I finally decided to purchase an Off-Axis Guider (OAG) for my astrophotography set-up. Even though I am a QHY camera guy, I decided to purchase a ZWO OAG. The ZWO OAG is is a little less money than the QHYOAG-M and the ZWO OAG has the ability to easily rotate the camera…within limits. I also decided to buy a new guide camera. I chose the QHY5L-II-M because I have written lots of software that uses the QHYCCD.dll to access my cameras so I didn’t want to start over using a different vendor.
I had been using a QHY5-II-M camera with my 177mm focal length guide scope. The QHY5-II-M has 1280 x 1024 pixels with a quantum efficiency of 50%. The large pixels are 5.2uM square which gives me a pixel scale of 6 arc-seconds/pixel(a-s/px). I plan to continue using this arrangement for polar alignment and mount calibration. I need a large field of view for this tasks and this arrangement has a 2.1 x 1.7 degree field of view.
The QHY5L-II-M has 1280 x 960 pixels with a quantum efficiency of 74%. It has smaller 3.75uM pixels and fits quite nicely into the ZWO OAG. According to PHD2 Guiding, this should give me a pixel scale of 1.52 a-s/px when using my 80mm refractor. When using my 8” EdgeHD @F10 the pixel scale will be 0.76 a-s/px(binned 2×2) and @F7 the pixel scale will be 0.54 a-s/px.
Achieving the correct back focus for the Imaging Camera
Back focus is a physical distance set by the telescope manufacturer that corresponds to the distance between the optical connection point on the back of the telescope to the imaging chip in the camera. Because there are so many DSLR astrophotographers, most telescope makers set the back focus to 55mm. Adapters for Nikon and Canon are readily available to easily achieve the 55mm back focus. Things get complicate when you decide to add filter wheels or an OAG which is why I didn’t bother trying when I was using my Ha modded Nikon D5300.
I may end up using my Nikon D5300 at some point but for now I am going to use my QHY294C. The QHY294C astroCAM has a physical dimension of 17.5mm which QHYCCD calls the Back Focal Length. This is the distance between the surface of the imaging chip to the mating surface of the 42mm T2 threaded connection on the front of the camera. To calculate the back focus I need to add up all of the optical path lengths of all of the components to determine if it matches the back focus stated by the telescope vendor. The physical lengths of these components can be larger than their optical path lengths simply because there are threaded connections that can protrude from one side or the other.
The main components I want to use are the ZWO OAG and the Filter Slider. The ZWO OAG has an optical path length of 16.5mm. The Filter Slider has a stated optical path length of 16mm but my micrometer measurements indicate an optical path length of 16.5mm. The only restriction I want here is that the OAG needs to be closest to the telescope so that the OAG guiding camera is not influenced by whatever filter I use.
The other complication with these devices is that they have a mix of M48 and M42 connections. The ZWO OAG has a M48 female (M49F) thread on it’s telescope side and it has the choice of a M42 male (M42M) or M48M thread on the camera side. The OAG comes with a M42F to M48M adapter which has a 0.0mm optical path length. The Filter Slider has a M48F thread on its telescope side and a M42M thread on its camera side. I also have a M42F to M48M connector that I purchased with the Filter Slider which has an optical path length of 1.5mm.
So the task becomes how to achieve the correct back focus while being able to thread everything together. I have three telescope arrangements and each one has a slightly different back focus requirement. My 8” EdgeHD SCT @ F10 (2032mm FL) requires an optical path length of 133.35mm. Celestron supplied a 78.5mm adapter with a M42M thread for this arrangement.
I have focal reducers for my 8” EdgeHD SCT and for my BK80ED 80mm refractor. Focal reducers are fussy in that they have a back focus requirement that needs to be strickly adhered to. The 0.7 focal reducer for my 8” EdgeHD SCT has a back focus requirement of 105mm. Celestron supplied a 49.5mm adapter with a M42M thread for this arrangement. The 0.85 focal reducer for my BK80ED refractor has a back focus requirement of 55mm. So…here is what I came up with….
I chose to use the M42M threaded arrangement for the camera side of the ZWO OAG. In all of my configurations I found I needed more shims so I “invented” M48 2mm plastic shims. I like them because they were easy to make and they give me lots of surface area which I believe helps to stiffen this arrangement. The plastic shim between the OAG and the filter slider is achieved using a combination of a 1mm OD wire and the ZWO supplied M42F to M48M adapter.
The 1mm OD aluminum wire acts as a spacer that stops the M42F to M48M adapter from fully threading onto the ZWO OAG adapter plate. When I place the 2mm M48 plastic shim over the M42F to M48M adapter it will come to rest against the ZWO OAG adapter plate. A lot of the M48 threads of the M42F to M48M adapter protrude beyond the plastic shim.
When I thread on the M48F side of the Filter Slider I have plenty of M48 threads available to achieve a robust connection.
Here is a graphic that explains how these pieces fit together. The 1mm wire prevents the M42F to M48M adapter from fully threading onto the OAG M42 adapter plate. The 2mm M48 plastic shim slips over the M42F to M48M adapter. When I thread on the M48F Filter Slider it cinches up tight against the 2mm plastic shim. Later, in this post, you will see that I can accommodate a 3mm plastic shim in this location.
This photo shows this assembled connection. The 2mm M48 plastic shim is now firmly sandwiched between the Filter Slider and the ZWO OAG M42 Adapter plate. This assemble can now be installed into the ZWO OAG body.
The ZWO OAG adapter plate is secured to the OAG body using three thumbscrews. These allow this connection to rotate a bit and also be installed in three different orientations. I hope this gives me some fine adjustment for easy framing of my astrophotos.
Here is what these components look like when installed onto my QHY294C. You can see the thumbscrews on the ZWO OAG and the helical focuser for the guide camera. You can also see the knob of the Filter Slider. The Filter Slider can hold one 48mm filter. There is some mechanical interference between the Filter Slider and the OAG’s guide camera mounting but I am hoping this will not be a big issue…famous last words!
BK80ED 80mm Refractor and 0.85 Focal Reducer
Here is what this assembly looks like when attached to the 0.85 focal reducer and my BK80ED 80mm refractor. You can barely see the QHY5L-II-M guide camera installed in the helical focuser. At this point I had already confirmed that I can simultaneously bring the QHY294C and the QHY5L-II-M guide camera into focus with the QHY5L-II-M installed as shown.
You may also notice that I installed a second 2mm M48 plastic shim between the 0.85 focal reducer and the ZWO OAG. This ended up being very, very necessary! The 0.85 Focal Reducer has M48M threads which are too long to thread directly into the ZWO OAG. The threads are so long that they would end up binding against the pick-off mirror assemble…very nasty indeed! The 2mm M48 plastic shim is all that’s needed to avoid this problem.
The 0.85 Focal reducer for my BK80ED refractor requires 55mm back focus. Here is my calculation of the achieved back focus
2mm Plastic Shim
16.5mm ZWO OAG
0.0mm 1mm Aluminum wire
0.0mm M42F to M48M adapter (came with OAG)
2mm Plastic Shim
16.5mm Filter Slider
17.5mm QHY294C Back Focal Length
———————————
54.5mm back focus Short by 0.5mm
So I have a little wiggle room with adding shims
Another aspect of using an OAG is how the pick-off mirror is oriented with respect to the imaging chip. This is a photo looking thru the 0.85 Focal Reducer down to the imaging chip. The QHY294C has a 4:3 imaging chip which is smaller than an APS-C chip. You can see the pick-off mirror located above the horizontal face of the chip. I hope this location works because it does not interfere with the 4:3 chip in any orientation. I have not testing this under the stars yet. You can also see the three tangs of the OAG adapter plate. Those tangs interfere with the pick-off mirror assembly so there is some angular adjustment but not a complete 360 degrees.
8” EdgeHD @ F10 (2032mm FL)
This set-up requires an optical path length of 133.35mm. Celestron supplied a 78.5mm adapter with a M42M thread for this arrangement. I used my M42F to M48M adapter to interface the Celestron adapter to the ZWO OAG. The 2mm plastic shim at this location is not needed. The M42M threads on the Celestron adapter are very long but, in this case, I also need to convert from M42F to M48M and my M42F to M48M adapter has shorter M48M threads. This arrangement gives me the following back focus.
78.5 mm Celestron T-adapter
1.5mm M42F to M48M adapter
16.5mm ZWO OAG
0.0mm 1mm Aluminum wire
0.0mm M42F to M48M (came with OAG)
2.0mm M48 Plastic Shim
16.5mm ART8147-48 Filter Slider
17.5mm QHY294C
———–
132.5mm Short by 0.85mm
Again…some wiggle room with shimming. You may also notice my home-made stepper motor focuser. It has lots of resolution and some back-lash that I overcome using software. I control it using a Moonlite focuser controller.
8” EdgeHD @F7 (1422mm FL)
The 0.7 focal reducer for my 8” EdgeHD SCT has a back focus requirement of 105mm. Celestron supplied a 49.5mm adapter with a M42M thread for this arrangement. The Celestron adapter is actually two adapters that are combined together for F10 and only the first section is used at F7.
Here are my calculations for the back focus of this arrangement:
49.5mm Celestron T-adapter
1.5mm M42F to M48M
16.5mm ZWO OAG
0.0mm 1mm Aluminum wire
0.0mm M42F to M48M (came with OAG)
3.0mm M48 Plastic Shim
16.5mm ART8147-48 Filter Slider
17.5mm QHY294C M42F
———–
104.5mm Short by 0.5mm
I have some wiggle room with shimming.
When I tallied up the back focus using the 2mm M48 plastic shim I found I was 1.5mm short. I scrounged around in my workshop and found a piece of 3mm thick plastic. I cut it into a 3mm M48 shim and found I had enough threads on the M42F to M48M adapter to safely thread the components together. Later on this year I will post with some feedback.
Peter