Backfocus calculation for QSI690 and TSRED379 ============================================= My setup is on the telescope side a reducer TRED379 from teleskop-service. This reducer needs for my focal length of 714mm a backfocus of 56mm. The image train consist of a QHY600PH c-mos camera, a QHYCFW3-XL filter wheel, a QHYOAG-L on axis guider and the necessary adapter (s). To define the optical length of the image train here are the drawings and calculations. QHY600 ------ Drawing at: https://www.qhyccd.com/uploadfile/2020/1103/20201103052311996.jpg .. image:: image/qhy600ph.png :align: center :scale: 71% It has a distance from sensor to the support surface of the dovetail from 17.5mm. Unfortunately QHY does not tell how thick the protection glass is. So you could guess and choose if you would like to take the optical reduction of the glass into acount. QHYCFW3-XL ---------- Drawing at https://www.qhyccd.com/wp-content/uploads/2021042888-scaled.jpg .. image:: image/qhycfw3-xl.png :align: center :scale: 71% The filter wheel has a thickness of 21.5mm. The dove tail support surface is 12mm - 9mm = 3mm above the surface of the filter wheel which leads to a total optical length of 21.5mm + 3mm = 24.5mm. QHYOAG-L -------- Is quite simple as this on axis guider has a defined optical length of 10mm. Filter ------ When using glass filters, they reduce the resulting optical length by 1/3 of their thickness due to the different refraction of the glass. My filter are 3mm thick, so you have to the optical length is reduced by 1mm. Calculation ----------- camera + filter wheel + oag - filter 17.5 + 24.5 + 10 - 1 = 51 So my optical length for the image train is 51mm. With a needed backfocus of 56mm I need an adapter to manage the interface oag -> TSRED379 within 5mm. This parts are not commonly available, but could be custom made. I choose a bolted (62mm diameter screw circle) on the filter wheel to screwed M48x0.75mm on the reducer. Remarks ------- .. epigraph:: Remarks about tuning the backfocus: There are a number of variables that come into play when using refractors with CCD cameras. The large chips and small pixel size in today’s cameras make things even more challenging. Field flatteners are designed with the goal to exactly counter the inherent inward curvature and bring the star size down by a factor of about 50 (down to about 5-6 microns), and make it perfectly round. By and large that's what our field flatteners are designed to do, but this exact cancellation is dependent on the exact focal length of the scope and the exact placement of the flattener optics with respect to the chip. A small difference in the scope focal length from ideal can require a different chip to flattener distance. If the distance is too short or too long, you will get imperfect cancellation of the field curvature/astigmatism, and you will get slightly larger stars and small amount of oval astigmatism and or coma. Therefore, to get better star shape for large chips, the chip to flattener distance will need to be modified somewhat (perhaps a few mm inward or outward). The first thing would be to determine the shape of the left over field curvature and whether it is inward or outward (under corrected or overcorrected). There is a very easy way to determine that by simply measuring the perfect focus point of the corner stars versus the perfect focus point of the stars in the center of the chip. If you have simple field curvature, then all the corner stars will have the same focus points where they are smallest, but if your camera has any tilt, then the field flattener will not correct all the corners the same. (Roland Christensen) Finetuning ---------- First determine the exact focus point of stars near the center. Then choose a corner star and focus on it. If you have to move the focuser forward (toward the objective lens), then your field curvature is under corrected and you will need to move the chip back a few mm more. If you have to move the focuser outward to get sharp corner stars, then your flattener to chip distance is too great and you will have to reduce the distance between flattener and chip. In either case, just looking at an image with defocused stars in the corners one cannot determine whether the field is inward or outward curving (under or over corrected). Inspection programs like CCD inspector also cannot tell which way the field is curved, but by noting the focus difference and direction between center and corner stars, you can tell which way you need to change the spacing.