guidescope guiding a 8SE SCT
Posted 03 February 2013 - 01:48 PM
Have any of you had any luck guiding your SCTs with a guidescope? or do you run into the problems like mirror flop and flexure they talk about?
and also, could someone explain to a beginner like me, exactly what mirror flop and flexure are and how they're caused? i have an idea, but i want to know these things.
Posted 03 February 2013 - 03:17 PM
Well, there's mirror flop and there's mirror shift. Both are due to the same underlying construction of the SCT where the mirror is glued to a tube in it's center, and this tube is slipped over a second tube which is fixed to the rear cell of the SCT. The two tubes slide relative to each other for focusing and have grease between them to allow them to move smoothly. But, the two cannot be an exact fit or it would not move so there is a slight difference in their diameters. This allows the mirror to move sideways a very small amount if force is applied one way or the other.
When an SCT is used on a GEM mount and it moves across the meridian, the shift in the mirror's weight from one side to the other causes it to flop from one side to the other to take up the slop between the two tubes. That is called mirror flop and it only happens when moving through the meridian.
Mirror shift is what happens when the SCT is tracking. You'll notice on a GEM mount that the SCT rotates as it tracks the night sky. You can easily see this if you have an eyepiece in the scope because it ends up in all kinds of weird directions as you slew from target to target. Since the scope's orientation relative to the pull of gravity is changing, the position of the mirror slowly changes as well so the tube slop is always oriented the same direction with respect to gravity.
With either mirror flop or mirror shift, the imaging camera sees the image move, but the guide scope does not. So guiding with a guide scope cannot compensate for it. An off-axis guider sees the same image as the imaging camera so if the guide star shifts slowly due to mirror shift, it should be able to compensate. Mirror flop can happen quickly and may move the image too far and too fast for even an off-axis guider to work.
Flexure also causes the guide scope and imaging scope to see different movements in the image, but due to a different underlying cause. All materials, even metal, bend under force. There are a number of metal parts that connect each piece of your gear to each other and they all flex differently when the mount moves during tracking. The more solidly all the parts are connected to each other, the less differential flexure will occur. The guide scope can think it is keeping the guide star perfectly centered, but the main scope is, over time, seeing the image shift a little relative to what the guide scope sees. It can be difficult to figure out what part of your setup is allowing the most flexure to occur and to figure out how to improve it. But going heavier and more solid always helps - like using Losmandy size dovetails instead of Vixen size.
Posted 03 February 2013 - 03:39 PM
Just one guy's opinion, you understand, but based on my experience with the Celestron Off-Axis guider, my advice is to steer very clear of this piece of kit.
Coupled to my Meade DSI, I think I would have exhausted the patience of a saint before I was able to get any alignment on a guide star let alone par-focus it and the object I was trying to image in the capturing camera !
Now my present system of coupling the Meade DSI to my 104mm refractor and using Stark Labs. PHD free software for guiding purposes is an entirely different affair in its simplicity to set up and the accuracy with which it tracks. I would therefore recommend a guide 'scope combination every time !
As to mirror shift and mirror flop, and again based on my own imaging experience, I think their potential effects are vastly exaggerated judging by the fact that they have never hampered my imaging with my Nexstar 8i OTA/GEM combination.
Referring though to their definitions: mirror shift is a phenomenon you may well have experienced yourself when focusing your 8SE. The image may well shift in the EP as a result of the focusing thread and fork movement when you turn the focusing knob .
Mirror flop on the other hand, is a function of gravity : the potential for the mirror to "flop" or "drop" when in such a position on an equatorial mount as to permit this to happen under gravity.
I hope that the above explains. As I said, it's only my opinion. Others may well differ.
Posted 03 February 2013 - 05:15 PM
I had read, from Tel and many others here on CN, that the Celestron OAG was to be avoided. Seems like many have had trouble using that particular one. I purchased a Taurus Tracker III instead and have been reasonably pleased with it. It's a quality-made piece of gear, provides a large pick-off mirror, and allows some in/out motion of the pickoff mirror for better placement of the guide star.
Having said that, it is still an OAG and has the downsides that are shared with all OAGs. The area of view of the pickoff mirror is limited and depending on your target it can be difficult to find a decent guide star. Since you're guiding at the same focal length as the imaging scope, you have the advantage of more accurate guiding, but it also means you have a smaller field of view than with a guide scope making finding an appropriate star additionally difficult.
The times that I've used it, I have been able to find a guide star and it has worked well. But I also have an Orion 50mm min-guider and I prefer to use that on smaller refractors where mirror shift is not an issue. Once I get my observatory (this summer, hopefully!) and can image more regularly, I may end up getting a side-by-side setup so I can use a guide scope if desired. That will let me use the OAG if there's good guide stars nearby, or a guide scope if not.
As Tel says, many of these issues of flexure and mirror shift can be overblown. Certainly if you're a perfectionist, you'll want to consider them. But you can get some pretty nice images without worrying about them, and only your picky astro-imaging-buddies will know the difference.
Posted 03 February 2013 - 11:52 PM
Posted 04 February 2013 - 02:20 PM
Posted 07 February 2013 - 09:23 AM
Posted 07 February 2013 - 11:53 AM
Posted 07 February 2013 - 12:41 PM
Posted 07 February 2013 - 01:29 PM
There seems to be some sort of strange caveat associated with the Orion "mini- package" in as much as the ad. states that it is a :--
"Complete and easy-to-operate autoguiding system for precise astrophotographic guiding with instruments up to 1500 mm focal length".
The 8SE has of course a focal length of ca.2000 mm but, (and maybe I'm missing something here), as far as I'm aware from my own experience, the focal length of the guide 'scope has no influence whatsoever on the imaging 'scope's focal length.
The main criteria for a guide 'scope is surely that it should be of sufficient aperture to produce sufficient brightness in the available and most favourable guide star and have a relatively short focal length in order to exploit a wide field of view and thus improve the guide star choice.
Posted 07 February 2013 - 02:16 PM
The common wisdom has been that the focal length of the imaging scope should be no more than 5x the focal length of the guider. That means that, in theory, a guide scope should be at least 400mm for the 8SE. However, PHD and other guiding programs can now do sub-pixel guiding, so some say that the mini-guider works fine on an 8" SCT.
My personal take is that sub-pixel guiding is more difficult and less accurate (and it has its limits), and that the closer your guide scope's focal length is to the imaging scope, the better. I think staying with the old 5x-focal-length advice is still a good idea. But, as with all things AP, it depends on how particular you are. For many, the images you get with a mini-guider and an 8" SCT will probably be just fine.
At the very least, it's a good place to start because it's inexpensive, it mounts easily, and adds little to the weight of the gear. Later, if you want to move to a larger guide scope, the SSAG guide camera from the mini-guider will work with that too. And if you get into AP, you'll likely have other imaging OTAs in the future. The miniguider will work well with them if they are shorter focal length.
Posted 07 February 2013 - 09:58 PM
Posted 08 February 2013 - 05:35 AM
I was both puzzled and curious but many thanks to Dan for providing that very comprehensive explanation. As they say; "I never knew that" !
However, to me, Dan hit the nail on the head when he said ,"But as with all things AP, it depends on how particular you are".
Taking therefore as an example, my own, (more Bohemian), approach: I have a 4" refractor placed in parallel with my Nexstar 8i's OTA on a dual dovetail bar, all sitting on a Skywatcher HEQ5 Pro GEM.
Depending then, on what I want to image, and with guidance from Rod Wodaski's "New Astronomy Calculator", which assists me in establishing which 'scope to use by providing the FOV size to expect, I make my choice between the two as to which will be the guide 'scope and which will be the "imager".
When using the 4" as the guide 'scope, I always "halve" its 590mm focal length by using a simple, Antares X0.5 reducer. (In actual fact, I think the amount of reduction is nearer to ca. 33% which provides for a revised focal length of ca. 390mm).
The reason for the reduction is to produce the widest number of guide star options possible within camera focus. My guide software is incidenrally PHD.
Conversely, if an object, such as the Orion Nebula, demands a very wide FOV, I use the 4" to capture and the N8i, suitably focal length reduced with a 6.3 FR to guide. (I have not yet tried it, but in always using a Meade DSI Mk.1 CCD camera to guide, I could use it in combination with my Meade 3.3 FR).
The above may be a far cry from a perfect mathematical model of how an imaging/guiding system SHOULD be set up, but I find it effective enough for my aspirations, given that my viewing range lies only between a restricted Northwest to a restricted Southwest, (rooftops), through East; I have to contend with the unpredictability of the English weather on any seemingly crystal clear night; and that I can almost always apply corrective measures to an image via the various pieces of software at my disposal.
I'm therefore mostly happy with my results.
Posted 09 February 2013 - 06:35 AM
My question is, are these good rings? Is there any part of the rings/mounting shoe/collimation bolts that are of poor design or material? I'm not sure if there'd be flexure with these rings and shoe. Would all of this be a sufficient autoguiding setup I can use with my setup to start getting into AP? I have yet to shoot my first photo, as i just received my first equatorial mount (celestron AVX), and just got my first DSLR (canon T2i). So now it's time for my first autoguiding setup. Looking for something cheap, light weight, and good enough to get me 5-10 min exposures if possible.
And also, would I need anything else like an extender for focusing? Or would just the ring kit, ST80, and SSAG be all I need?
Thanks in advance guys! You're all helping me learn 10x as much on here than I do trying to figure it out on my own
Posted 09 February 2013 - 08:28 AM
At that time it was $249 shipped. I haven't installed it yet, time and weather are not on my side! It comes with everything you need, including rings, mounting hardware, and extension tube.
Good luck with your decision.
Posted 09 February 2013 - 08:31 AM
Thanks in advance guys! You're all helping me learn 10x as much on here than I do trying to figure it out on my own
And helping you spend your money !0x faster!
Posted 09 February 2013 - 10:33 AM
The astrotech setup looks pretty solid, and looks like it might be a bit heavier-duty than the set that comes with the Orion awesome autoguider package. If you get that, don't forget to take advantage of the Cloudy Nights discount. It's not a lot but every bit helps.
Since the Orion package comes with an extension tube, I'm assuming you'll need one.
I think the choice between the Orion and the Astro-tech comes down to how you want to mount it. The Astrotech mounts on a finder-like-shoe which will attach to the rear cell. This is easier to attach, but limits any fore-aft movement of the guide scope and may be more difficult to balance.
The Orion requires the attachment of a full rail from end to end on the OTA. I'm not sure if the rail they provide is long enough for an 8" SCT and if not, you'd have to get a longer one. If it is, it will take a bit more effort to mount the scope, but provides a lot of adjustment room to help with balancing.
Posted 09 February 2013 - 11:25 AM
It is too long in fact. It will overhang the rear of the ota by about 4-5 inches if I remember correctly. Nothing a jigsaw can't handle though, and a few squirts of black spray paint.
Posted 09 February 2013 - 11:59 AM
Posted 09 February 2013 - 12:07 PM
Posted 09 February 2013 - 12:54 PM
without the scope and rings:
Remember this adds more weight, so only go this route if the mount can support it. For AP, I am at the limit of payload for the Sirius EQ-G, and even then I have to get a counterweight bar extension to balance the load properly.
The celestron los-D rail for 8":
Astrotech adapters for mounting rings:
Another option you could try that lets you use the stock bar with the SSAG package is ADM radius blocks. These are spacers that enable you to get a more secure fit for the bar. No matter what you decide to do, you will be looking at some extra cost associated with adapters to mount a dovetail bar. Hope it helps,
Posted 09 February 2013 - 01:12 PM
do any of you know if celestrons 125mm rings they sell for their guidescope, will fir the ST80?
I just measured my ST80 and the outside diameter is 90mm. I don't know what the range of sizes is for scopes that fit the celestron rings. But, Orion has a variety of rings and they recommend their 102mm rings for scopes of up to 80mm aperture.
ADM sells very solid and well made guide scope mounting kits which you can buy with the parts needed to mount to your particular OTA:
Mini-dovetail bar: http://www.admaccess...descope_kit.htm
Vixen dovetail bar: http://www.admaccess...descope_Kit.htm
Posted 09 February 2013 - 01:38 PM
This will mainly be a guidescope, but i'd like to tinker around with AP through the guidscope if possible. I'm assuming the ST80 would be better in that case since it's at f/5 compared to celestrons 7.5?
But my main imaging will be coming from my 8SE OTA until i can upgrade. So as mostly just a guiding scope for my OTA, which would you guys take??
Posted 09 February 2013 - 02:56 PM
Probably not big difference one way or the other. I don't think either one will be particularly great for AP because they're both achromats and will show a fair amount of false color. But you can certainly use either to play around with it. But I think I'd give a slight edge to the ST80 just because of the lighter weight.
Keep in mind that with mounts in this rage (30 pound capacity), the weight rating is for visual use. For AP, it is recommended to load the mount at 1/2 the rated capacity for best results, 2/3 at most. That means that the mount should be loaded with 15 to 20 pounds of gear. When you start adding up the weight of the main OTA, camera, guide scope, autoguider, mounting hardware, adapters, finders, etc. ... well it adds up quick!
Some would say that an 8"SE with a full compliment of gear is pushing the limit for this kind of mount, especially given its long focal length. On the other hand, going back to that "how particular you are" statement, there are many who do use this kind of setup on a CG-5, Sirius, or HEQ5 mount and get good results.
But in any case, weight is a concern and the lighter you keep your rig the better. That would make me lean towards the ST80.