I think you main "general" issue with OAG is going to be FOV of the guide camera at the focal length you are using and then being able to find a guide star within that FOV. It may mean you sacrifice your image presentation/FOV to find a guide star with an OAG.
If you chose a guide scope then differential flexure is your main issue to deal with, this can be overcome though with good mechanics.
Using an OAG with the Orion SS autoguider and a 110 mm f7 refractor, there are almost always guide stars in the field of view. Maybe 1 in 10 occasions I have to hunt around a bit. With a longer focal length, it may be a bit trickier.A guide scope is certainly easier to use, as long as you're sure of beating flexure
whats already been mentioned pretty much sums it up...
OAG Pro's
Less load on mount
Guiding at similar or better pixel resolution
Minimizes differential flexture
Con's
Can occasionally be difficult to get a star... I have never had to hunt around with the QHY5 + TMB/Lomo 80mm F/6. Not once. However it was sometimes tricky in the GSO 8" RC @ 1634mm FL.. The only thing required usually with the 8RC was longer guide exposures...
I have not used a guide scope in quite a while, and I don't see myself ever going back..
It all depends on your image scale IMO. If it's anything around 2-3asp don't bother. If you're at F10 with a 0.5asp then you need to real tighten your guiding and then OAG works well. Otherwise don't sweat it.
Well, image scale yes, or your wasting the res at 0.5asp with bad guiding, but also FL counts.
Thats the funny thing actually, OAG is easier with a refactor, but its not so important (wide field,but if you can easily, as Alex does, why not!), a refractor guide scope on a refractor imager bolted down well wont have much flexure cause they both flex the same way ie they are both heavier at the same end, and the average mount can handle a 2 refractor load better.
Its at long FLs with big heavy SCTs, you NEED OAG cause flexure becomes very obvious, but its much harder, as guide stars are harder to find, and dimmer, and a typical SCT is heavier at the opposite end than a refractor guide scope, making flexure worse.
I found at 2m FL (or less) F6.7, an external guide scope was ample and very convienient if the guide scope/cam were all bolted down hard, not adjustable, to 20 min exposures, just.
Now at 2270mm, and 20min exposures (.62asp image scale) I have to use OAG, and its a serious pain in the neck.......... until lately I found the secret of success, took a long while, but well worth the effort IMO.
I spent hrs at times, even with an auto rotator, tring to find a guide star randomly and preserve a set composition, very frustrating. The FOV indicator in the sky was wrong, the SBIG internal guide box was nothing like reality, so I tried like hell to make the FOV indicator see the same as the OAG. In the end, I took long guide exposures, plate solved them, and compared them to the plate solved main image, the OAG pic was WAY off, like 3 times as far as the internal guide chip, and 180deg rotated !. I modified the FOV indicator, with painfull trial and error, untill it was dead right.
Now, finding a guide star is so stupidly quick, you adjust the FOV indicator so it shows the view you want (in the sky), with a guide star where you want it, nudge the scope image to the same view, and voila, instant guide star, ready to go. I was so often sooo close to a guide star randomly looking, but always just out of view without knowing. There are plenty of obvious guide stars for the picking in the sky, but near impossible to find just randomly poking around.
With the rotator linked to the sky, its a joy just to rotate the sky FOV indicator to taste, the rotator follows, and bingo, a guide star pops out just where the sky FOV indicator says it should be. And, you can select the one you want in advance based on magnitude shown in the sky.
This all is just as relivant, in fact more, if you dont have a rotator, just set the sky FOV angle to your OAG angle manually in the sky, and nudge the scope to position.
Never again will I randomly poke around for a guide star, its well worth the effort to get the sky FOV indicator just right..........NOW its nearly as convienient as a guide scope ;-).
Starry nights is fine, it has the same FOV feature you can customise.
You dont need anything else to use an OAG, just SN and the OAG, and set up the FOV manually. With no rotator, you just set the FOV rotation to the same as the cam and leave it. Even though the "DSLR" indicator doesnt have the seperate guide chip indicator, you can build a "CCD" (which has the guide option) from scratch with the DSLR chip dimensions and the guide chip.
I plate solved because I could, but you dont need to, manually finding the OAG position is fine. I suppose I should have actually physically measured the OAG pick-off mirror distance before I assembled the image train, that would have been a lot quicker ;-).
I like SN more than Sky, but the PME needs the Sky so im stuck with it.
BTW, so yes you can use an OAGer with a DSLR or CCD, no diff really, so long as the OAGer is large enough for the pick-off mirror to be out of the DSLRs FOV. Ive only used a MOAG, is was easily big enough.
My Lumicon OAG is also plenty big enough to not impede the light onto the imaging sensor.. I did as Fred mentioned, I measured the OAG's mirror offset from the center of the barrell and created a FOV in Starry Nights to show me the ST8300's FOV, and the relative FOV of the guider... This has made manually setting up shots easy... I can frame them how I want within the imaging chip whilst having the guider at the right angle to detect a star...
As for other equipment.. You may need extension tubes or a parfocalizing ring ( I use a parfocalising ring ) in order to focus the guider.. You may also require different spacers depending on what flatteners you use etc.
I currently have the OAG before my flattener, which sucks as 1/2 of the guide chip gives me distorted stars that aren't too useful for guiding.. I'm looking at upgrading to the TS OAG9 from Teton Telescopes in America. It only takes up 9mm of backfocus, so I could easily fit it between my CFW and my flattener. This would provide me with a full field of sharp round guide stars....
The only other consideration you need to make is whether or not your focuser will handle the load... I've got a 2" FT on my TMB refractor and the focuser sags under the load of my setup.. I need something better.. but can't think of what will do it? My whole setup, (Camera-CFW-extender-flattener-OAG-Guide cam) weighs in at about 3.1kgs.. The focuser does not like the load at all... I dare say the focuser in your 10" RC wouldn't handle it well either...
Best option in that respect would be to add extension tubes to get you within 1.5" of the focus point, then add something like the FLI PDF focuser.. That way your whole setup is a single, thread on, solid piece.. with the PDF allowing for 0.5" of focus movement.. This way your setup would have 0 sag, and focusing would be automatic too....
OAG's definitely have their upsides, but there are negatives to consider.
If you have short focal length scope (to 1200mm) then using a finderscope as a guide scope works very good, it weighs almost nothing and gets plenty of stars in view at 1 Second.
I'm looking at upgrading to the TS OAG9 from Teton Telescopes in America. It only takes up 9mm of backfocus, so I could easily fit it between my CFW and my flattener. This would provide me with a full field of sharp round guide stars....
Alex I have one of these units and am not overly impressed. Sure they are very thin but have no provision for rotation if placed directly after the FW (yours would be worse then mine). Once you fit them they are stuck and you have to turn the whole focuser to find anything worth using. The light path is also severly restricted with the aperture from the prism to the camera (hole through the stalk to mount guide camera) being about half the size of the QHY5 chip. The focus mechanism is fairly rudimentry to say the least and unless you really really really tighten it will allow the guide camera to flop about. As with all things both good and bad are present.
Interesting.. Thanks for the comments Mark.. I was under the impressions from the website that the OAG does support rotating the guider independently of the imaging camera... I was aware of the mirror being smaller than the QHY5 sensor, however I figured better to have less area, but more usable stars.. Currently, about 40% of my sensor is getting useless stars anyway, so the difference is minimal really.. As for focusing, the Lumicon OAG has no focusing mechanisim at all... I just manually pull the QHY5 outwards in the assembly and tighten the grub screw, have a look at the screen to judge the focus, manually move it a little more etc... I did get a parfocalizing ring so now I have the correct focus position locked in place..
The upside is that with the super short focal length of my setup, I rarely rotate my guider anyway (not that the lumicon one supports rotation of the guider) But I've never had to move mine in order to find a guide star.. Shame you live ~3700km away... Would love to have a play with the TSOAG9 and compare it to the other OAGs I've have / still have..
Interesting.. Thanks for the comments Mark.. I was under the impressions from the website that the OAG does support rotating the guider independently of the imaging camera... I was aware of the mirror being smaller than the QHY5 sensor, however I figured better to have less area, but more usable stars.. Currently, about 40% of my sensor is getting useless stars anyway, so the difference is minimal really.. As for focusing, the Lumicon OAG has no focusing mechanisim at all... I just manually pull the QHY5 outwards in the assembly and tighten the grub screw, have a look at the screen to judge the focus, manually move it a little more etc... I did get a parfocalizing ring so now I have the correct focus position locked in place..
The upside is that with the super short focal length of my setup, I rarely rotate my guider anyway (not that the lumicon one supports rotation of the guider) But I've never had to move mine in order to find a guide star.. Shame you live ~3700km away... Would love to have a play with the TSOAG9 and compare it to the other OAGs I've have / still have..
Alex I was using mine last night which is why the pain of it all is so fresh in my mind. With my little refractor (F6) pointed straight at the guts of eta I could not find a suitable guide star using a QHY 5 as a guide camera. The stars were so dim that it kept losing them. I will try to explain what I mean about no rotation. The OAG is made up of 4 parts. The main body is a hollowed out cylinder with a M48 thread at the front, three set screws set at 120 degrees intervals and a square to V slot cut into the top for the prism holder. You then have the prism holder which is in essence a retangular stalk about 50mm long, 20mm wide and 7mm thick with a V cut on one side and left square on the other side. It slips into the main body and is adjustable being fixed by a thumb screw. The third part is a circular fitting with a T thread for the camera and this slips over the prism holder and can move up and down being fixed by a thumb screw. This setup is not very stable unless you screw it down really tight and even then a small bump can losen it up again. The last part is the interchangable back plate which can be swapped depending on the thread you want to use to attach the FW or what ever. This has three tabs perpendicular to the face which are about 15mm wide and set at 120 degrees apart to match the set screws of the main body. The metal has been machined away between the tabs to allow the prism holder to pass through and it fits snuggly into the open side of the main body. Now once you screw the adapter to the FW you have three choices (restricted by the tabs) in which you can position the main body to allow the prism holder to clear the FW. 2 of these will be redundant as the camera mounting plate will strike the filter wheel so you only really have one choice. If you try to rotate the main body it will jam as the prism holder hits one of the three tabs and you are stuffed. The only option left is to rotate your whole focuser to find a guide star and that ruins your framing as there is no other way to decouple the camera from the train if you are using a solid screw together setup. It could be over come by using an adapter to move the OAG further away from the FW but there goes your 9mm right. Even then you would only be able to move it to three positions and would not have the capacity to place it anywhere around a full 360 degree circle.
Glad you bought this up Mark, as I was becoming swayed by the "9mm" aspect. I have had a love/hate relationship with OAG's for a while, and at present am using an older 50mm finder, converted to accept the SXV Guide-head.
Hmmmmm, maybe I just leave it as is.
Thanks
Gary
I agree with Gary, I was ready to jump on one in a few months time.. I might just stick with what I've got.. .If it ain't broke, don't fix it I guess..
Your result looks good though Mark...
I don't find OAG's at all difficult to use if you plan your image using planetarium software with field of view indicators... It will show you your imaging FOV with the guider FOV offset at the right distance etc.. I use Starry nights... Works a treat, and gives me a very good idea of where to find guide stars with my setup - see the attached images.
Alex I have always had a love hate relationship with OAG's. I love the idea but hate them in practice (I have 3 of them now all different types gathering a lot of dust ). I am trying to set up a nice grab and go kit using a sphinx SXW for the mount with a little F6 meade 5000 80 apo. I have a side by side setup which I use with a ZS 66 as a guide scope with the QHY5 and this allows me to keep tracking inside 1 pixel deviation in MaximDL. Unfortunately the telescopes and camera weigh 7.4kg which is right on the 2/3rds limit for the mount so it really does not handle the wind well. I really want to get the OAG to work for me (~ -3kg) but am not happy with it's performance so far (i.e. fiddly @#$%^&* ). So I either buy the SXD and chuck a FLT 98 CF (3.8 kg) along with the side by side setup or get the OAG working. What method did you use to determine the field of view settings in SN??? I am not ready to give up yet .
When you add your cameras sensor as an FOV indicator, there is a checkbox saying "Internal guide chip" Check that box..
This then allows you to input the details :
Sensor size : I used the QHY5 sensor size, as the Lumicon OAG mirror is bigger than the sensor hence - full illumination.. With yours, use the size of the mirror.
guide chip offset : This is how far offset the guide chip is from the imaging chip. Measurement from the center of the guide chip to the center of the imaging chip. I measured from the center of the OAG's aperture to the center of the pick off mirror.
On first light, with everything at 90 degree alignment, I found the SN FOV I to be incredibly accurate.. I rely on it for guide star acquisition 100% and it has never let me down... If it says there is a star brighter than Mag10 within the guider FOV, I know I can image that target..
Ps - Your 5000 series APO gives you the same FL as my TMB, What guide cam are you using? With my QHY5 I've never had a problem finding guide stars... Previously I used a WO ZS70ED as a guide scope, and it did work exceptionally well, I had it hard mounted with no adjustable rings etc and the guiding was always flawless, but I really do prefer the OAG setup.. Portability is no big deal to me... I used to rock around with an EQ6, C11, 102mm APO, and about 30 other odds and ends to make it all work and never complained about it... This setup is a dream.. I can pick up my mount with counterweights, telescope and camera attached and move it should I need too...
Thanks Alex I will give it a shot to see what happens. I am using the QHY5 same as you but was thinking I might try the QHY2 to see if that was any better . I think the dim stars are due to the very small prism used in this OAG (exit is 5mm wide). I have never had a problem when using the ZS 66. My car is very small (read 2 door convertable) so room is at a premium for me. I bought a little honda generator over Christmas which belts out 1 KW of power. It runs everything at idle (50dB, 8 hours endurance) and is about the size of a shoebox . So got me power station just need to get the rig sorted and I am off to the bush.
. I did mange to get this quick eta with it.
). I am trying to set up a nice grab and go kit using a sphinx SXW for the mount with a little F6 meade 5000 80 apo. I have a side by side setup which I use with a ZS 66 as a guide scope with the QHY5 and this allows me to keep tracking inside 1 pixel deviation in MaximDL. Unfortunately the telescopes and camera weigh 7.4kg which is right on the 2/3rds limit for the mount so it really does not handle the wind well. I really want to get the OAG to work for me (~ -3kg) but am not happy with it's performance so far (i.e. fiddly @#$%^&* ). So I either buy the SXD and chuck a FLT 98 CF (3.8 kg) along with the side by side setup or get the OAG working. What method did you use to determine the field of view settings in SN??? I am not ready to give up yet 
. I think the dim stars are due to the very small prism used in this OAG (exit is 5mm wide). I have never had a problem when using the ZS 66. My car is very small (read 2 door convertable) so room is at a premium for me. I bought a little honda generator over Christmas which belts out 1 KW of power. It runs everything at idle (50dB, 8 hours endurance) and is about the size of a shoebox 
. So got me power station just need to get the rig sorted and I am off to the bush.
