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Old 28-01-2019, 08:28 PM
BlakPhoenix (Rohan)
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Question SCT vs Refractor

Hi all,


I have been playing with my dad's Celestron 8" SCT/AVX combo on an off for a few years now and finally feel ready to get a scope for myself to make this hobby more accessible and enjoyable. My dad is only into visual, but I have attempted imaging on his gear and find that the OTA/mount struggles when pushed in this direction. I have got myself a SkyWatcher EQ6-R Pro mount and now need an OTA to sit on it (along with a myriad of other accessories).



I have been doing as much research as I can and have had my eyes on the Celestron 9.25" EdgeHD SCT for a while since it appears to be much better for the imaging side of things, without being too large and heavy to still be portable with 1-2 people. I have read that people do not like this route due to the lack of focal reducer, however Celestron has since released this, so thought that should it it solved...



Since then I have been looking at Astrobin, and after noticing that a lot of fantastic photos come from refractors, started to second guess the SCT. I have had a look on Bintel for an APO with a similar price to the 9.25" and found the SkyWatcher Esprit 100mm Triplet Super APO. I understand that the refractor is ~2 stops faster than the SCT, and of course it has a much shorter focal length for fitting in large DSO's and will make tracking much easier. However, I'm worried about it's ability to image planets, and how planets/the moon would look visually through it at outreach events compared to the SCT.



I'd love to get your experienced opinions about this.



tldr: I plan to get into planetary and DSO astrophotography (stating with a full frame dslr), while also wanting to use this for outreach events (so portability/viewing matters). Price limit ~$3,500 for the OTA to be paired with a EQ6-R Pro mount. What do you recommend?
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Old 28-01-2019, 08:33 PM
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Ukastronomer (Jeremy)
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Horses for courses.

I have both see attached, I have the 120 esprit, superb scopes

I chose the 8" edge on Evo mount for THREE reasons, first it is (EDGE) the best for imaging, not only because it has clutches on the mirror to stop any flop when imaging and second because (see last image) the mount takes other scopes (and a wedge) and third it and the mount are a lot lighter than the 9.25 for travel (it also has extra elements on the build to provide a perfect flat field.

I also have the focal reducer, you can only use the one made for the edge on this scope, remember that.

https://www.rothervalleyoptics.co.uk...-8-edgehd.html

If were into long exposure imaging I would get a different mount, this one

https://www.widescreen-centre.co.uk/...oto-mount.html

As anyone will tell you NEITHER scope is perfect for all work which is why people have both
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Old 28-01-2019, 08:42 PM
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Ukastronomer (Jeremy)
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Have you seen this on the forum

http://www.iceinspace.com.au/forum/s...d.php?t=173042

http://www.iceinspace.com.au/forum/s...d.php?t=173039
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Old 28-01-2019, 09:17 PM
BlakPhoenix (Rohan)
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Quote:
Originally Posted by Ukastronomer View Post

I hadn't, thanks Ukastronome.


The main reason I was leaning towards the 9.25" was for it's ability to fill full frame sensors. From what I've read the 8" EdgeHD isn't able to do this?
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Old 28-01-2019, 10:14 PM
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Ukastronomer (Jeremy)
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Quote:
Originally Posted by BlakPhoenix View Post
I hadn't, thanks Ukastronome.


The main reason I was leaning towards the 9.25" was for it's ability to fill full frame sensors. From what I've read the 8" EdgeHD isn't able to do this?
I had the same questions, I got a PST (now sold) that came with a supposed lunt solar eyepiece 130 (free) no difference over a decent eyepiece
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Old 28-01-2019, 10:37 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by BlakPhoenix View Post
The main reason I was leaning towards the 9.25" was for it's ability to fill full frame sensors. From what I've read the 8" EdgeHD isn't able to do this?
The Edge 8 will somewhat illuminate a full frame but expect plenty of light drop off towards the corners. That won’t be the case with the 9.25 (maybe a teeny bit).

For planetary imaging, you don’t need the Edge HD as you’ll always want the subject in the centre of the FOV.

If you’re wanting to get started in DSO astrophotography, I’d suggest staying away from long focal length scopes like SCTs to start with. The shorter focal lengths, such as that of the Esprit 100 and similar, are much more forgiving to get going with and the FOV is very satisifying.
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Old 11-02-2019, 01:16 PM
tvandoore (Tim)
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I started out with a C8 (non edge) but went to small refractors to try my hand at DSOs. Once off the reasons was simplicity - no collimation worry about, so one less thing to go wrong. Given how many issues I had early on, I wish I went the "stay simple until you have some form of result" path earlier.

Something like the little ED80s are great $/result and a great stepping stone, but not really appropriate for planetary (super portable, though). I believe the ED80s with 3" focusers will fill a full frame. They are easy to pick up second hand.

C9.25s great for planetary, but a much bigger spend, less portable, and you already have access to a C8 to compare any other scope with to see what you prefer.
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Old 12-02-2019, 02:39 PM
Wavytone (Nick)
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Quote:
Originally Posted by BlakPhoenix View Post
I'd love to get your experienced opinions about this.
The first question is whether you are a primarily visual observer, or aiming at photography.

If the former, I'd suggest either a 10" dobsonian, C8 (small and light) or C11 (aperture rules, but its a big heavy beast).

If the latter, stick to small refractors upto 100mm to start with - and forget SCTs, they are a whole world of pain. While it is possible, its far from easy and you have a lot to learn.

Disagree re the C9.25 for planetary BTW, the slow primary means a big fat secondary obstruction to achieve f/10 and the result is a scope that is not ideal for high power (planets), and not good for low power/wide field stuff. And SCT's don't achieve much better than 0.90 strehl on DPAC tests.

If you want a serious lunary and planetary scope find a big Maksutov f/13 ... f/15 with small central obstruction and high strehl (0.95 or better).

These are like unicorns - they are legendary, very few have seen one, and a very lucky few own one.
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Old 14-02-2019, 04:44 PM
assbutt94
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I just upgraded to the 8"Edge from the normal 8se OTA.
I use the EQ6R pro.

I think a 9.25 will be getting a bit too heavy for a EQ-6r. You have to keep in mind the weight of a rail, a decent guide-scope and guide-rings, as well as a camera.
You might be able to save weight with OAG.
Also the weight of all the accessories like star pointers, finders, dew shield and all that.

Also you will experience vignetting with a full frame AND focal reducer if that bothers you. Check out these flat frames showing it.
The Vignetting is fine without a reducer, and planetary is not an issue as you use the centre of the camera chip.
Collimation isnt too difficult once you do it a couple times, but it's one extra thing to keep in mind, especially since it's far more critical with planetary. I messed up capturing Mars at opposition last year due to collimation being out on my imaging nights that came before the big dust storm that blurred detail.

I think SCT are great for planatary, I use either a 2x or 3x barlow to get the right sampling with my ASI120MC. Using fire-capture to image and guide works really well for me.

Last edited by assbutt94; 14-02-2019 at 09:01 PM.
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Old 15-02-2019, 01:41 AM
raymo
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People use C9.25s on an HEQ5, let alone a 6. The 6 will carry it easily.
Raymo.
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Old 15-02-2019, 11:04 AM
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Yep, I have mine on an Orion Altlas Pro, an AZEQ6 with a coat of black paint.

I am chasing my tail a bit guiding it for imaging, but that is more a case of tweaking the mount mechanically to be as good as I can make it and sorting out the guiding settings to get PHD2 working sweetly. Currently it tends to oscillate on both axis but guiding at 1500mm focal length is never going to be particularly easy with consumer level gear.
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Old 15-02-2019, 12:37 PM
tvandoore (Tim)
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There's a useful calculator here that may help:
https://astronomy.tools/calculators/ccd_suitability

Eq6-r will be fine for a 9.25 for planetary but starts to find it's limits for long exposures at that focal length once it's loaded up with all the bits.
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Old 15-02-2019, 02:00 PM
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Plonk my setup in to that calculator (C925 with focal reduction working out to about 0.68 and an ASI294 camera) and for OK seeing it returns "Slight over-sampling. Will require a good mount and careful guiding"

I would agree with that, my primary work at the moment is getting guiding going more sweetly as PHD2 using out of the box settings seems to be chasing the seeing, leading to oscillations on both axes, the worst being the RA. If I get some clear skies this weekend I plan to set up and work on guiding to refine settings and calm it down.
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Old 23-02-2019, 09:01 AM
Sunfish (Ray)
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I get an answer of 0.7 for c8 with apsc DSLR.

I love my old C8 now it has been tightened and checked over.( all the little bolts that hold it together come loose) and collimating. The images have detail. The 800 refractor views have great crisp and intense colour. They weigh about the same.

But collimating SCT is everything and takes up a lot of time. Once right and tight it is great but without perfect collimating , not so good for images. Not all new SCT will collimate easily either I think.

It would be good to hear how you get the guiding happening as I too have struggled with PhD.

Quote:
Originally Posted by The_bluester View Post
Plonk my setup in to that calculator (C925 with focal reduction working out to about 0.68 and an ASI294 camera) and for OK seeing it returns "Slight over-sampling. Will require a good mount and careful guiding"

I would agree with that, my primary work at the moment is getting guiding going more sweetly as PHD2 using out of the box settings seems to be chasing the seeing, leading to oscillations on both axes, the worst being the RA. If I get some clear skies this weekend I plan to set up and work on guiding to refine settings and calm it down.
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Old 26-02-2019, 09:41 PM
Wavytone (Nick)
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Rohan,

As Jeremy suggested "horses for courses", there is no scope that will "do it all".

If you want to wow an audience at outreach nights with the moon and planets as the targets, the C9.25 is the way to go.

But forget imaging DSO's with it, you will find an SCT frustrating and it will be outgunned by the faster, smaller refractor for which the cameras are most suited, given the seeing constraints affecting most.

For imaging there's not much point using a big heavy setup that can resolve 0.6 arcsec visually in conditions where the FWHM as recorded by the camera will be 1-2 arcsec or worse.

OTOH the planets in a short 100mm refractor... meh.
An f/15 refractor with perfect optics is another matter, but nobody makes these anymore, and no good for imaging.

Last edited by Wavytone; 26-02-2019 at 09:53 PM.
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