Science In Your Hands!

4.25 Inch Telescope Mirror Blank

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Quick Overview

A 4.25 inch telescope mirror blank is great size for beginning and experienced amateur telescope makers.

- Schott Supremax® 33 borosilicate glass
- Low thermal expansion
- smooth finish on each surface
- virtually no bubbles
- 3/4" thickness

4.25 Inch Telescope Mirror Blank

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  • 4.25 Inch Telescope Mirror Blank


A 4.25 inch aperture telescope is a great size for the beginning or expert amateur telescope maker. The first thing you will need to make is your primary, or objective, mirror. With this top quality mirror blank and the appropriate abrasives, polish, and pitch you will be able to make a telescope mirror to your own specifications that can potentially out-perform many mass produced telescopes.

Our 4.25 inch blanks are made from Schott Borofloat® borosilicate glass, a glass very similar to Pyrex®, that has become the standard for premium telescope mirrors. The advantage to this glass is its low coefficient of thermal expansion which helps your mirror to stabilize quickly as temperature changes through a long night of astronomical discoveries.

Each blank is water-jet cut from 3/4" thick float glass. This provides a nice smooth finish on the side and a polished finish on each surface. They have straight sides, parallel surfaces and virtually no bubbles. Preparing the backside of these blanks is not required before grinding your mirror; however, a couple minutes of grinding with 320 grit silicon carbide will help to differentiate it from the main face.

Additional Information

Glass Type Supremax® 33 Borosilicate Glass
Diameter 4.25 in.
Thickness approx. 0.75"
Coefficient of Thermal Expansion 3.25 x 10-6 K-1 (20-300°C / 68-572°F)
Density 2.2 g/cm2
Technical Information Supremax® 33 Borosilicate Glass
Weight 1 lb.

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