How to Correct perpendicularity error?
Use the adjustment screws on the back side of the index mirror till the true arc and its image coincide.
What is Side Error?
Cause: Horizon glass not perpendicular to the plane of the mirror.
How to check side error?
- Clamp index arm at 0° 0’.
- Hold sextant vertical and look at a small heavenly body
- Gently turn the micrometer on either side to move the image vertically up and then down and then up again.
- If there is no side error, the image of the body will pass exactly through the true object.
How to correct?
Use the second adjustment screws mounted at the back of the mirror.
What is Index Error of sextant?
Cause: Index Mirror and horizon glass not parallel to each other when Index is set at 0°.
How to check index error?
- Set index at 0°0′.
- The reflected and the direct image should be in perfect horizontal line.
How to Quantify the index error?
If the object and image are not in perfect horizontal line, it means there is index error. There are three methods for quantifying the error.
By Observing the Horizon
- Adjust the micrometer till true and the reflected horizon coincide exactly.
- The reading off or on-the-arc will give the error.
- (If it reads 0°3′ on the arc, it means the error is +3′ and you will have to subtract the error from the reading.
- If it reads 57′ off the arc, it means the error is -3′ and you will have to add the error to the reading.
By Observing a Star
The procedures is similar to the horizon method give above because of the sharp and pointed nature of the star, it is more accurate.
By Suns Observation
- Clamp the index at about 32′ on the arc.
- Look at the sun through appropriate shades.
- The reflected image would appear below the direct image.
- Manipulate the micrometer till the limbs touch each other.
- Note the reading on the Arc.
- Now set the index 32′ off the arc (means set it at 60-32=28′)
- Look at the sun through appropriate shades.
- The reflected image would appear above the direct image.
- Manipulate the micrometer till the limbs touch each other.
- Note the reading off the arc.
- Obtain the difference between the On and Off the arc readings (Suppose the readingare 32′.8 and 33.4, the difference is 0′.6 off the arc. )
- Difference / 2 is the IE. (Here it is 0′.6 /2 = 0′.3)
How to correct index error?
- IE upto 3′ may be left on the sextant and allowed for.
- If IE exceeds 3′, it is removed by adjusting adjustment screw Z.
- This will effect horizon glass which will upset the side error settings.
- Back to adjusting for side error.
- Back to Index error correction.
- Keep on doing it till, side error is 0 and Index error is minimised.
Care of a sextant
- It must be handled gently.
- Never touch the arc. Do not polish the arc.
- After use, keep it back in its case after wiping it with chamois leather or a soft dry cloth.
- Stow the sextant in a place which is free am vibrations.
- Ensure that the worm and the arc are clean.
- Never grind the gears.
- Use soft cloth for cleaning the optical and glass surfaces.
- Take great care while removing and then refitting the telescope.
