The Askar FRA500 90 mm astrograph boasts a 500 mm focal length, f/5.6 focal ratio, and is an excellent fit for those astrophotographers looking to do deep-sky imaging. The FRA500 Askar OTA is lightweight and compact, making it portable and a great choice for outdoor observation or imaging sessions, whether you are in your favorite spot or on the go. FRA, which stands for "FlatField Refractive Astrograph" provides built-in flat field correction, allowing the astrograph to deliver quality flat frame images during astrophotography sessions. The air-spaced quintuplet APO optical structure. The front lens group consists of three lenses, two with ED glass, for excellent correction of aberrations. The other group is composed of two lenses with small space separating to provide fine sharpness across the whole image. The Askar 90 mm FRA supports an image circle of 55 mm, much larger than the standard 44 mm. The special back focus design eliminates the need to calculate the back focus or use an additional spacer.
This modern optical design includes a retractable dew shield with a bright red bezel ring on the front of the OTA. When the dew shield is extended, the total length of the OTA measures 452 mm, and 410 mm when it is retracted. At the back of the optical tube, you will find a large 3.5" rack-and-pinion, high-precision focuser with dual-speed knobs, one for coarse adjustment and one for 1:10 adjustments. On the left side of the focuser, there is a standard finder base with a silver thumb screw, allowing the FRA500 to piggyback finders, guiders, and other compatible accessories. The body of the APO FRA500 has a smooth coating and even finish. Inside the focuser, there is an easily readable graduation.
Included with the astrograph are a pair of tube rings, red anodized to match the FRA500 astrograph body. There are various practical attachments available on three sides of the tube rings so you can expand your imaging setup as needed. This is an incredibly convenient feature when it comes to organization and getting the most out of your sessions. Attached to the handle are the included Losmandy dovetail bar and the multi-use handle.
Objective lens：Quintuplet air-spaced APO (including two ED glass)
Back focus：158mm（from base of M48×0.75 thread on the conical extender to focal plane）85mm（from base of M86×1 thread on the rotator to focal plane）
Total length：452mm（410mm when the dew shield is contracted）
Rear-end thread type：M86×1
Package items：FRA500 OTA, a pair of tube rings, a handle, an Askar Losmandy dovetail plate, a conical extender, a manual