Been doing some reading about an OAG compared with a dedicated guidescope and was wondering if it's a worthy change for my setup.
As I understand the OAG significantly reduces weight on the setup however it's tricky to get the guide camera in the OAG and imaging device (say a DSLR) both in focus.
I tried guiding for the second time the last few nights. Overall it was ok but when taking long exposes my images came out a bit blurry. Also I noticed with some shots every 2 photos it would be blurry so unsure if there is a tracking issue with my mount or it could be the guidescope setup.
Wanted to ask around and get peoples opinions on using an OAG for an SCT and if it's a worthy change?
Stefan,
For the past ten years I’ve been using effectively an OAG on my C11/ NEQ6 with a Lodestar and PHD2 to guide 10 subs for spectroscopy. No issues, no drama.
I started using an OAG when I got a RC6. Can be tricky to set up as you suggest but only have to do it once. The OAG now stays with my camera and filter wheel and just swap the whole lot out to both my RC6 and 65Q.
I use a QHY OAG with a QHY 5L II.
I was using a guidescope which was working OK but decided to follow the crowd because I read OAG was Better (which I'm sure it is for some). Purchased and swapped out a few extensions and had perfect focus......found good guide stars easily every time.......changed all my PHD settings to suit......calibrated no worries.......but guiding was absolute crap! Struggled with it for a couple months, pulled my mount apart thinking it was issues with that, tried everything. I then put the guidescope back on and was immediately able to do 10 minute subs with 1 pixel variations....almost perfect.
OAG may be great for some.....but it lost me money and a couple months imaging......
Guess it goes back to the old adage - If it aint broke don't fix it.
PS - get a slightly used OAG for sale if you're interested????
Is it possible to swap out the camera for a diagonal and eyepiece if you want to do visual or an alignment or do you need to remove the entire OAG?
Also recommendations on one?
I've got a 2" nosepiece on the front of my OAG so its easy to remove camera, FW, guidecam and OAG all as one unit and move to other OTAs.
They can be fiddly to set up, especially if you throw a focal reducer into the mix. But I just got lots of spacers and shims and parfocal rings and worked out the right combo.
Started off with the ZWO OAG (11mm thick) but managed to break it and replaced it with the TSOAG27 (27mm thick) as I have plenty of back focus and it is easier to adjust. I also made sure I have all the right size allen keys (little ones) so the screws can be tightened up firmly. The main advantage of an OAG is elimination of differential flexure but if the OAG bits are themselves able to move you've got the same problem.
An OAG eliminates differential flexure. It is not often that anyone could image at moderate to long focal lengths and use a guide scope without some differential flexure. An OAG is the way past all problems. The first time I setup an OAG it was difficult but the main thing to remember is that you need to have the guide camera a similar length from the pick off prism to the sensor on the main camera from the pick off prism. So focus the main camera first then focus the guide camera. It does not need to be perfect focus, slightly off is ok. Guide settings will vary too because you will be guiding at the same focal length as the imaging scope. CCDware have a great tool that helps you determine what your guide settings should be for your system. It's worth chasing it up. Try aggression about 8/10 first and then back it off or up it from there.
I would never go back to a guide scope. I have used OAG with an TSA, FSQ, with an RC and now with a Newtonian. Persistence pays off.
FWIW, having a sturdy 1.25' helical focuser or 2'-1.25' eyepiece adapter can really help finding the focus point with the guide camera. It can be very fiddly trying to adjust the OAG mechanism to find focus the first time, especially when you are being eaten alive by mozzies!
Also, I've had situations imaging galaxies off the galactic plane (ngc 300 for example) where I had zero useful stars in the OAG. A very sensitive camera and longer exposure times might be needed to guide properly in these situations. Also, I'd not recommend the Orion thin OAG as the flange camera side attachment seems very flimsy. I upgraded the finger bolts to hex bolts to give more clamping force on the camera.
I only have one imaging train so to save the hassle of changing the setup each time, I use the OAG on both my RC8 and ED80. Bit of overkill at 480mm FL on the ED80 but it still works perfectly well.
I got an orion thin oag a few months ago, once the two camera's are setup for mutual focus you don't have to do it again.
The pic shows mine with a Baader coma corector fitted (for giuding on my 10" newt), but I can remove that and fit a 2" nosepiece to use it with my RC scope without any adjustments to the oag spacings.
I have the TOAG from Orion that Rick mentioned, and you can shift the prism in/out and side-to-side to find guide stars. But I find that if you have a reasonably sensitive guide camera (I use the ASI120MM) then you'll never be without plenty of options. It can be a bit disconcerting to see the comet-like appearance of the stars (you're picking off light at the edge of the field, so get coma and other edge-related nasties), but as Paul noted, it doesn't affect guiding since it is still fairly straightforward for your guide software to estimate the centroid, even if the FWHM looks terrible!
The Orion TOAG has a narrower profile (15 vs 16.5 mm) which can be important if backfocus is tight. It also has an adjustable prism position, whereas the ZWO is fixed. As a trade-off, the ZWO one looks to be a little more robust. I've only used the Orion one, for 3 years, and it's still performing superbly, FWIW.
I went though the same proccess choosing one, reading lots of reviews etc.
A lot of the bad reviews were about an older version of the orion toag, apparently they have now beefed it up with more grub screws to lock things in tight. I am happy with my Orion.
There is not a lot of choice in oag's if you want to use a DSLR for imaging.
