Ritchey Chretien vs Edge HD

RC telescopes have good reputations and for good reasons but I am a huge RASA fan! The RASA 8 is only 400mm and while I wish it was a bit more, imaging at F2 with great optics is a true pleasure. You will need different filters because of the speed, even your light pollution filter should be rated for f2. With regard to your question about recollimation of the Edge HD when switching configurations; typically recollimation is not necessary because the Hyperstar camera mount screws onto the threaded receptacle of the secondary mirror. The keyed design of the mirror receptable ensures that the mirror is returned to its original position leaving collimation undisturbed. At least that is how it has been working on my Celestron SCT.

Chris Sullivan:
I'm not sure I can add anything worthwhile to this question, other than that I've been consistently impressed with the optics of my 8" carbon fiber RC.

Hi Chris, I can see why.  Your images are amazing!  If I go with the RC, not the Edge HD 11, I'm leaning towards the TPO RC 10 Truss.  Do you have any thoughts on that type of scope?

Chris Sullivan:
I don't have any first-hand experience with SCTs, RASAs, etc., but I have seen people post images with odd reflections due to the front corrector or with bloated stars from the RASA's corrective elements (the ones in front of the camera).

Yes, I've seen the large light circles around bright stars in some of the RASA images.  I understand it is the reflection of the main mirror in the image.  I'm not sure what RASA users are doing to avoid that.  Definitely need to understand that better before investing in a RASA.

Roman Pierce:
With regard to your question about recollimation of the Edge HD when switching configurations; typically recollimation is not necessary because the Hyperstar camera mount screws onto the threaded receptacle of the secondary mirror. The keyed design of the mirror receptable ensures that the mirror is returned to its original position leaving collimation undisturbed.

Hi Roman, thanks for that confirmation, I'm very glad to hear that.  Of course the need for recollimation after switching configs would be a non-starter.

Wei-Hao Wang:
I'm interested in this topic too.  I would like to hear from those who use RC in portable imaging, about whether it holds collimation well, how frequent you have to re-collimate, and how hard it is to collimate.

Hi Wei-Hao, great questions, thank you!
By the way, probably a topic for a separate forum post, I found one of your posts regarding asin stretching to be very helpful in finishing my Lagoon Nebula image.  After consulting with Roger Clark regarding the use of his rnc-color-stretch (by the way I think Roger and his website are an absolute treasure), I was not able to successfully stretch with full image size.  I had to reduce the image size for the rnc-color stretch to work.  It was then that I found your comments about asin stretch for linear stretching of the image, and it is that stretch that I used to finish the image.

Trace

Chris Sullivan:
Just that I'd be jealous?

;)  well that will make two of us.  10" RC or not, I think I'll be frustrated for a while trying to capture & process to the level of the images in your bin!

You're far too kind. I used it at the native focal length from February 2018 (first light) until August 2018 (even though I bought the reducer with the scope) and I vowed never to use it again without the reducer. Because my light pollution (Bortle 7-8-ish) is so bad, I need to gather a lot of data to get a decent SNR. I just found f/8 to be painstakingly slow. F/5.75 feels a lot more manageable - I haven't changed my camera angle since this past March.

As far as needing a flattener - it might depend on how large your sensor is. I do recall reading some people use a Hotech SCA with the scope, but I'm not sure if it's really needed. I might ask on Cloudy Nights.

Chris Sullivan:
You're far too kind.

Well may be Chris but your Imaging with an 8" is remarkable.  I've been browsing and admiring your images for a few days now since deciding that an RC was a strong contender for my next scope.
Regarding f/8, I hope I'll be ok since I'll be Imaging mostly in dark sky locations, Bortle 1-3.
My sensor is an APS-C, don't know if that's going to need the flattener with the 10"f/6 RC.

Wei-Hao Wang:
I'm interested in this topic too.  I would like to hear from those who use RC in portable imaging, about whether it holds collimation well, how frequent you have to re-collimate, and how hard it is to collimate.

I used my RC for years in portable imaging.  I have not collimated it.  Now, it's in an observatory.    But, I should mention, it has a reasonable build quality, and that makes a difference.

Regards,

Thanks Dhaval, very helpful points going both ways. Clearly these are both great setups.  I was not aware of the difference in cost/availability of reducers for RC vs EHD.  Again this is great info and I'm learning a lot through this thread.
I'm a very long time newt user, and I would definitely miss the star spikes.  One real joy of this hobby for me is sharing pictures with friends and family, and in almost every case, the first comment I get is "look at those stars!, referring to the brighter spiked ones.  I guess if I go with the EHD, I can always add some vanes to the dew shield if that's the result I'm after.

Trace

Interesting.  I assume the resulting spikes are not super straight and sharp.

iHi Trace,
Well I guess I can chime in whereas I have at one time owned all 3 of these OTA's, albeit my RASA is an  11".

Edge 11" HD with Hyperstar…I had it a while back and found it  to be an awesome  and extremely versatile scope.   I had a Hyperstar and a FR/FF from Celestron. At prime focus @ F10 I found it obviously slow and could not control star bloat. Also I had to buy  a bigger capacity mount  just to handle  the weight.  With the  hyperstar I found it  to be quite rewarding shooting at F/2.  What I didn't care for was loosing the remote/automated capability as I had to manually change the filters.  I found collimating  the hyperstar to be a nightmare. I could not tame this for what ever  reason. I can not say  too much about using the FR/FF as I only had it for a short while and didn't get much usage.  Never tried planetary  either.  Dew shield and heater is a must have!

Celestron RASA…In my case I have an 11" and still own it. Overall the performance is ok. Once  again you need a stout mount. I have been quite satisfied with my iOptron CEM 120 EC2.  I found that using   a Starizona filter slider worked for mono  but vignetting was terrible. I have upgraded to the Baader UFC system  and that solved the vignetting and it  is  a nice system to dial in the very critical back focus for the RASA.  Also 2" filters are a must!!!  Getting the back focus correct is a must with scope!  I can't  emphasize that enough!!!  My problem with this scope is  the focusing  SUCKS!!! Mirror flop is terrible. So bad that after focusing I need to plate solve to get  back to my  position. Off by sometime thousands of pixels. Now to the mirror reflections on the stars… some people don't mind them.  I personally think the ruin an otherwise great image. This is the deal breaker for me.  I wasted many nights imaging as I found  the reflections to be really bad after stacking.! Once again a dew shield and a heater is a must have! I do know  that  Celestron has admitted that the focus system sucks and have released a version 2 of the RASA 11".  Unfortunately they will not allow a trade in or upgrade to this new model even though mine is only 6 months old. Also  they lowered the price so much on the original version that I can't sell mine on the used market for what they want for it new. I guess I will be stuck with it sitting in my garage.

Now for the RC…I have an iOptron 8" Photon and just recently got a GSO 10" Truss.
First collimation…I was worried about this prior to purchasing an RC as I read too many bad reviews. I can say  this. I have owned the 8" for 2 years now and I have NOT had to touch the collimation since I originally set it up.  Now I do use it in my obs but I have had it on and off of my mount several times transporting it to my garage while using other OTA's. I have used in in high heat and extreme cold and the collimation is spot on!  I bought a Howie Glatter Laser collimator for this but actually found a $15.00 Cheshire on Amazon that put me spot on!!!
Diffraction Spikes!!! I love them….Main reason for wanting an RC.  Dew is not  an issue with either of these OTAs I do use a shroud on my truss. It also helps keep stray light out.  As has been  stated prior, the stock focusers are garbage. I have a Moonlite CSL on both with their mini V2 controller. Focusing is exact and repeatable over and over. These OTA's hold focus all night with little or no adjustment needed.  I do use a Stellarvue FF/FR on both OTS . .8X reduction get s me to a manageable f/6.4. I can use my 1.25 " filters on both of these OTA with no vignetting.  I have never tried visual observation on the RC but I have been told that is a no no as these are strictly an astrograph!

I only have  a short amount of time on  the 10" truss up to now. I got it for Christmas this past year and the weather has been horrible. When I have had opportunity to use it the seeing has been dreadful.  Collimation on this OTA was quick and spot on. The advantage the 10" truss has over the the 8"  besides FL is the capability to center the focuser on the secondary  with a separate  adjustment unlike the 8".   This can come into play if your focusing system has any sag or tilt. You can buy a separate tilt adjuster for the 8 from AstroTech. Always seems to be out of stock for some reason!!! I wonder Why?
The RC is my favorite of all of these. Performance is awesome.  After all, there is a reason the Hubble Space Telescope is an RC design.

In conclusion:
Versatility—-Edge HD
Speed- RASA
High Performance- RC

CS/ Bob

Erlend Langsrud:
My understanding is that  the focuser on the RASA 8 is of a new and improved design. I havent noiced any mirror flop at all.

You are correct about the focuser on the RASA 8. As I stated above I was talking about my RASA 11 which I believe  the new version has the same focus system in it that the RASA 8  has.  The fact that  Celestron has admitted  to a flawed focuser  design  with this model and they won't stand  by it with a 6 month old OTA has me quite annoyed!

Erlend Langsrud:
I am not able to spot this problem at all in your excellent images (M33, Pelican nebula) nor in my own Rosette image.
I did notice this problem around a bright star in Orion Nebula, so it can certainly be an issue with bright stars. I'm guessing M45 with several bright stars would be affected by this problem. I guess Trace will have to look at the pictures and judge by himself.

Thank you  very much for the compliment on my images.  Yes  the RASA's are capable of excellent quality images. I found  that I just have to select my targets  carefully to avoid any slightly bright stars. My Pelican for instance is in HA only because I had to throw the O3 data away. The stars had enormous mirror reflections around them in addition to the expected halos inherent with O3.  Here is a link to an image of the Double Cluster.    Double Cluster w RASA 11  I kept it because I am quite pleased with it, but if you look you will see the reflections  and these stars are not what I consider all that bight!

And yes I agree with you..Trace will definitely have to look at images and judge by himself.

CS/Bob

Wow! These are really fantastic responses.  Thank you all very much for sharing your experiences, expertise and suggestions.  This is turning into a reference, not just a topic thread.  At least it is for me, I will be returning to it often to hone my understanding.

Erlend Langsrud:
There is a circle/disc around vert bright stars, but vey faint with my setup. You can look at my Rosette nebula image to get the idea.

Hi Erlend, your image looks fine, I see no obvious discs. I've seen some RASA images where it's very pronounced, and I must say I'm not a fan of them.

Erlend Langsrud:
I think this "problem" with my setup is comparable to the diffraction spikes from a Newtonian or RC. I accept it as a inherent effect of the scope design. I will not try to fight it, except for coosing suitable filters.

I find them to be too distracting even if they are faint. But that's just me, of course everyone is different.  I know some people really don't like the star spikes, but like Chris, they are an aspect of the RC that I like over a SCT.

Erlend Langsrud:
The one really huge drawback IMO is that you will not be able to use a filter wheel. You can use a filter drawer, but it makes narrowband imaging more of a hassle.

For now my interest lies with one shot color imaging and broadband.  While I respect the amazing images (and extra work I must say) that comes with narrowband work, it's not for me at least not yet. I have used a 40 year old Lumicon Deep Sky Filter (Ha,Hb,OIII) in my backyard for the whirlpool galaxy, but the results were very poor (image has strong blue signal and no matter what I did in post I couldn't get natural color in the image).

Erlend Langsrud:
I would go for the RC pluss RASA. I suspect that hyperstar is a hassle, and not quite as good as a RASA even if you get the collimation right. It is also hyper-expensive.

I think the Edge is an amazing scope and I will continue to keep it in mind, and having an 11" model + Hyperstar I'm sure would be fantastic.  Equinoxx seems to do pretty well with it ;) (pure magic!!).  However, the RC does appear to be lining up better with my interests.

Erlend Langsrud:
That said:  EdgeHD would be my first choise for long focal length scope  if price was not an issue.

Well, long focal length is what I'm after, but f/10 is just too slow.  Not sure if I can reduce the Edge 11" down to f/8, and how it would perform there compared to the 10"f/8 RC.  I think that's a key question for me.