Askar SQA70 F/4.8 Petzval Astrograph

Askar SQA70 F/4.8 Petzval Astrograph

Highly corrected lens with Petzval optical design with five lenses, including two ED lenses with low colour dispersion, and air spacing. Diameter 70 mm, focal length 336 mm, focal ratio F/4.8. Sturdy 2.5" double-speed focuser with 32 mm travel and high capacity even for fairly heavy cameras.

Performances

On-axis stellar radius remain confined within 1.84 microns RMS, while at the edge of the corrected field, 22 mm from the optical axis, stellar radius remain within 2.4 microns RMS. Vignetting is very low and MTF data are excellent.

Optical design

The Petzval Askar SQA70 F/4.8 astrograph features a five-lens optical design, two of which are ED, offering a smoothed field of no less than 44 mm, thus also suitable for full-frame format sensors. Since the lenses are integrated inside the telescope body, and not movable with the focuser shift, there is no need to position your camera in a pre-calculated position: simply connect your camera to one of the adapters supplied as standard, find the correct focus point and you are ready to shoot!

The optical design has been fully computer-optimised with special software, finding a combination of special lenses combined with low-dispersion (ED) elements to ensure optimum results.

Support ring

A support ring with a hollow design in the centre is included to lighten the weight while maintaining excellent structural rigidity. An oversized knob allows the ring to be opened and closed without additional tools.

There are four threaded M4 and M6 bolts designed for attaching various accessories such as an additional clamp or directly to another dovetail bar and thus connect different accessories. You can, for example, mount an instrument in parallel.

The upper part of the ring shows a dovetail slide clamp, which is very useful for installing a guide telescope for example.

Finder bases

On both sides of the focuser are mounts for standard mini Vixen / Skywatcher finder mounts, for mounting finders and small, lightweight guide telescopes.

Retractable dew-shield

The dew-shield is retractable, making the Askar SQA70 Petzval Astrograph extremely compact to carry around. When pulled out, it protects the optical beam from unwanted light and helps delay the formation of condensation on the front lens.

Dual Speed 2.5" Focuser

The focuser is 2.5" with rack-and-pinion movement. It is compatible with the most common focuser motors on the market. The focuser travel is 32 mm and the focusing tube has a high-precision scale engraved on it. The entire focuser body is CNC-machined and special care has been taken to suppress unwanted reflections, also by blackening the internal surfaces.

360° Rotator

The focuser end is equipped with a high-precision 360° field rotator with graduated scale for perfect repositioning. A knurled knob allows it to be locked.

High-quality machining

The optical tube of the Askar SQA70 F/4.8 Petzval  Astrograph is entirely manufactured with the aid of numerically controlled (CNC) machinery, with parts anodised and painted. The upper part of the handle and the focuser bear an instrument identification plate, giving that extra personalised touch that denotes attention to detail.

Included adapters

The instrument is equipped with various photographic adapters, all with a knurled edge so that they can be easily screwed and unscrewed. These adapters allow, for example, the direct connection of cooled cameras with M42 (T2), M48 and M54 threads.

By means of additional optional adapters it is also possible to connect Canon, Nikon, Sony or other brand cameras.

Optical design

The optical design of the Askar SQA astrograph is a modified Petzval with five lenses, two of which have low chromatic dispersion (ED).

MTF Diagram

The MTF (acronym for Modulation Transfer Function) diagram is a quantitative description of the clarity of an image produced by a lens and reports a quantitative measure of resolution and contrast. The vertical axis reports the MTF value, while the horizontal axis reports the distance from the optical axis and thus the centre of the frame.

The curves below correspond to the low and high frequencies, up to the edge of the 44 mm image circle. The sagittal lines and the distance between the sagittal lines of the Askar SQA70 lens always remain very small, while the low-frequency line corresponding to 10 lines/millimetre is overall higher than 0.9, demonstrating the high resolution and sharpness of the Askar SQA70.

Low vignetting

The relative illumination diagram shows the light loss at the edges of the field in relative terms (%) to the centre. The vertical axis shows the percentage of relative illumination, while the horizontal axis represents the distance from the centre of the image.

As can be seen, the Askar SQA70 F/4.8 Petzval Astrograph provides more than 90 per cent illumination even at the edges of the 44 mm image circle. Vignetting is basically non-existent.

Spot diagram

The image quality of an optical system can be assessed by analysing the spot morphology in a spot diagram.

The Askar SQA70 F/4.8 Petzval Astrograph has a central spot with a radius of only 1.84 microns RMS at the centre, while at the edge it has a maximum radius of 2.4 microns RMS. In summary, stellar pointiness is excellent.

Longitudinal chromatic aberration curve

It is a representation of the deviation of different wavelengths of light at the focal point. The different curves in the diagram correspond to the different wavelengths of light.

Longitudinal chromatic aberration (LCA) occurs when the different wavelengths of light (colours) do not converge at the same focal point along the optical axis of a lens or telescope. This results in inconsistent colours in the image, usually manifesting as blurred edges or colour separation.

The longitudinal colour curves of the Askar SQA70 F/4.8 Petzval Astrograph have a very small gap and even coincide, the curve curvature is also very slow, indicating that the Askar SQA70 lens' ability to control chromatic aberration is very good.

Specifications

Size

Example