From the telescope inward to the camera: telescope → flattener or reducer → off-axis guider (OAG) → filter wheel or drawer → spacers → camera. Convention places the OAG on the telescope side of the filter wheel, so the guide camera picks up stars before they cross any filter. Keep filters as close to the sensor as your back-focus budget allows.
The canonical assembly order
Read from the telescope toward the camera, the sequence that shows up again and again in working imaging trains is: telescope → field flattener or reducer → OAG → filter wheel (or filter drawer) → spacers → camera sensor. That order isn’t arbitrary — each stage exists to correct, guide, filter or fine-tune the light path before it reaches the pixels, in that order, and moving one stage past another usually breaks the reason it was there.
ZWO’s own connection diagrams show this arrangement for every cooled camera it documents — the OAG sits between the corrector and the filter wheel in each one ZWO. What ZWO has never published is a single sentence of prose stating the order; it exists in the company’s documentation only as diagrams, not as an instruction you can quote.
The clearest prose statement of the same order comes from a retailer, not the manufacturer. High Point Scientific describes imaging accessories as sitting “after the OAG,” and gives a worked figure for the resulting stack: a ZWO CAA combined with a camera, an OAG and a filter wheel needs roughly 87.5mm of back spacing to assemble High Point.
Swap the wheel for a drawer and High Point documents a shorter variant of the same order: telescope → corrective element → spacer → filter drawer → camera High Point. On ZWO's own specs the drawer and the wheel eat an identical amount of back focus — the table further down puts real numbers next to that — so the drawer's real advantage is fewer moving parts and a simpler stack, not a shorter one.
If you haven’t settled on a camera yet, that choice comes first — see our guide to the six specs that actually determine which dedicated astrophotography camera to buy. The rest of this train gets built around that decision, not the other way around.
Why the OAG goes on the telescope side of the filter wheel
Manufacturer documentation gives you the order but not the reason for it. ZWO’s diagrams show the OAG ahead of the filter wheel, and ZWO’s own hardware is physically built for that sequence — the OAG-L’s mounting flange bolts directly to the telescope-facing side of the EFW body ZWO, so the two parts are machined to stack in that order. What ZWO does not publish anywhere in its manufacturer documentation is a stated reason for it.
The explicit rationale lives at Tier 2 and Tier 3, not at the manufacturer. Practitioners on Cloudy Nights explain it as a guide-star budget problem: put the OAG behind a narrowband filter and you’re guiding on whatever light makes it through a few nanometers of bandpass, which can eliminate guide stars a clear-aperture OAG would have found easily Cloudy Nights. The same logic appears in the build notes for an open-source OAG-focuser project’s documentation GitHub OAG-Focuser Project, and SBIG built its self-guiding filter wheel product line — which puts the guide chip ahead of the filters by design — on the same premise in its own marketing copy SBIG. None of these three is ZWO; treat the order as manufacturer-documented and the reasoning as practitioner-documented, because that’s genuinely where the two live.
This matters most on mono rigs, since a filter wheel is only in the train at all because mono cameras need one to shoot color or narrowband — see our mono vs. OSC comparison for why a one-shot color camera skips the wheel entirely. ZWO’s OAG-L is the guider built for this stack — check current price at Agena.
Filter wheel vs. filter drawer
A ZWO EFW 7×2" electronic filter wheel carries seven 2" (50.4mm) filters on a motorized carousel and is 20mm thick ZWO. A filter drawer swaps in one filter at a time by hand instead. ZWO’s own M54 filter drawer is also 20mm thick ZWO — on a spec sheet, the two components cost you an identical amount of back focus. What differs is what you get for it: a wheel holds every filter you own and switches remotely mid-session; a drawer holds one filter, swapped by hand, which is exactly what a one-shot-color rig running a single dual-band filter needs and nothing more.
A component’s listed thickness is its mechanical footprint, not necessarily the full optical distance it adds to the train. The Gibraltar Astronomical Society pegs a generic filter wheel at about 20mm mechanically, but roughly 21mm once you count the extra optical path length a filter’s own glass adds once it’s sitting in the beam Gibraltar Astronomical Society. Hold those as two related but different numbers — the mechanical spec-sheet figure and the figure you should actually budget for — rather than treating them as interchangeable.
How much back focus each component eats
Every stage you add between the flattener/reducer and the sensor consumes part of your back-focus budget, and the components in this article’s dispute — OAGs, wheels and drawers — are some of the biggest single line items in that budget.
| Component | Back focus consumed | Notes |
|---|---|---|
| ZWO OAG (standard) | 16.5mm | 8×8mm prism ZWO |
| ZWO OAG-L | 17.5mm body / ~22.5mm with the included 5mm tilt plate | 12×12mm prism — hold the body-only and with-tilt-plate figures apart ZWO |
| ZWO EFW 2" (7×2") | 20mm | Motorized, 7-position ZWO |
| ZWO M54 filter drawer | 20mm | Manual, single filter ZWO |
| Generic filter wheel + filter | ~20mm mechanical / ~21mm optical | Mechanical thickness vs. optical path length are two different figures Gibraltar Astronomical Society |
| ZWO camera body (APS-C/full-frame) | 17.5mm | Smaller 1"/4/3-format bodies use less — see our back-focus guide for the full breakdown ZWO |
Landing the whole stack on your corrector’s specified back focus — the 55mm convention most flatteners and reducers are built around — is its own piece of math, and it’s owned by our back-focus explainer, which also hosts the back-focus builder tool that totals a stack for you. We won’t re-derive that arithmetic here.
For the last few millimeters of fine-tuning once the major components are in place, fine-increment spacer rings are the standard tool — Agena’s spacer and extension-ring selection covers the common M42/M48 increments.
The real disputes
Most of the imaging train has one defensible order. Three points genuinely get argued over — one is a real minority build practice, the other two are places where forum confusion is common even though the underlying rule isn’t actually contested.
OAG before or after the filter wheel
Position A — the majority build, and the one ZWO’s own hardware is machined for: OAG on the telescope side of the filter wheel, so the guide camera picks off a star before it passes through any filter — the narrowband guide-star reasoning above Cloudy Nights. Position B — a real, if less common, minority practice: some imagers instead run the filter wheel on the telescope side of the OAG (OAG on the camera side of the wheel) — a build documented on Cloudy Nights as genuine practice, not a mistake, though the forum thread doesn’t spell out a rationale distinct from Position A’s Cloudy Nights. What both agree on: the OAG’s only job is to find and hold a guide star, and everything else in the stack is arranged around that requirement, not the other way around.
Treat Position B as demand and context rather than as a competing canon figure — it’s a real practice some imagers choose deliberately, documented at forum level, not a manufacturer-endorsed alternative.
Prism placement and clearing the sensor
Two practical rules from High Point Scientific govern where the OAG’s pickoff prism ends up in the stack: keep the guide camera as close to the prism as the helical focuser allows, since every extra millimeter of air path between prism and guide chip dims an already-faint pickoff star High Point; and place the OAG ahead of tall accessories in the stack, or the guide camera can run out of inward focus travel before it ever reaches focus High Point. Cloudy Nights threads are full of the failure mode this avoids — a guide camera racked all the way in that still can’t reach focus — which we treat as context for why the rule exists, not as the source of the rule itself.
Reducer before or after the filter wheel
This one isn’t a genuine either-way choice the way OAG placement is. The reducer or flattener has to sit on the telescope side of the entire accessory stack, because it’s the element that sets the corrected focal plane every downstream spacing measurement is built from; move it, and you haven’t just relocated the filter wheel — you’ve changed what a correct back focus even means for that setup High Point. Filters and wheels inserted between the corrector and the sensor simply add to the spacing that has to land on the corrector’s spec, which is what the table above is for. High Point notes this as a common point of confusion in build threads on Cloudy Nights, not a live technical debate High Point.
Parfocalizing the guide camera
“Parfocal” here means the guide camera reaches focus on a star at roughly the same imaging-focuser position the main camera does, so refocusing the main camera mid-session doesn’t throw the guide camera out of focus along with it. In practice: focus the imaging camera first, then adjust the OAG’s helical focuser until the guide camera is also sharp, and lock that position — most ZWO OAGs use a knurled locking ring on the helical focuser for exactly this. Once the two are parfocal, autofocus routines that touch only the main imaging focuser leave guiding undisturbed for the rest of the night, instead of forcing you to re-find a guide star every time the main camera refocuses.
FAQ
Does the OAG go before or after the filter wheel?
In the great majority of builds, the OAG sits on the telescope side of the filter wheel — the arrangement ZWO’s own connection diagrams and hardware are built for ZWO — so the guide camera picks off a star before that light passes through any filter, avoiding the narrowband guide-star issue practitioners describe Cloudy Nights. A minority of imagers reverse it, running the filter wheel on the telescope side of the OAG instead — a real, if less common, practice documented on the same forums, not a manufacturer recommendation either way.
How much back focus does a filter wheel use?
A ZWO EFW 2" wheel is 20mm thick ZWO, the same as ZWO’s own M54 filter drawer ZWO. Budget the extra optical distance a filter itself adds on top of that mechanical figure — see the comparison table above.
Should I use a filter wheel or a filter drawer?
A wheel makes sense once you’re switching between more than one or two filters in a session, which describes most mono rigs — see our mono vs. OSC guide for why. A drawer is lighter and simpler, and fits a one-shot-color rig running a single dual-band filter that rarely comes out.
Where do spacers go in the imaging train?
Last, immediately before the camera — once the flattener, OAG, and filter wheel or drawer are all in place, spacers make up whatever’s left to hit your corrector’s specified back focus. Our back-focus guide has the full math and a builder tool that totals the stack for you.
Does the reducer go before or after the filter wheel?
Before — the reducer or flattener always sits on the telescope side of the whole accessory stack, because it defines the corrected focal plane everything downstream is measured from High Point.
What’s a ZWO CAA, and where does it fit in the train?
ZWO’s Camera Angle Adjuster (CAA) is a motorized camera rotator that sits at the camera end of the train, used to fine-tune framing and correct sensor tilt — not a focus device ZWO. High Point’s worked example — a CAA plus camera, OAG and filter wheel — lands around 87.5mm of back spacing for that combination High Point.