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9.25 SCT or Refractor for imaging?


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8 minutes ago, Adaaam75 said:

Would the EQ6 handle the weight of the 9.25 plus the potential weight of a guidescope, reducer, ccd and filter wheel comfortably without sgruggling?

 

It isn't just about weight and, for DS imaging with an SCT, I think you can do a lot better than an EQ6 but only at a considerably higher price. Very often these conversations revolve around weight, which is not even half the story. When you are sampling the sky at a fine pixel scale (because you have a long focal length or small pixels or both) you need a very accurate mount. A standard EQ6 unguided is a very inaccurate mount with about ten times the periodic error of a premium one (costng five times as much, let's be clear.) Under autoguiding an EQ6 usually becomes a fairly accurate mount but the fewer arcseconds per pixel at the imaging camera, the more marginal it becomes. It's impossible to say at what pixel scales an NEQ6 would start to struggle because they vary considerably and so does the seeing and the skill of the person setting up the guider, etc etc. However, I have a couple of NEQ6 mounts which I lend to guests and I generally feel confident that they will guide out well enough to make about 1.5 arcseconds per pixel possible. People image below that, even well below that, with an EQ6 but.... it will very likely stop being 'plug and play' and may well require fine tuning, mount rebuilding etc. to work reliably.

You can find out your imaging pixel scale here: http://www.12dstring.me.uk/fov.htm

A sweet spot, especially for beginners, is considered to be about 2 arcseconds per pixel. The extremes, at least those which I've used, are from about 0.6 to 3.5. I only tried 0.6 on a premium mount.) This is the kind of resolution you get at around 2"PP. I find it suits me but for small galaxies you do need more.

Nice for nebulae:

M42%20TEC140%20LRGB%20V3-X3.jpg

...but a bit lacking for small galaxies...

NGC3718%20etc-XL.jpg

If you want to play safe on the mount front don't go for an imaging pixel scale much below 1.5 to 2 "PP. (As a provider I have to be as sure as is possible that what I offer will work, so be aware that I'm on the conservative side in my estimates.)

Olly

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I'm also thinking that for a SCT (Especially a 9.25) you'd be wanting an OAG rather than a separate guide-scope.

.....mutter, mutter...10 Micron.....mutter, mutter....ASA......mutter, mutter... :evil4::D

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13 minutes ago, DaveS said:

I'm also thinking that for a SCT (Especially a 9.25) you'd be wanting an OAG rather than a separate guide-scope.

.....mutter, mutter...10 Micron.....mutter, mutter....ASA......mutter, mutter... :evil4::D

Yes, you are absolutely right about the OAG for an SCT.

As for the muttering, I'd also advise muttering 'Mesu Mesu Mesu....'

:Dlly

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Would the EQ6 handle the weight of the 9.25 plus the potential weight of a guidescope, reducer, ccd and filter wheel comfortably without sgruggling? 

Mine carries a C9.25 and ED80 with no problems. I was suggesting taking the C9.25 off to do imaging with the ED80 though - using the ED80 on the EQ6 as many others do  ;)

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19 minutes ago, Mr Spock said:

 

 

Mine carries a C9.25 and ED80 with no problems. I was suggesting taking the C9.25 off to do imaging with the ED80 though - using the ED80 on the EQ6 as many others do  ;)

Sure, and that would work sweetly in all probability. If the OP goes for DS imaging with the C9.25, though, accuracy becomes the big deal.

Olly

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My Avalon Linear Fast reverse coped with the C9.25 ...... What it couldn't account for sadly was the wasted nights where the sky conditions would have been fine for a short focal length scope, but was useless at 2.5m, or the nights where there was a breeze and it was useless to try to guide the larger scope when the smaller scope would have been fine, or even high humidity where the claggy sky just wouldn't support a long focal length.

Never underestimate the difficulties of imaging at longer focal lengths and more to the point, never under estimate the data that you will discard or the nights that will be missed as the conditions are just not up to the job..... even if you are champing at the bit.

That's my experience anyway and it doesn't get massively better at 1.7m either.... I still waste nights where conditions are unsuitable specifically for long focal lengths :(

 

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I know it's been a while since I been here so i'll just say suggestions above have been very good suggestions . I myself have a C8 SCT on a german eq5 with tracking motors , no go-to . When I align it's self manual align with a crosshair ep . What I will agree with is for close-up personal the 9.25" I've heard for many years is an excellent scope especially for planets , but also for small galaxies and GCs' which are my favorite . I viewed more than image . I also have a original Stellarvue AT1010 ( called the Tank ) . I also agree for large FOV imaging a small 60mm-102mm refractor are great . But what I wanted to mention more or less ask is if you have a great refractor for viewing and you want to image more why not check out or consider a astrograph scope ? I don't have one but from what I have searched and read they are perfect for DSO imaging but not so good for viewing . But they are way less than you are proposing to spend and you could put valuable $$$ in other needed areas . Below is just a couple suggestions . The Astro-Tech are popular and have been said to have very good mirrors and glass . Just might be something to check out .

Astronomics has some Astro-Techs as cheap as $299.99

https://www.astronomics.com/astro-tech-imaging-newtonian-telescopes_c4.aspx

Orion has one as cheap as $529.99

http://www.telescope.com/Astrophotography/Astrophotography-Telescopes/Orion-8-f39-Newtonian-Astrograph-Reflector-Telescope/pc/-1/c/4/sc/19/p/101450.uts

 

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1 hour ago, swag72 said:

My Avalon Linear Fast reverse coped with the C9.25 ...... What it couldn't account for sadly was the wasted nights where the sky conditions would have been fine for a short focal length scope, but was useless at 2.5m, or the nights where there was a breeze and it was useless to try to guide the larger scope when the smaller scope would have been fine, or even high humidity where the claggy sky just wouldn't support a long focal length.

Never underestimate the difficulties of imaging at longer focal lengths and more to the point, never under estimate the data that you will discard or the nights that will be missed as the conditions are just not up to the job..... even if you are champing at the bit.

That's my experience anyway and it doesn't get massively better at 1.7m either.... I still waste nights where conditions are unsuitable specifically for long focal lengths :(

 

What an excellent post. We have so many people quoting arcseconds per pixel (a habit of mine! :eek:), the Nyquvist theorem (for which I care not a jot) and a pile of other tosh and here we have the sudden appearance of... what's it called???... I used to know this, dammit.... Oh yes, reality!

Sara is not saying that you can't image with a long focal length SCT, and there are lots of other things that she is not saying as well. But I would strongly advise any would-be imager to read what she IS saying.

Olly

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This has gotten heavy! Although it's only heavy for me because although I was aware of issues like periodic error and accurate tracking I hadn't really considered just how difficult imaging with a SCT could potentially be. In the name of fairness and to give me a sound understanding as possible i have some more questions generated from all your helpful and in depth replies

1) considering my budget is around the £1600 (I understand I don't have to spend that) would the EQ6 out perform the AVX and CGEM for accurate tracking?

2) Would a focal reducer/8" EdgeHD Fastar not reduce imaging times provide more opportunities for imaging sessions?

3) The way I've read the advice does increasing the speed/reducing focal length mean that accurate tracking issues mentioned above aren't such an issue as they would be for a SCT? although obviously still an important necessity for photography.

4) Will an offaxis guider be as accurate as a guidescope? It seems to good to be true! Would it have an affect the image?

5) What real time scale differences  are we talking here for the same DSO between a refractor and a SCT if the whole object fits into both fields of view? Or is it just then down to seeing conditions?

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An OAG is the only thoroughly reliable way to guide a reflector. You might get a guidescope to work but I'm specifying 'thoroughly reliable.' However, OAGs can be a bit of a pain and the ability to use a guidescope reliably is another good reason for imaging with a refractor!

The Fastar system is fast when you have it working but in my view it is too fast to be sensible. The depth of field is too shallow for the focuser, the light cone is too steep for many filters, collimation and tilt become absurdly critical and - the killer for me - I just don't ike many of the published images I see. The stars are large and rather blobby in broadband.

The effect of focal reducers on imaging time is complicated. In a nutshell a focal reducer only saves time if you want to keep everything that appears in the new image. If you are imaging a small object that fits in the frame withut the reducer then all it does in a given time is give you a smaller image of the object. There will be no significant difference between this and the image without reducer resampled downwards. For  detailed discussion of this Google The F ratio Myth plus the names Stan Moore and Craig Stark.

The fewer arcseconds per pixel captured at the imaging camera the better your tracking has to be. Putting it that way removes all ambiguity.

Olly

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43 minutes ago, Mr Spock said:

Is the £1600 for mount only? I ask this as the 8" Edge + focal reducer is over £1300, EQ6 £950, plus camera and guider, which is a substantial amount of cash.

Te budget is for the scope and mount as i am only future proofing for when i decide in the not too distant future (I'd imagine a year or so) to consider another budget for my astro imaging equipment. Within my budget i can afford the 8" EdgeHD on an AVX mount or potentially the 9.25 SCT. I was leaning towards a SCT prior to joining this site however now you call all understand the questions! I will not be able to buy say the SW80ED and a SCT and initially the aim is visual.

http://www.365astronomy.com/Celestron-Advanced-VX-8-EdgeHD-Computerised-Telescope.html

http://www.365astronomy.com/Celestron-Advanced-VX-9.25-Schmidt-Cassegrain-Computerised-Telescope.html

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More seriously, you say that your budget is a maximum of £1600. What camera are you intending to use? Do you have it already? When are you thinking that the initial visual use will turn more to imaging? Are you prepared to open the floodgates of spending if you start imaging?! It is seriously addictive and appears to have no limit to the budget required........

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13 hours ago, PhotoGav said:

More seriously, you say that your budget is a maximum of £1600. What camera are you intending to use? Do you have it already? When are you thinking that the initial visual use will turn more to imaging? Are you prepared to open the floodgates of spending if you start imaging?! It is seriously addictive and appears to have no limit to the budget required........

The budget is for the scope and mount only as i dont feel like ive learnt the sky enough yet and visually there is still more than enough to keep my enthusiasm going for long after i even start imaging. I am aware of the cost of ccds filters guiders guide cams etc and this is why i am doing this in what i think is the sensible way but also like my title, i am seeking advice on the the scope and mount first. What i am thinking is that maybe ill get a SCT first and then from the advice on this thread, take the hit a in acouple of years and get a SW80ED for DSO's and be a two scope family!

I currently have a Canon DSLR with adaptors to use afocally. Here is one of my first ever moon mosaics using my SW 130P on an EQ2 with my Canon and also my first official star Trail effort, I have the itch.....

stitch 1.jpg

Startrails 10th Feb 16 Edited(1).jpg

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Uh oh... Those are two quality images. I can see that we have another poor soul about to slip down the astrophotography slope. Enjoy the ride my friend!

As with all equipment purchases, buy the absolute best that you can afford and you won't be sorry.

Good luck!

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  • 11 months later...

Wow, there has already been so much great info given I really can't add to any of it . But I will say that each scope has a reason for its own . If I could I would have more than I have already . Right now my main scope is a Celestron 8" SCT . I love for viewing and I have taken several good images from it but not great . Reason is I'm more into observing than imaging . My other scopes are  a Stellarvue AT1010 refractor 80mm . It's not an APO and I have taken a few images with it but it being a chromatic refractor produces alot of CA so images have alot of purple fringe around stars and the moon . But again  I really like the mobility for moving around quickly and I can use it on a camera tripod if I wish . My next scope is a Bushnell Newtonian which had its own mount but a cheapy . I spotted my first DSO , M3 a Globular Cluster . The thrill of that blew me away and that's why I bought the Celestron C8 cause I wanted to look deeper in space but as mentioned it had a smaller field of view (FOV) . What I never owned but have looked through is a Orion 12" Dobsonian . It brought out the Veil Nebulae like a 3D picture , beautiful ! M57 the Ring neb was fantastic ! 

All in all what I'm saying each type of scope has its own place . For DSO's a SCT can be used for all imaging but with a narrow FOV not all DSO'S will fit but all planets will for sure . The refractor is a choice scope for WFI (Wide Field Imaging) but only if it's a APO or at least a ED with good glass . Newtonian are good from 6" and up , 6" best for WFI . I agree tho SCT's best for planets . M31 the Andromeda galaxy will not fit in the FOV of a 8" SCT . All scopes are great , but if you only want one I suggest a SCT but someday if you can you need a refractor in your arsenal . Good luck deciding and buy what you want . Just make sure you will be pleased with it ! 

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