Backfocus spacing Review
Add approximately 0.6667 mm of physical spacing. Verify any adapter before purchase.
The astrophotography workbench
BackfocusLab: telescope backfocus & compatibility calculator
Your next clear night starts here
Check your imaging train.
Before you calculateFollow the evidence trail
- 01ScopeSet focal length and reducer
- 02DetectorCheck scale and field
- 03ConnectionsAudit spacing and threads
Saved on this browser
No cloud sync. Other people using this browser profile can see these plans.
Live rig analysis
On-deviceYour rig, in focus.
1 check needs attention
A planning check, not a compatibility guarantee. The telescope-side interface and physical clearances still need verification.
Your spacing budgetTrain is short
mm
Add this much physical spacing to reach the target.
- Entered train
- 55 mm
- Physical target
- 55.6667 mm
- Filter correction
- +0.6667 mm
Thread handoffs Nominal pass
2 nominal metric-thread handoffs agree. Engagement and clearance still need verification.
Sensor coverage Nominal pass
28.26 mm diagonal is inside the 44 mm image circle. This is not a vignetting or corner-quality guarantee.
Sensor endpoint Nominal pass
Train ends at “Sensor.” Confirm the final length is the flange-to-sensor distance.
Pixel sampling Nominal pass
2.06 arcsec of seeing per pixel scale unit; well matched. Sampling is close to or exceeds the seeing limit. Drizzle is unlikely to add resolved detail; native integration is the reference choice. Reference values only; verify against your own guiding and focus quality.
The light path
Not to scale- 01Spacer / thread adapter16.5 mm
- 02Filter drawer21 mm
- 03Camera flange to sensor17.5 mm
Reference planeImaging sensor
Inspect every thread handoff 2 connections
- Nominal matchSpacer / thread adapter → Filter drawer
M42 × 0.75, opposite genders
- Nominal matchFilter drawer → Camera flange to sensor
M42 × 0.75, opposite genders
Framing, not guesswork
Sensor meets image circle.
The sensor fits inside the entered image circle. This does not model vignetting from filters, adapters, or baffles.
SensorImage circle
Keep the evidence
Rig bill of materials
0 of 3 components have a source link. Linked specifications still require verification.
| Spacer / thread adapter | 16.5 | M48x0.75 female | M42x0.75 male | User-entered; verify |
| Filter drawer | 21 | M42x0.75 female | M42x0.75 male | User-entered; verify |
| Camera flange to sensor | 17.5 | M42x0.75 female | Sensor | User-entered; verify |
What this tool cannot verify
- The telescope-to-first-component interface is not checked. Verify it separately.
- Nominal thread matches do not verify engagement depth, shoulders, clearances, or tilt.
- Apply filter-path correction once, only if the specified target excludes it.
Calculation notes & limitations
Effective focal length = native focal length × reducer or Barlow factor. Image scale = 206.265 × pixel size in µm ÷ effective focal length in mm.
Field of view = 2 × atan(sensor dimension ÷ (2 × effective focal length)), converted to degrees. Sensor diagonal = √(width² + height²).
Physical target = specified sensor distance + optional glass correction. Glass correction = thickness × (1 − 1 / refractive index). Spacing gap = physical target − summed component lengths.
Metric threads reconcile only when diameter and pitch agree and genders are opposite. Unrecognized labels stay unresolved. The first component’s telescope-side interface is not modeled.
Ready for a second opinion?
Keep the whole picture.
Plan JSON restores your rig. Result JSON includes inputs, units, checks, and limitations for an agent or another observer.
Saved only in this browser. Export a copy to keep a backup.
Tonight's sky
Pick a city to load live conditionsOptional. Cloud cover, seeing, transparency, humidity, and wind from 7Timer's astronomical forecast, plus moon phase, for a city preset, a search result, or your location. No request is sent until you choose one.
Planning calculators
Reference formulas, computed locally from values you enter. Not tied to the rig above; verify for your case.
Sub-exposure & integration planner
Suggests a sub-exposure length where sky background dominates read noise, then how many subs fit a session.
electrons
electrons / second / pixel
hours
Signal-to-noise estimator
CCD-equation estimate for a stack of sub-exposures.
electrons/pixel
electrons/pixel
electrons/pixel
electrons
Autoguider settings
Guide scale and a PHD2-style minimum-move starting point from your guide scope and camera.
mm
µm
Eyepiece magnification & exit pupil
Visual magnification and exit pupil for an eyepiece on this telescope.
mm
Refocus interval
Rough interval before refocusing is worthwhile, from temperature drift and the critical focus zone above.
°C / hour
Field rotation (alt-az mount)
Approximate rotation accumulated over an exposure on an alt-az mount, from a widely cited practical formula.
°
°
°
seconds
Dust cap / filter clearance
Checks whether a filter fits inside a lock ring without interference.
mm
mm
mm
A millimeter is a component decision.
Worked example: 0.67 mm to add
With the illustrative scope and sensor inputs, the effective focal length is 640 mm, the corrected target backfocus is 55.67 mm, and the entered train is 55.00 mm. The result therefore asks for +0.67 mm. The displayed 2.1° × 1.41° field and 1.21 arcsec/px are geometry outputs, not a seeing or guiding promise.
Audit the ordered train
Read the bill of materials from scope to sensor. Confirm each component’s optical length and input/output thread against its manufacturer drawing before ordering a spacer. The app preserves user-entered evidence labels so an unknown connection stays visible.
Use the image circle as a boundary
ZWO publishes product manuals and drawings through its documentation downloads. Compare the sensor diagonal with the entered image circle and keep any reducer or filter correction in the spacing check.
Math you can follow.
Checks you can question.
Spacing, sampling, and coverage answer different questions. We keep them separate, show the assumptions, and never hide a mismatch inside a compatibility score.
The starting train is illustrative. Replace its dimensions with the specifications for your own equipment.
Explore the formulasNominal is not guaranteed.
A dimensional check is not a compatibility guarantee. Verify manufacturer reference planes, thread engagement, clearances, tilt, and optical performance.
Source links provide a path to documentation, not proof that a component has been independently verified.
A little clarity goes a long way.
FIELD NOTE / 01
Telescope backfocus: where every millimeter goes
Build a spacing budget from the correct reference plane, including the camera and filter path.
Read the field note ↗FIELD NOTE / 02Image scale and field of view, without the confusion
See how focal length, sensor dimensions, and pixel size affect framing and angular sampling.
Read the field note ↗FIELD NOTE / 03M42 and M48: why diameter is not enough
Check diameter, pitch, gender, and the parts between them before ordering an adapter.
Read the field note ↗Good questions. Clear answers.
Is 55 mm backfocus always correct?
No. Use the distance and reference plane specified for your exact reducer, corrector, and camera arrangement. The starting 55 mm example is not a universal requirement.
Does a matching thread mean these parts will fit?
It means the entered metric diameter, pitch, and opposite genders agree. Engagement depth, shoulders, physical clearances, and the telescope-side interface still need a manufacturer check.
What does a positive spacing gap mean?
Your entered train is shorter than the calculated physical target. Add spacing only when the difference exceeds your chosen tolerance and you have verified the reference planes and filter correction.
Can I use my own components?
Yes. Edit the component cards, reorder them using the arrow buttons, or import a CSV. No catalog lookup is required. Unrecognized connections stay unresolved rather than receiving a false pass.
Are my rig details uploaded?
Not by the calculator. Saves use this browser’s local storage. A shared rig link includes inputs, component names, and source URLs in its fragment, readable by anyone who has the link. Exported files are yours to manage.