here are some ideas that I would have wanted to be aware of, if I were to make a decision:

• the scale is important, you need to make sure your mount can handle the small scale when imaging at 1500mm when reduced. I own an ASI1600, which has a pixel size of 3.8um, that puts me at 0.5” when reduced, and 0.3” at F10. This leaves little to no room for errors, unless you’re ok with trailing stars or bloating.
• Collimation is not that bad. I’ve had some experience with newtonians, and SCTs are a lot easier. Don’t let this be a factor when you decide.
• Dew is a problem. The corrector plate collects dew unlike anything I’ve seen, even compared to my refractors. Dew shield is a must, heater strap might do the job as well, combine both if you can.
• The reducer is not perfect. You will see in some stars some artifacts, which I don’t see without the reducer. Some stars, will have a chunk of their outer circle in different colors. I cannot explain it further than the experiments I’ve done. You can see what I mean in my Messier 92 photo
• The reducer is not perfect #2: I had a hard time getting the stars to look normal in the corners. They look horrible in the oag due to having picking the stars from the edge of the field.
• OAG is a must!
• If I could go back in time, I am pretty sure I would have went with an 8” RC instead of the EHD, as its focal length is manageable, reduced or not.
• My limited experience with the esprit holds me from dumping conclusions on you, but it is my first scope with near perfect stars all across the field. Although it is much heavier than the edgehd, if that matters to you, and much longer, if that matters to your mount

I hope this helps. Let me know if there is anything else I can help you with

Clear skies,
Michel

Michel Makhlouta:
Hi Ian,
That’s a tough decision, although I am not sure they fit in the both category. I categorize my OTAs based on their FL, and the esprit 120 FL is half of what the E8HD would give your while reduced. I have both, but haven’t used the esprit 120mm yet as it has been mostly cloudy since I got it recently, but I have done a lot of work with the C8. You can check what I was able to achieve with it in my gallery (propeller to owl are with the edgehd, as well as the crab. All reduced except M101)

Here are some ideas that I would have wanted to be aware of, if I were to make a decision:

• the scale is important, you need to make sure your mount can handle the small scale when imaging at 1500mm when reduced. I own an ASI1600, which has a pixel size of 3.8um, that puts me at 0.5” when reduced, and 0.3” at F10. This leaves little to no room for errors, unless you’re ok with trailing stars or bloating.
• Collimation is not that bad. I’ve had some experience with newtonians, and SCTs are a lot easier. Don’t let this be a factor when you decide.
• Dew is a problem. The corrector plate collects dew unlike anything I’ve seen, even compared to my refractors. Dew shield is a must, heater strap might do the job as well, combine both if you can.
• The reducer is not perfect. You will see in some stars some artifacts, which I don’t see without the reducer. Some stars, will have a chunk of their outer circle in different colors. I cannot explain it further than the experiments I’ve done. You can see what I mean in my Messier 92 photo
• The reducer is not perfect #2: I had a hard time getting the stars to look normal in the corners. They look horrible in the oag due to having picking the stars from the edge of the field.
• OAG is a must!
• If I could go back in time, I am pretty sure I would have went with an 8” RC instead of the EHD, as its focal length is manageable, reduced or not.
• My limited experience with the esprit holds me from dumping conclusions on you, but it is my first scope with near perfect stars all across the field. Although it is much heavier than the edgehd, if that matters to you, and much longer, if that matters to your mount

I hope this helps. Let me know if there is anything else I can help you with

Clear skies,
Michel

Hi Michel,
Thanks - this is great.  Much appreciated.
Ian

But if you are happy with what your 70mm does with wide fields and want to extend your arsenal to include a galaxy hunter, perhaps you should look for something more specialized to that kind of target.

Thanks, I am really happy with the 70mm, its a great little scope.   I just want to increase my reach, and increase the number of targets I can catch with a longer fl.   Your input is great.
Ian