Update, 06/04/2012: I’ve received the replacement prism from Teleskop Service. Great support, it took only a few days to receive it. The prism that came with my original OAG9 was cracked and was the reason stars appeared “crossed”.
Update, 05/20/2012: It wasn’t easy but I was able to get this setup working and working well! I thought I had my cameras par-focal or maybe I really did have them par-focal but for some reason the Lodestar would reach focus. Since I really want to image M101 and can’t come back next week-end I decided to hook up and the AT65EDQ to guide with the SSAG. Guiding started out pretty well and it did seem as it was getting worse, which might be a sign of flexure. My friend Dan Watt was out there and with his help we decided to try and put these cameras really par-focal. The helical focuser was just a little too long and we were able to reach focus using a T-thread to 1.25″ compression adapter. This allowed quite a bit more freedom in adjusting the Lodestar. This is how we got it focused. We did notice that the stars although rounds were crossed by some kind of thin refection. PHD using the star centroid to calculate deviations and corrections wasn’t affected by this. I started right away with a 10-minute exposure and it came out perfect. The PHD graph showed a RMS value of about .70 px. While this number seems big, it’s actually pretty good as both cameras are imaging at about the same scale (0.87 arcsec / px Vs 1.52 arcsec / px or a ratio of only 1.74x): .70 px of correction on the guider is equivalent to only 1.2 px on the 500D. For comparison, when using the 65 mm refractor, the imaging scale ratio is 2.93x. With a typical RMS value of .60 px, the equivalent correction on the 500D is 1.75 px.
I will have to make some minor adjustments of the adapters and of the location of the OAG in relation to the camera, but now I know precisely how much back focus the Lodestar needs.
Update, 05/15/2012: The SX Lodestar arrived yesterday. I mounted it to the OAG and installed all the drivers (windows drivers and Ascom). Tonight I hooked up the cameras to the 8″ RC to make the two cameras par-focal, with day light. The bigger pixels on the Lodestar require much less back focus than what I had with the SSAG. It’s a good thing, the closer the better. Although I am not too concern about flexure as it’s a pretty solid assembly. I don’t regret using the helical focuser either as really I don’t like adjusting the two set screws that allow the displacement of the guiding camera. [I won't be able to anyway when my new cooling system is finished... in June/July]. This focuser isn’t much heavier than a M42 extension, and I won’t have to struggle finding one of the perfect length.
So I am now focused on both cameras, they’re par-focal. And I know I can reach focus on the stars. I actually have quite a bit of room as I am still using a 1″ rear cell extension between the scope and the focuser. PHD recognizes the SX Lodestar with Windows Drivers just fine. I did try to run it with the Ascom driver but it is currently bugged: the SX-Ascom group suggests it is a interlacing problem. It’s not a big problem for PHD, but it pretty much means I will have to stick with the SSAG on the AT65EDQ for my polar alignment routines with EQalign. I am sure this will get fixed soon enough.
Overall I can’t say it’s a success yet as this setup hasn’t really seen its first light… but at least I know all of the optical/mechanical problems have been fixed!
Update, 05/12/2012: I have installed the thin rotating ring what came with the Orion Deluxe Off-Axis Guider in between the OAG and the M42 extension that clears the cooling system… I can now adjust the guide camera 360 degree (at the cost of slight focus re-adjustment). Also, because the rotating ring M3 thumb screw was interfering with the OAG itself, I’ve replaced by a M3 set screw with brass tip.
After playing with the OAG during day light, I’ve finally been able to reach focus on the 500D with the Focal Reducer installed! Success! I also realized that I need quite a bit of extension to reach focus on the guide camera. I have a non rotating helical focuser that I used on my Mini Borg 50 mm guide scope. I installed it in between the camera and the OAG, this will make focusing the guide camera an easier task than playing with thumb screws. Speaking of thumb screws, I’ve also replaced one of the M4 screws that hold the Aluminum tube which holds the prism. It is way too close from… anything to be usable. Using a set screw here is much more adequate as it can be reached with a Allen screw driver very easily.
Notes: It is very difficult to estimate the sensitivity of the SSAG during day light. I decided to order a SX Lodestar to replace it. I am going to sell the SSAG. It served me well and I am sure it will find a new owner. I will install it in a couple of days and will make final “static” pictures before first light next week end.
Some early conclusions: I am pretty satisfied with the setup now. I can reach focus on both cameras, it is very sturdy. The prism is installed pretty far out to the edges and cause no vignetting. I will calculate the actual focal reduction that I have with this setup after another plate solving but it shouldn’t be far from the .68x that I had with just the FR and the 1.25″ long T-adapter. I have some new modifications of my cooling system in mind… and I made sure that this OAG isn’t interfering with my plans. More on this very soon, all I can say is that I think I will be able to bring the sensor down to what CCD cameras do!
Update, 05/09/2012: I’ve received and installed the OAG9 on the 500D along with the AP-CCDT67 Focal Reducer.
So far I am pretty satisfied. After carefully measuring back focus distances on all these items, it seems like my OAG9 setup is shorter than the T-adapter / CCDT67 setup that I had when I shot the Iris Nebula and M5. This means I should have no problem reaching focus! The only “problem” is the fact this assembly leaves no freedom to rotate the guide camera around. If I can’t adjust it 360-degree I would like to have it stay perpendicular to the longer side of the body. In this direction the sensor is shorter which should allow more flexibility in finding a guide star whilst minimizing risks of vignetting. The OAG does allow three positions but currently none of them are satisfactory. There are other solutions: first I am going to try using a M42 thin nut which would allow me to lock the guide camera in a specific position but I am not sure if the M42 male thread on the OAG9 will be long enough. I could also stack a rotating ring in between the M42 extension and the OAG. If I have no back focus problem, this might very well work.
Orion Deluxe Off-Axis Guider Vs. Teleskop Service OAG9: I am using the opportunity to make a quick comparison between these two Off-Axis guiders.
- Price wise, there is a pretty big difference. The OAG9 is about twice the price of the Orion. They’re both available in the US (Opt carries the TS).
- In terms of package, once again Orion can’t really be beat: three male-female M42 extensions, a rotating ring, a short T adapter (2″ nose piece with a female M48 thread for filters or reducers…). TS doesn’t include anything but on the other hand they were really smart about their choice of male and female threads, you shouldn’t any adapter in most situation. I did need some but it’s because I am using a somewhat over sized cooling system.
- DSLR’s and back focus problems: You can read below for more details about what I went through but basically, sensors on DSLR cameras are pretty far back into the body. This a lot of back focus wasted. In other words if you are going to use an OAG on a DSLR back focus is at the very least going to be a concern. If you want to use a Field Flattener or Focal Reducer, it will be a problem. The Orion is about three time thicker than the TS (or 20 mm longer). That’s already about 80 mm of back focus if you unlike me don’t have an over sized cooling system. At 80 mm you are already well above the maximum back focus allowed with a field flattener such as the AT2FF (55 mm max). For focal reducers, it becomes a question of what the maximum back focus you can do with whatever telescope. Also note that the Orion has a female M42 on the telescope side of the OAG. Focal Reducers with a male M48 thread you will need an adapter. I got a very short one from Agena for $30. The short T adapter Orion provides is 28.5 mm and won’t be of any use for this purpose. The three screws around the guider assembly can actually be troublesome and interfere with your camera.
- Adjustments: Orion’s design offers slightly more freedom in adjusting the guide camera: 360-degree, prism’s tilt in X and Y. The OAG9 can only be installed in three positions and doesn’t offer any prism tilt. However, the prism assembly can be pushed in or pulled out on a pretty long range. While I haven’t tried this in practice, it does seem like a better idea.
In conclusion, if you are using a DSLR, unless you have (and don’t plan on having!) no back focus concerns with your telescopes I would probably consider a thin OAG such as the OAG9. Orion offers a great and somewhat inexpensive package but its design may prevent you from using Field Flatteners or Focal Reducers.
Update, 05/02/2012: Thanks to Astrometry, which plate solves astro pictures on Flickr, I was able to calculate precisely the actual focal length of my setup with 500D / T adapter / AP-CCDT-67 / AT8RC: 1110 mm. Considering the native focal length of the AT8RC the actual reduction is .68… which is equivalent to a sensor to FR of 78 mm. The flange to flange distance between CCDT67 and T adapter is 28.5 mm. All in all, it means that the sensor in DSLR is really, really far in the body (about 50 mm)… No wonder that I couldn’t reach focus with the OAG in the equation…
At 78 mm (or .68x reduction), I still had to use a 1″ long extension between my focuser and the back of my AT8RC. The draw tube of the focuser was also rack out by about 15 mm. Increasing the “Sensor to FR” distance will probably mean dropping the 1″ extension. What I am hoping right now, is to be able to use a thinner OAG like the OAG9 from TS which is only 9 mm thick. Based on the specifications it has a T2 thread on the camera side which is perfect as I need a small extension to clear the cooling system (CP30T). On the telescope side it already has a M48 female thread which is exactly what I need for the AP-CCDT67. The Orion OAG has the mirror tilt screw protruding quite a bit at the back, this made me use an extension even longer just to prevent this thumb screw from interfering with the front plate of my cooling system. The OAG9 doesn’t have that screw. I think I should be able to end up at 80 mm or just a little over that. And with a little bit of luck, I believe this will still be good enough to reach focus. Well, let’s hope so.
One good thing is that the SSAG should work. At .68x, the focal ratio is about f/5.4 which should be fast enough.
I won’t be able to go back to Anza before two weeks and hopefully the OAG9 will be there by then!
Update, 05/01/2012: That second attempt didn’t work either. This time I just couldn’t reach focus on either camera with no extension and everything racked in. I am guessing that the distance between FR and sensor exceeded the minimum back focus available from the telescope. I ended up having to remove the OAG once more and used my new focal reducer with the 65 mm refractor as guide scope. Good news is that the focal reducer worked pretty well.
I am going to have to reconsider using the OAG as it really seems like that I would have to drop the field flattener and use a more sensitive camera such as the SX Lodestar. There are targets on which I probably don’t need the field flattener basically all of the very small targets, like planetary nebulae. But for everything else, I would probably want to keep the FF. Now, I’ll probably want to try this Ap-CCDT67 at a smaller reduction ratio.
Update, 04/25/2012: Last week end, I was able to test this new setup. Here are the problems I found:
First, focusing during day light on a distant object wasn’t enough. Practically, the object I aimed at was probably too close. Basically, I used too much extensions and even with the focuser fully racked in I could not focus on any star. I was able to fix that by just removing the large extension that came with the 8″ RC.
Second: once I focused the 500D, I did a couple test shots and noticed that the field curvature wasn’t corrected at all. After checking the recommended back focus for the AT2FF I realized that having this specific Field Flattener in front of the OAG was a mistake. Unfortunately, this AT2FF connects to the front with a 2″ nosepiece which makes attaching it to the back of an AOG a real challenge. If I was to do that (FF to back of OAG) then I would also need substantially more extension on my guiding camera.
Third and last: I then fired up PHD and the SSAG was just unable to “see” any star. I knew about the low sensitivity of the SSAG and kind of expected this.
So, after much thinking I decided to buy a different Field Flattener. I actually ordered the Astro-Physics CCDT67 which is Focal Reducer and Field Flattener at the same time. It has a lot of back focus which will allow me to keep it in front of the OAG. I haven’t installed it but I expect a FF to Sensor distance of about 70 to 80 mm, which according to the CCDT67 Specifications should bring the focal ratio down to about f/5.6. I’ve been thinking of getting a Focal Reducer because imaging with the 500D and 1600 mm result in a lot of oversampling (at least from my dark site). What I am hoping is that the SSAG might actually be able to pick up stars at this focal ratio. If not, well I’ll have to invest in more sensitive guider like the SX Lodestar. But even if I have to, I probably won’t regret the CCDT67.
Original Post, 03/04/2012: Ever since I got this AT8RC I’ve been thinking of getting more accurate guiding. [Not that my setup with the AT65EDQ as guider was inadequate] – I wanted to see what I could produce when guiding at a resolution close to my imaging resolution. I am still struggling understanding what all the PHD settings are for and by that I mean “trial and error” has been the most reliable way for me to get good results. I don’t expect this to change anytime soon, though.
So basically I am hoping this OAG setup is going to help me improve my guiding or at least allow me to do 15 or even maybe 20 minutes exposures… What I am also looking forward to with this setup up is some kind of transition to long focal length guiding. In a hopefully near future I will be getting a CCD camera (with dual chip) and this new setup should help learn – especially as I plan on getting Maxim DL before I get a CCD. Last but not the least, although I couldn’t notice any flex with my AT8RC/AT65EDQ combo I should be able to pretty much avoid any flex all together.
Here is a list of the parts I’ve used in this assembly:
- Canon 500D
- CP30T-EOS cold finger cooling system
- Orion Deluxe OAG
- 1″ Extension from AgenaAstro
I started putting everything together and installed the whole assembly on the 8″ RC. I was able to reach focus on the 500D with none the large focuser extension. However, I had to almost rack out the focuser which I don’t like. I ended up using the 2″ extension which allows me to reach focus with about 3/8″ on the ruler.
I then installed the SSAG with and without the 2″ extension (the one that came with the ST80 “Awesome Autoguider” Package from Orion) and neither configuration would allow me to reach focus on the SSAG. Doing this during day light, I had PHD setup for .05s exposure which allowed me to estimate I needed about 1″ of extension on the SSAG. I bought a $20 1″ extension from AgenaAstro and after some adjusting I was able to reach focus on both cameras.
As you can see on the pictures, I had to use one of threaded extensions provided with the OAG to clear the nose piece of the camera as well as some of the custom parts of my CP30T cooler. The AT2FF screws directly in the front side of the OAG. I had some concerns about that as normally these are supposed to be really close to the camera. During day light it didn’t seem to create any problem but I’ll only be sure of that when I try the setup under the stars.
Overall it fits, barely, but it does. The cooling system didn’t help in this regard but I wasn’t ready to trade the cooler for an OAG anyway!
I’ll make sure I post something when I get to test this setup under the stars which should be in about two weeks!