Yeah I got my bahtinov from this guy on ebay too. Think I paid like $20 and it works just fine. This is the link to his page:
https://www.ebay.com.au/itm/25490285...UAAOSwwiZbblJT
Calibration frames may be a bit different depending on what software/process you're using - but for eaa on asiair app you do it BEFORE you start livestacking so that the live results are calibrated.
Bias (to remove hot pixels etc):
1) Lense cap on
2) Fastest frame exposure your camera can take
3) At least 40 frames
4) Should be taken at same/similar temperature and gain settings as your actual imaging.
Dark (to remove amp glow):
1) Lense cap on
2) At least 20 frames
3) Should be taken at same/similar temperature / gain settings / sub exposure time as your actual imaging.
Flat: (to reduce dust bunnies/vignetting/uneven illumination)
1) Lense cap OFF
2) Image train fully setup for the image you about to take (incl filters)
3) Telescope focussed (even the slightest change in focus requires new flats)
4) I use a flat light panel I hold at front of telesope / dew shield
5) Use image preview to determine exposure time so that the AVERAGE is about half way along the histogram
6) Take at least 20 frames
7) Should be taken with everything setup EXACTLY as per your imaging - but the exposure time as per (5)
You can save the calibration stacks - but I only ever re-use the darks and bias ones.
You can get away without calibration shots, buuut, if you want to download the final live-stacked image and have a play with it the result will suffer.
I was looking through my phone and found a preview image I took of the pencil nebula, then another one of calibrated live stacked (a bit messy cause I didn't adjust my darks/bias for increased gain), then my final version after a play on the photoshop express app... Don't zoom too much into the corners...that's the price you pay for using a 8" sct at f4 with a 4/3 sensor!! Gives you a bit of an idea why I do the calibrations tho...
Hope that helps...