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Astrophotography questions from a nitpicker.

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#1 Myth

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Posted 10 February 2013 - 04:39 AM

Hello again!

I posted some images here a while ago, showing a few simple DSLR-lens wide-field attempts. For reference while you read this following wall of text!

Now, I'm not sure I fit the "Beginner" bill entirely, I'm one of those people who like to absorb information like it's the last thing I do on this good Earth, and I study things in stupendous detail for no good reason at all. So I've learned a lot in the past year or so, which is how long I've owned my telescope for.


And I've come to the point that I want to spend some money on new gear, because at least as far as I can figure, I need more stuff to get further in my imaging. The problem is, I've read too much, studied too many concepts, crammed too much information into my addled brain, without real-life experience to back it up.

So let me just lay it all down for you, and hopefully the collective wisdom of this forums' membership can help me out!


Currently, I do own this;

Celestron CGEM mount
Celestron EdgeHD 8" SCT
Canon EOS 60D, stock
Variety of EOS lenses, some very fine examples (L series)
Celestron NexImage (yay)



I'll start out by saying I think the Edge is probably a good telescope. I hope it is - it cost me a lot! But the problem is, I think, the CGEM is not the greatest mount when it comes to astrophotography, at least when running un-guided. Periodic error is obvious, and I've never been able to keep stars round for longer than 30-60 seconds at most, and that's on a great day with a lot of effort spent on setup and alignment.

The camera is alright - I started out, and still am, an avid terrestrial photographer, so modding it is not an attractive option at the moment. But I think it's got a lot to give still - it'll be a while before I strictly need a proper CCD camera.


Now the problem is I've spent so much time reading about cameras, mounts, telescopes, all kinds of optics, software, image processing theory, et cetera, that I don't know where to go next.

I want to get some kind of autoguiding going - that's for sure. The ability to expose for minutes, rather than seconds, will probably keep me busy for a long time.

But autoguide; how? OAG? Guide scope? What camera, OSC CCD, Mono CCD, stick to the NexImage, how big, how small, how much to invest, etc.


I've eventually boiled it down to what makes sense in my head, after overloading myself with information, so, lets have a sanity check! This is what I think makes sense so far, keeping in mind a rough roadmap into the future (The more equipment I can keep for the future without necessarily compromising the now, the better):


I really really really like the SBIG ST-i Autoguider Kit + CCD. This thing seems pretty ingenious, it's small, lightweight, well made, I suspect SBIG knows a thing or two about CCD cameras.

And furthermore, I can use the camera as a planetcam, I can OAG guide with it in the future, I can also combine it with something like an SBIG STF-8300 if/when I go down the "real" CCD imaging route.

But my concerns are these;

Will flexure be a problem? To me it looks like flexure is often a bit of an overstated problem; I can't see how this tiny lightweight contraption mounted properly on my 8" Edge would cause a problem, but you tell me!

Is the 100 mm F/2.8 lens too wide to guide accurately for my 8" SCT? It does claim sub-pixel accuracy guiding capability, and I can believe that - but I don't know how well that translates to my Canon 60D mounted to an 8" SCT. I've done a lot of searching, and I seem to find good, solid, arguments for both sides. I myself am particularly mathematically challenged, so I can't really sit down and do the math to figure out the implications myself. On the other hand I am good at understanding logical concepts - I understand there's a difference in image scale between the two optical systems, and that the SBIG will have a different arcsec-per-pixel value than the Canon, and that it will somehow affect how the guiding accuracy of the ST-i translates into imaging accuracy for the Canon. I just can't read the numbers!



Another idea I had was to get some mounting rails onto my Edge (I will have to, to mount the ST-i anyway), and try some more piggyback wide-field photography. I own a few really great lenses for my 60D, and I think they'll be able to do a great job given proper guiding.

So my idea was this; slap some ADM Vixen rails onto the telescope, get my hands on one of these camera platforms (It's beautiful!), and do some wide-field work while using the 8" Edge as my guide scope.

Is that a sensible idea? Will the ST-i working through the SCT make for an accurate guide setup, or will the periodic error throw the ST-i off it's guide star before it has a chance to correct? Maybe a focal reducer (Good luck finding one for the Edge, maybe a standard one will do since image quality is not of great concern for guiding?). In a perfect world I guess this would result in incredibly good tracking for the piggyback camera, but my world is not perfect, so let me know how you think this would work out in reality...

Bottom line though, is that getting an ST-i guider kit, or alternatively if you think it's better; a larger guide scope, plus a CCD guide camera, will kill two birds with one stone - wide-field is sorted as I can guide through the SCT, and I can also reach the DSO's through guiding with a guide scope.

The reason I don't mention OAG's is that it just seems like a bother. I understand the benefits! But it just seems like a lot of fuzz. If doing wide-field imaging with a refractor I would probably think it was a great idea, but it seems to me that an 8" SCT will need a lot of twisting and turning of the OAG to find a reliable guide star in some cases, and that it's a lot of weight being tacked onto the back of my telescope, upsetting the balance on my already periodic-error-challenged CGEM mount. It just seems like a lot of work, especially considering the ease of guidescope-guiding, and the fact that flexure seems to be a bit of an overstated problem if you avoid scope rings like the ST-i kit does, and if the imaging train is solidly mounted. And I think the ST-i guide kit can achieve an extremely solid mounting, as does my Canon on the T-adapter at the back of my telescope.


Next up is the leap to actual CCD imaging. It's a but further into the future than getting my Guiding sorted, but it's still something I really want to do.

And it seems I have a lot of options. Many, many options. And I guess there's no straight answer. But hear me out.

I have the option of buying a reasonable DSLR, and modding it - full spectrum photography seems really really really cool even for terrestrial use, so that might be a fun idea. I have an IR-pass filter laying around somewhere, that'd be fun.... Canon 450D maybe? It's cheap, got larger pixels, and seems to have a reputation for great (low) noise characteristics. And all those megapixels on my 60D aren't doing me any favors at 2032mm anyway; if I understand oversampling/under-sampling correctly. I might not! In any case, it's probably the cheapest alternative out there, for someone who refuses to give up those megapixel resolutions.

I understand that you can do great photography with cameras under the 1 megapixel mark, but allow me to be stubborn and shallow, I really want higher resolution results. I'm more into this for the great looks and putting prints on my walls, rather than finding that rare fuzzy and having little to show for it.


My other option is getting an OSC CCD. QHY seem to offer some good deals here - their cameras aren't bad, they won't put you in debt for the rest of your life, and they have a good camera body format, if you want to ever give f/2 imaging a try. Since my Edge is Fastar/Hyperstar capable, I might just do that one day - and it's my understanding that if you want colour imaging from a Hyperstar setup, OSC CCD is your only option? At least on a small 8" SCT, since a big camera body would ruin your light gather ability. Something like a QHY8L however fits perfectly, giving the same obstruction as the secondary mirror normally does. I also suspect that since a Hyperstar setup gives a much larger image scale, the ST-i guiding might actually work really well with it - if their image scales are more similar, that should mean the guiding accuracy of the ST-i kit should translate more closely to the Hyperstar image, right? It makes sense to me at least.

Good idea, bad idea? It would be able to fit both ends of the telescope, and it seems to do a decent job in any case. There are other options of course, but in the OSC CCD department it's a tough performance/price ratio to beat it seems. Starlight Xpress cameras seem nice, but they have some weird solutions sometimes, and seem really expensive for what they offer. The Celestron Nightscape seems like a fair deal too, but I'm not sure I trust Celestron as much as I do more experienced OSC CCD brands; even if the sensor inside is all from the same manufacturer, the camera electronics are not.


At last, my third option, is to go all the way to a big-name CCD camera with all the bells and whistles. I'm thinking of something like the SBIG STF-8300, or maybe go as far as the STT-8300 with it's self-guiding-filterwheel-goodness. This in itself presents another choice; Monochrome vs OSC CCD.

I'm a bit of a perfectionist, so the Monochrome + Filter Wheel option is very attractive to me. I enjoy the idea of control it gives, and being able to get as much data as at all possible out of my imaging time. But on the other hand, I also realize that I do not live in a place with great seeing conditions. Yes, I have little light pollution - yes, I live 800m above sea level. Unfortunately fog and clouds and wobbly atmosphere is common here. So maybe LRGB imaging will be a humongous waste of time? I live far north, so winter is the only time I can realistically image as well. Summer nights here in the north are basically bright as day. We only have a few months per year of actual astronomical night.

Part of me feels like going down the OSC CCD route may make me happier in the long run. It's not like the images will be ugly - maybe not quite as good, but not ugly. And, versus LRGB imaging, OSC CCD imaging has a larger chance of success given the circumstances anyway - limited time per year to image, and limiting seeing conditions, and chance of clouds rolling in all the time.




So, that's a lot of putting my thoughts down in text. It outlines my current roadmap though; Achieve autoguiding first; get some kind of CCD imager next. And I want to do that with as little compromise as possible, and wasting as little money as possible. I don't consider buying an expensive CCD imager to be a waste - buying a guider that I use for 3 months then replace, is a waste though... so you see what I mean.


Does this make sense to anybody else? Have I got my theory screwed up somewhere, or have I spent the last few months reading online actually learning something useful? Any input would be much appreciated. I realize I ask a metric ton of questions here, but there's nobody else I can really ask, and I can't afford going down the trial-and-error route to learn by doing. As I said - I am essentially a beginner, with one too expensive telescope standing around not doing anything, because it's owner believes she needs more gear before she can go any further.

Thanks for reading!

#2 GlennLeDrew

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Posted 10 February 2013 - 05:34 AM

I'll limit to just a couple of points, based on observation and experience.

Strive for simplicity.

To me, the OAG notion does not fit in this philosophy. It's not required from an optical leverage standpoint, and it can introduce problems with finding a guide star. I tried this scheme back in my film photography days, and gave it up in very short order. The frustration wasn't worth it.

The small, lightweight, 'finder' class guiders work. Two friends have 11" and 14" SCTs, and use the Orion SSAG, with its wee little 50mm f/3.8 objective, to good effect, albeit so far with some focal reduction on the scope to f/7. There is always a guide star in the field, and flexure is not a problem (when the cables are 'braced' on the OTA so as to not be tugged as the scope slews about.)

Were I to go with autoguiding, this is the kind of system I'd choose. I'm that impressed.

#3 Tel

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Posted 10 February 2013 - 08:53 AM

Hi Myth,

Reading between the lines, you appear to be poised to go down the same road as I regretfully did when I began astrophotography in that any one of your proposed options might overcome the basic problem of your CGEM's inability to track unguided for any time much longer than a simple Alt./Az. mount is capable of doing before the onset of field rotation.

In contrast, I have a Nexstar 8i OTA mounted on a Skywatcher HEQ5 Pro which will give me at least two minutes unguided; three on "good day" as you put it. Guided, however: I have not taken the set-up beyond 10 minutes, but I think, at least as it stands, it's capable of ca.15 minutes; maybe a few more.

Spend money by all means if necessary, but, fundamental to my own experience and quickly dispersed optimism as a result of the misguided route I took in doing so, invest it, first and foremost, in addressing the overriding question as to why the basic CGEM, without any "trimmings", tracks so poorly.

Best regards,
Tel

#4 ghataa

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Posted 10 February 2013 - 10:40 AM

Hi Myth,

The Orion SSAG is a great place to look for autoguiding. Most folks don't do much better unguided on the CGEM-class mount than what you described but with guiding typically the sky becomes more limiting.

Best,

George

#5 psandelle

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Posted 10 February 2013 - 11:22 AM

Myth - maybe I missed it (forgive me if I did), but what do you MOST want to image? Me, I'm a wide-field kinda guy. Love the images Tak 106's give you with a large chip. I went a more economical route, but, essentially, my goals dictated what I bought.

You have a real nice set of gear, and with guiding, and maybe a reducer to make things easier, you should be set for a lot of good imaging (and there's always Hyperstar if you get the wide-field bug. Also, you could do some good planetary work as well at longer focal lengths).

But WHAT do you really want to image? If it's galaxies, you probably do want an OAG (because you're going to be using longer focal lengths, probably, to get those dim small ones). Wide-field? A guidescope will probably be less painful to get the hang of (and work just fine).

Will you be shooting from suburbia/light polluted skies? Gotta get mono (yeah, yeah, some people don't and do great pics) for narrowbanding.

So, what's your first goal for imaging? Then you can figure your first setup. Your scope can be very versatile, and that's a good thing if you later decide to expand your imaging range (you might start wide-field, then go for them dim galaxies, and with your setup it's not much problem, and not much extra to do that - try going widefield after you've bought an f/12.5 MakCass...much tougher).

Oh, and the SBIG guider camera, or the Lodestar are top-of-the-line and should hold you in good stead whatever way you go now or in the future.

Just my 2 cents,

Paul

#6 Tel

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Posted 10 February 2013 - 11:35 AM

Hi Myth,

Most folks don't do much better unguided on the CGEM-class mount than what you described but with guiding typically the sky becomes more limiting.

Best,

George


Sounds like a really classy piece of kit if it won't track any better than that, George !

It's certainly difficult to believe that, at best, it performs so badly when unguided, considering the price it commands and the famed logo it bears ? :idea:

Best regards,
Tel

#7 ghataa

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Posted 10 February 2013 - 12:28 PM

Tel, seems sad but true from what I have read! If folks want to go deeper than 1-2 minutes on this class of mount especially with longer focal lengths than guiding is a necessity. I have read the Astrophysics class mount for instance can do very well unguided but the cost of entry is high! At least the finderscope guiding solution isn't outrageously prohibitive cost wise but still adds an additional expense.

Best,

George

#8 Tel

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Posted 10 February 2013 - 12:41 PM

Thanks, George.

From what you say therefore, maybe finder 'scope guiding is indeed the most effective and economical means by which to control this wayward mount ! :waytogo:

Best regards,
Tel

#9 CounterWeight

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Posted 10 February 2013 - 12:57 PM

Unguided imaging... you are looking entirely at the 'mechanical' of the mount. For that amount of $(yes it's a lot of $ in isolation, but for imaging grade mounts it's in the less expensive, less accurately machined / constructed end) I wouldn't expect a lot more than you're getting. I am not saying anything is impossible, just IMO it's what I'd expect.

Autoguiding will certainly improve things once you dial it in. I too have had far better than expected results with the mini-guider and no problem recommending.

I agree too that you can't roll it all up into anything simple. Deep space high resolution is different that deep space widefield. RGBL or Ha/SII/OIII is different than OSC imaging. Planetary yet again different in all respects.

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Broad side of the barn when talking about imaging and guiding I'd say from easy to difficult-

'easy'<--------------------------------------------------------->'most difficult'

planetary...........widefield..........deep space widefield.........deep space narrow field

focal ratio('fast')<------------------------------------------------>('slow')

short exposure<----------------------------------------------->long exposure

most forgiving<----------------------------------------------->most demanding

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


Within the above there are of couse all the caveats about mount quality and scope aperture, OTA weight, focuser to load qualities, arcsecond per pixel, exposure length to light pollution threshold, many considerations.

Hope this helps with your considerations.


.

#10 RedLionNJ

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Posted 10 February 2013 - 04:58 PM

If the PE is so bad with your CGEM, why not train it, first? It won't make it 100% perfect, but it will certainly reduce the amount of corrections any subsequent auotguider will have to issue.

As far as autoguiding goes, I would get a mid-range 80mm short-focus refractor and a really inexpensive cam. I picked up a Point Grey FireFly MV off eBay for $45 - works like a charm.

Grant

#11 Myth

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Posted 10 February 2013 - 06:46 PM

Thanks for the replies, folks!

I'm glad to see that I'm at least on the right track with my ideas / conclusions.


Speaking of the CGEM mount, it's a bit of a sore spot. When I started out with this madness of a hobby, the first telescope I bought was the CGEM + 8" Edge kit. Bold move for a first scope? Definitely, but I've always figured myself a fast learner that more often than not end up just being frustrated by advices of "start small" and such, with no offence intended towards friendly advice-givers.

And the Edge is a nice scope, definitely. The CGEM is not a good astrophotography mount however. It'll do a fantastic job for visual observation though! But that wasn't really my goal. And as was said above - it's not a particularly expensive mount either, given it's role as a beginners "serious" mount.

Celestron in and of itself is not really that big of a name once you start looking for quality - they're like a Jaguar; they seem exclusive, but you realize they're frankly not that special at the end of the day. I own both, so I can speak of experience!

At least that's my understanding of it after much reading.


So, the CGEM can be remedied by PEC training, to some degree. I'll admit I don't quite understand how that works yet - I tried fiddling with it once, but it seemed that you need to "playback" the PEC recording to make it work - which seems heinously *BLEEP*-backwards from a logical standpoint. Shouldn't it just be programmed into the drive logic so that it always corrects every revolution of the worm gear, by that PEC profile?

Or maybe I'm just missing something.

I've also read that the CGEM has a partially un-correctable pattern to it's periodic error; every 3rd worm revolution, there's another periodic error chiming in. Which can't be corrected for through PEC - since PEC only plays back one worm gear revolution.

So in the end, guiding seems the only way to tame the mount to a workable state.


Makes me regret I didn't research more before buying. I guess I fell for the advertisement and admittedly good job Celestron did at branding their products and making sure you get the impression they're the best of the best. Which is what a mass producing brand should rightfully do, that's how they earn their money.

In hindsight I may have been better off saving up a little more money, and getting a mechanically better mount. Valuable lesson learned, though! At least I got some great optics out of the deal.


Wow that was a negative tangent! Back to the original intent of the reply...!


My goals for imaging is basically; Anything that's pretty. Like has been said, the 8" Edge is very versatile, so I'm happy I have it. Between default f/10 imaging, f/7 with the HD Reducer (once they push it out the door..), and f/2 Hyperstar, that scope can cover a heck of a lot different image scales. I have no goals to image particularly difficult objects, that's not my purpose so to speak. I just want to get my own images of many of the common, popular, and indeed beautiful objects, that are out there and within reach of my current telescope. I think my Canon 60D + some of my lenses can do a decent job at wide-field imaging for now, riding piggyback on the Edge. It might not have the sensitivity in all the interesting light spectra, but with enough exposure you can pick out enough to make for a pleasing image. It'll get me going at least!


I live in a mountainous area, and in excellent conditions as far as light pollution (Or indeed any pollution) is concerned - there is none what so ever. So, narrowband imaging is an option, but not my only option. I would definitely want to go Monochrome + OIII / H-a imaging some time, but that's something I consider a future project. I think going down the larger-resolution OSC CCD path will grant some more instant gratification, and keep things interesting until my pocket can take the hit of acquiring a Monochrome setup.

And also I realize I can probably make things cheaper and / or easier for myself in some regards, but the thing is, and this has always been my way of thinking anyway; buying cheap things usually end up meaning you buy something better further down the line. And thus, it wasn't that cheap after all. Some people call me a snob, but matter of fact is I prefer getting "the good stuff" straight off the bat, knowing it will do a damned good job, for as long as I will ask of it. This is the reason I feel like going for the SBIG guider, and eventually perhaps CCD, since I know they're good, reliable, trustworthy cameras from a highly reputable brand. It's also why I drive a Jaguar - it may be 15 years old, but it sure doesn't look or drive like it's 15 years old. And hey, it didn't actually cost that much! Maybe I should go looking for a 15 year old camera that once lived a more exclusive life? :p


Jokes aside I appreciate the feedback so far. Indeed, many choices, many options!


Does anyone have any experience guiding through an 8" SCT? For use in wide-field imaging, as I mentioned in the original post; I figured it might be a neat idea to guide through the SCT while imaging with my comparably wide camera lenses, and maybe eventually a companion Refractor.

Seems like a great idea actually, having a reasonable refractor attached to my SCT would make it an extremely versatile setup - if each scope can be used to guide the other whenever needed. But, again, that's something for the future.

Does anyone have any hands-on experience with the SBIG ST-i Guider Kit? I've yet to find any sort of review or anyone with hands-on experience of the darn thing. I really have no doubts the CCD is an excellent piece, but it's more the rest of the kit that I'm curious about.

#12 psandelle

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Posted 10 February 2013 - 07:39 PM

I've seen a lot of people take some beautiful pics with a CGEM (just Google, or look around here, or on the Yahoo groups), but if you're not happy with it, just get a Mach 1 and be done with it. Most people in that size-range seem to like that mount tremendously (I'll probably move to one eventually, when I've learned everything I can with what I have). And since you want to get the "good stuff" straight off the bat, that would be the "good stuff." And since your mount's probably your #1 most important piece for imaging, that would be the first priority. [NOTE: I don't think anyone would call a CGEM a Jaguar by any stretch of the imagination. :)]

I've heard nothing but good things on the SBIG ST-i Guider Kit (a number of people here have written about theirs). I went the Lodestar/Borg 50mm route myself. Almost went with the ST-i instead of the Lodestar, but at the time the ST-i was just coming out and hard to get. Love my Borg & Lodestar unit - solid, great guiding with MaxIm DL, no problems.

Sounds like you're going to be pretty happy whatever route you go.

Off and on you get a lot of reports over the years of people guiding through their "big scope" and imaging through lenses or their widefield guide scope. Seems to work well, provided their main imaging camera is set up right. I played with it for a second to see if I could do it, and it wasn't a big deal with MaxIm.

Paul

#13 RedLionNJ

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Posted 10 February 2013 - 08:34 PM

As far as guiding goes - I use my fork-mounted 12" SCT for guiding my 80mm refractor, which is piggybacked on top. I've used all three of my DMKAU618, my Orion SSAG and my FireFlyMV for guiding purposes, hooked-up to PHD and a GPUSB. The only "trick" is to desensitize the autoguiding corrections so you don't end up chasing the seeing in the SCT.

Grant

#14 Myth

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Posted 11 February 2013 - 07:33 AM

[NOTE: I don't think anyone would call a CGEM a Jaguar by any stretch of the imagination. :)]


Indeed not, I was more talking about the fact the CGEM is advertised as being a lot more than it perhaps is! (Not unlike my 1998 XJ8, which did actually cost me less than my friends paid for their tiny little hybrid cars!)

I'm not saying it's not a decent mount, but it doesn't get far without help, is what I mean. It seems it's pretty rare for anyone to be able to shoot unguided for more than 30 seconds or up to a minute on the mount, which is perfectly fine for observing and such. I mean, I can point it at the moon, go inside and have a 15 minute coffee break, and when I come outside again, it's still pointed at the moon! This excited me a lot back when I first got the thing. But it frustrated me a great deal when I attached a camera to the back of it. At 60 second exposures, stars weren't even oblong - they were U shaped.

I think perhaps what makes it feel a bit worse than it is, is the fact it's sold in a kit with the EdgeHD SCT's, which are great telescopes, the difference in how good the scope is versus the CGEM, is sort of the point that let me down a bit.

Anyway..!


I'll probably go for the ST-i kit and some mounting accessories soon, then. The nights are already getting notable shorter here, and worst of all, Orion is starting to skim the horizon, I need to get a move on! I wouldn't want to miss out on this years M42...


Come to think of it, the ST-i will probably do a good job during summer as well with the Lunt LS60T/PT. Means I can get even more use out of it!

#15 Myth

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Posted 11 February 2013 - 07:37 AM

As far as guiding goes - I use my fork-mounted 12" SCT for guiding my 80mm refractor, which is piggybacked on top. I've used all three of my DMKAU618, my Orion SSAG and my FireFlyMV for guiding purposes, hooked-up to PHD and a GPUSB. The only "trick" is to desensitize the autoguiding corrections so you don't end up chasing the seeing in the SCT.

Grant



Sounds great. Is there any particular reason to use the GPUSB adapter rather than going [PC -> Guider -> ST-4 Port]? I guess you'd have to use an adapter if your guider didn't have an ST-4 port, but the ST-i does so... Or is there an advantage to guiding through the hand controller?

I read about different kinds of guiding, and I'm not sure I get whether "pulse guiding" is only possible through the hand controller, or if signals sent to the Guide port will be able to work that way.

Either way, basic ST-4 guiding doesn't seem to be a bad thing, so either way will work I guess.

#16 psandelle

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Posted 11 February 2013 - 09:48 AM

Myth - I was joking about the Jaguar and the CGEM. :)

Why do you want to go unguided? I mean, there's at least one guy who's doing some very long unguided work on his ASA mount, but I'd say that 90+ percent of the best imagers (and the rest of us po' folk) guide, regardless of their mounts (and that's including AP, SB and even those ASA's). What's the good of unguided for you? I'm not sure a Mach 1, the best in that weight-class, will get you all that great unguided times. Certainly better than the CGEM, but if you want 10 minute or more subs, you're gonna have to pretty much guide. And you're getting the ST-i already. Why not give what you have a chance and if you find the CGEM, with guiding, doesn't do what you want, THEN get a new mount? I mean, you already have it, and there are PLENTY of great images taken with CGEMs.

When you miss M42, you just get the Lagoon and Trifid and North American and Pelican for summer. It's all good....

Paul

#17 orlyandico

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Posted 11 February 2013 - 02:48 PM

a regular finder guider like the Orion package or the ST-i will not be enough to guide the EDGE. In my experience you need at least 500mm focal length on your guider to successfully guide a 2000mm FL SCT.

Mirror flop has not been a problem for me, surprisingly. This is for exposures up to 20 minutes (guided with a separate guide scope).

I would be a bit leery about getting long subs with the EDGE8 and CGEM. I have a CGEM, it doesn't play nice with my C9.25 (2350mm FL). Probably if you get the (spendy) 0.7X reducer for the EDGE. That would tame the FL a bit, bring it down to 1400mm.

The problem with the CGEM is that even when guiding, the rough gears in the motor gearbox means you can't get really fine tracking.

Oh - and I do have a Mach1. I still guide. Because, unlike the CGEM with its great ASPA routine, the Mach1 doesn't give you much in the way of polar alignment assistance. Since my polar alignment isn't great, I guide.

One of these days I will use the PEMPro alignment and get it dead on. But I don't expect to get > 10 minutes unguided. Maybe 5 with the C9.25.

In contrast, using the ASPA on my CGEM, I can do 6 minutes unguided. Albeit at 336mm focal length (short refractor). This CGEM has 40" of periodic error of which 25" cannot be corrected by PEC.

#18 John Wunderlin

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Posted 11 February 2013 - 05:28 PM

In my experience you need at least 500mm focal length on your guider to successfully guide a 2000mm FL SCT.


Have you tested this scenario? I've been able to guide my 2500mm scope without my FR using the Mag Mini other than a bit of mirror flop issues. I'd love to see someone get something close to a definitive answer to this question since it comes up so often. My experience is that 50mm with 1600 focal length works great for me- greater than 95% of usable subs most nights. I don't usually image without the reducer, so I can't say if that works all the time, but I've seen posts of individuals using 14" SCT's guided with the baby 50mm orion successfully.

#19 Myth

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Posted 22 February 2013 - 12:21 PM

I guess it comes down a bit to the individual mount behaviour and overall setup people have.

I've seen some convincing writing on why you don't really need a large focal length on your guider to guide a longer focal length scope. A lot of guiding software now can guide to sub-pixel accuracy with ease - in my mind, this means that even if each pixel on your guider cover the same area as 10 pixels on your imaging sensor - you'll still be able to guide accurately enough.

I don't recall the exact numbers, but the argument was that your guiding is much more limited by your seeing than by the focal length of your guide scope, and that unless your seeing was incredibly good, a meager 100mm focal length on your guider would be enough to guide relatively large scopes, since your resolution is limited anyway. If your seeing prohibits any more than 2 arcsec/pixel imaging, there's no need to guide better than that, either.

But like I said, every single part of your setup is going to act as a variable on how this works out in practise. Just saying it seems you don't necessarily need a large guide scope.


I received my ST-i + ST-i Guider Kit a few days ago, haven't had a chance to set up my mount & scope yet though. But I've taken a few images with the ST-i already to try familiarize myself with the camera and the software - and boy, that thing is sensitive, when you're used to imaging with a DSLR... Wow! Excited to try it for actual guiding in a few days.

#20 *skyguy*

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Posted 22 February 2013 - 12:53 PM

In my experience you need at least 500mm focal length on your guider to successfully guide a 2000mm FL SCT.


Have you tested this scenario? I've been able to guide my 2500mm scope without my FR using the Mag Mini other than a bit of mirror flop issues. I'd love to see someone get something close to a definitive answer to this question since it comes up so often. My experience is that 50mm with 1600 focal length works great for me- greater than 95% of usable subs most nights. I don't usually image without the reducer, so I can't say if that works all the time, but I've seen posts of individuals using 14" SCT's guided with the baby 50mm orion successfully.


I have absolutely no problems guiding my fork mounted 12" SCT (1920mm FL) with and 80mm f/4 (320mm) guidescope and my SSAG (imaging camera resolution 2.1 arc-seconds/pixel). However, part of guiding equation that's rarely mentioned ... is the size of the pixels in the guide camera. The SSAG has 5.2 micron pixels which translates to an image scale for guiding of 3.35 arc-seconds/pixel. Since my local seeing is 2-3+ arc-seconds ... guiding at a longer focal length will probably result in the SSAG "chasing" the seeing, just like in my bad old OAG days. From the results I'm getting ... I could probably get by guiding at a focal length even shorter than 320mm.

#21 rflinn68

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Posted 22 February 2013 - 01:10 PM

I've taken some pretty decent images with my CG5 using a small apo. I use my CGEM DX for my bigger scopes and I love the mount. There isnt a mount out there that will track well without a good polar alignment. What process are you using to polar align? The tried and true method is a drift-align but the Celestron All-Star polar align routine works pretty good for me now that I am using the camera and BackYardEOS with the crosshairs. I could usually go 3 minutes easy and sometimes 6 minutes with a very careful polar align but I got the Orion StarShoot autoguider and havent looked back. I'm not sure why you are reluctant to use a guidescope.

#22 Myth

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Posted 22 February 2013 - 04:44 PM

I think there's some misunderstandings here! I'm not reluctant to use a guide scope / guider at all - it's why I bought my ST-i guider + kit after all. And got them sitting here right now, just waiting for the weather to clear (and spare time to not be a rare commodity).

My initial concern earlier in the thread was whether or not the small (Tiny, in fact!) 100mm lens included in the ST-i guider kit was going to provide sufficient guiding for a DSLR mounted on the back of my 8" SCT.

But I think it'll be alright, I'll find out in a few days how things work out.






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