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Contemplating my next OTA purchase. Need advice.

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#1 jlecomte

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Posted 06 September 2021 - 06:24 PM

Hi everyone,

 

I currently own an AT130EDT refractor mounted on an iOptron CEM70. This setup performs very well as you can see in this image I recently took with a ZWO ASI533MC Pro: https://www.astrobin.com/45aeuu/. The mount performs flawlessly, and routinely guides between 0.3" and 0.6" RMS (using an OAG) depending on seeing, so all is good!

 

Next year, I think I will be ready for a longer focal length OTA to tackle smaller galaxies and nebulae. The only hard constraint that I have is that it must work with my existing mount, the iOptron CEM70, because I don't plan to get another mount for a long time. I am also planning to continue using my ZWO ASI533MC Pro, at least for some time, before eventually switching to something like an APS-C size mono sensor, in a few years...

 

I have been thinking about the Celestron 9.25" EdgeHD. It's a good value, and I can find plenty of reviews and lots of people have been successfully using it. But I am also looking at other options. For example, I wonder whether a GSO 10" RC truss OTA could be a good candidate. I like the fact that it is natively at F/8 and can be further reduced using the AP reducer down to F/5.4, i.e. a 1,340mm focal length. On the other hand, it has a much larger central obstruction, it has spider vanes, it is heavier and bigger, it is harder to collimate, etc. (the focuser will probably work just fine since my imaging setup is pretty light weight) Price wise, they are not too far apart (in the neighborhood of $2.5-$3K)

 

Let me know your thoughts. One thing that would likely keep me away from the RC is the lack of real reviews I could find. If you know of one, please share it smile.gif Thanks!

 

Note: I am aware that whichever I end up going with, I will likely have to wait many months before receiving it due to everything being on back order, but that's completely fine with me.


Edited by jlecomte, 06 September 2021 - 06:29 PM.


#2 mehdymo

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Posted 06 September 2021 - 06:43 PM

I was in the same situation and finally decided to buy EdgeHD 11". It would do everything such as planetary, widefield (withhyperstar), and glalaxies imaging and it is excellent for observation as well.



#3 Rasfahan

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Posted 07 September 2021 - 01:28 AM

Before you decide on a mirrored long FL scope you should check if the seeing at your site really supports the higher resolution. There is really no reason to shoot at 2000 mm FL with current CMOS when your seeing is 2.5“. When I switched to mirrored scopes last year it was like hitting a brick wall. I‘m finally coming around but it has been a long (and expensive) learning experience. Do not underestimate the mounting requirements. Your guiding is fine, but there‘s no guarantee that it‘ll keep this way with the larger OTA.

 

I haven‘t tried the EdgeHD optics and there are many nice images taken with these scopes, but on the other hand the design has some downsides, namely: Dew susceptibility, thermal management requirements, mirror flop and mirror shift. I am not fond of their reducers because they always produce CA (you can see it on all images). So they are not really hassle-free setup-and-go.

 

Another possibility would be the Tak Mewlons. They are slower but that just means you need to bin. Well-made. The Mewlon 250CRS are not produced currently as far as I know because of some COVID-induced shortage. 
 

Because of that I’ve decided on an RC by CFF who sell the 10“ with GSO optics (which are good). The mechanics are light years ahead of my 8“ carbon GSO RC, especially the mirror adjustments. It comes with a great 3“ FeatherTouch as standard. Service is great. The price is not much higher than the GSO offerings. From 12“ you can get CFF optics. For an APS-C chip I would recommend a field corrector, e. g. the TSRCFlat3 or the one from CFF.

Collimating an RC is an acquired taste: It is very unsavory at first and needs a lot of time to learn to like. I’m not quite at that point yet but I am down to 30 minutes for „really close“. When people tell you it ain‘t so, check the corner stars of their images. I‘m getting close to „really good“ now with my setup and hope to achieve perfection someday soon. If you set up each night, though, I would not recommend it. Until my progress this last weekend I was seriously considering selling the RC for the comparable ease of a 130mm refractor.

 

If you have the space (I don‘t), check out an imaging Newtonian. There are some nice offers around: TS ONTC, Lacerta, Noctutec, Farpoint (8“), Hercules (>12“). Newtonians are far easier to collimate than the RC. Of course if you use something like an ASA reducer to get them to f/3, the difficulty is high again. But since you want high resolution, f/5 (or f/4) seems a good choice. Of course, the big OTA will be wind susceptible, but people here take them up mountains for a „portable“ setup, so it can work.

 

For planetary imaging or as an all-rounder telescope the EdgeHDs are hard to beat, though.


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#4 jlecomte

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Posted 07 September 2021 - 02:10 PM

Thanks for the comments, that is exactly what I was looking for.

 

A few additional details I left out:

 

* I do most of my imaging from my (light polluted) backyard, so I set up and tear down my equipment only about once a month.

* Occasionally, every few months, I take my equipment to dark sites.

* I am aware of the collimation learning curve, but it does not really scare me. However, if I have to collimate it every single time I take it to a dark site, then that's a problem.

 

The CFF RC has been discontinued according to their web site: https://cfftelescope...chretien-250-mm

 

The EdgeHD is definitely a good all-rounder. The only thing is that at F/7 with a reducer, it's not super fast... Unless there are reducers out there I am not familiar with that will bring it down even lower?

 

A Newtonian with the focal length I am looking for (between 1,300 and 2,000mm) would be gigantic, and very sensitive to wind gusts (my 130mm F/7 refractor is already sensitive to wind due to the length of the tube) so that's not an option.



#5 Rasfahan

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Posted 07 September 2021 - 06:15 PM

CFF discontinued the 10“ RC with their own optics. They still sell them with GSO optics: https://cfftelescope...duct/rc250f8gso

Also, TS seems to sell them now under their own brand (TS RC Pro).

 

The collimation of my RC 8 does not survive car trips. But I think that holds true for most mirrored scopes. The adjustments needed to make are usually light and limited to the secondary. My RC10 does not travel.
 

Unless the FOV is too small for the object I wouldn‘t fiddle around with a reducer. You get the same „speed“ by binning, even in post. 

 

The question is, what do you want the 2000mm FL for? With the ASI533 you have a tiny FOV and are very much oversampled at 0.39“/px, so will want to bin/resample anyway. I wanted the aperture for light gathering and that works out, but I pretty much always resample to 0.8“/px. If I could get an affordable, compact 10“ (or 12“… aperture fever …) f/4 scope with good optics and mechanics, I‘ld jump on that instantly for the larger FOV.


Edited by Rasfahan, 07 September 2021 - 06:19 PM.


#6 Gregory

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Posted 07 September 2021 - 07:03 PM

How about a corrected Dall-Kirkham from AG Optical?

 

Gregory

 


Edited by Gregory, 07 September 2021 - 07:05 PM.


#7 Bean614

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Posted 07 September 2021 - 07:05 PM

"However, if I have to collimate it every single time I take it to a dark site, then that's a problem.".

I own Reflectors and SCT'S.   My skies are Bortle 4, or better, and I STILL check & tweak collimation before EVERY use, even though they are not traveling!

Yoshida you weren't afraid of collimation, so why is collimating every time a "problem" (as you put it).

Whether for Visual or Photography,  they ONLY way to use a scope is when it's been perfectly collimsted.

As Rasfahan said, your collimation won't survive a car trip.

Every experienced observer/photographer on CN checks collimation often, even several times per night!



#8 Gregory

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Posted 07 September 2021 - 07:15 PM

Every experienced observer/photographer on CN checks collimation often, even several times per night!

Um…both I and an astro-buddy transport our scopes (a 12-inch

and a 10-inch catadioptric) 90 miles to a darker sky site about

twice per month.  Our OTAs seem to hold collimation just fine.  

 

However, they do ride comfortably in the back seat of a sedan. 🙂

 

Gregory



#9 Bean614

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Posted 07 September 2021 - 07:25 PM

Here's the way I look at it:  If your scope isn't Properly and Perfectly collimated every time you use it, you will NEVER be able to tell just how really good it COULD have been.



#10 jlecomte

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Posted 08 September 2021 - 12:31 AM

 

CFF discontinued the 10“ RC with their own optics. They still sell them with GSO optics.

Ah, missed that, thanks! That looks great, indeed!

 

 

How about a corrected Dall-Kirkham from AG Optical?

At $10K, it's a little too high...

 

 

The ONLY way to use a scope is when it's been perfectly collimated

Agreed! But then, it's also a fine balance between obsessing for hours and getting some imaging time. I suppose with the right tools, knowledge, and practice, it can be done pretty quickly.

 

 

The question is, what do you want the 2000mm FL for? With the ASI533 you have a tiny FOV and are very much oversampled at 0.39“/px, so you will want to bin/resample anyway.

That is really the core of my question. In California, where I live, the seeing is often good to very good. Since I live in a light polluted area (if we don't count the dark site trips I take every few months), I go after resolution. I am interested in smaller targets, like planetary nebulae or small galaxies that are seldom photographed. Hence the need for a longer focal length. I mentioned 2,000mm, but that's really the upper limit of what I am considering. My current refractor at F/7 is 910mm FL. I wanted to have the next step up, and the EdgeHD with a reducer, which amounts to ~ 1,600mm FL, would work pretty well I suppose. An optional hyperstar would fill the lower end of the FL spectrum too.



#11 Bean614

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Posted 08 September 2021 - 06:41 PM

"fine balance between obsessing for hours and getting some imaging time".....???????

 

Nothing like a bit of exaggeration,  eh?

 

In actuality,  most folks, after getting over their fears, and doing a few collimations, find that it takes mere minutes to get collimation dialed-in.  NOT hours!



#12 jlecomte

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Posted 08 September 2021 - 08:58 PM

 

[...] it takes mere minutes to get collimation dialed-in [...]

I believe it. That's why I wrote that collimation does not worry me at all. I collimate my dob every single time I set it up and it takes only a few seconds with a laser collimator (granted it's much easier and quicker to collimate a newtonian than an RC)



#13 AtmosFearIC

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Posted 08 September 2021 - 09:24 PM

I have a Mewlon 250 that, although not going on dark sky trips, does get setup every available night and it’s been collimated 3 times in 3 years. It holds collimation very well, even when it has done some travelling but that isn’t often.

#14 Rasfahan

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Posted 08 September 2021 - 10:52 PM

"fine balance between obsessing for hours and getting some imaging time".....???????

 

Nothing like a bit of exaggeration,  eh?

 

In actuality,  most folks, after getting over their fears, and doing a few collimations, find that it takes mere minutes to get collimation dialed-in.  NOT hours!

Well, I‘m inept, then. Takes me hours with RCs.




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