Evening, I have a friend who shoots with a K70 and we both shoot astro a fair amount. So, let me provide some background first so that all of this makes sense.
A year ago, I saw that a local camera shot was sponsoring a 3 day astro class for $99. Friday evening 2 hour class, Saturday night on location class 5pm to midnight and Sunday afternoon post processing session. The instructor shoots for Arizona Highways. She starts off saying, that she doesn't know why half of us are taking the class since we have as much experience as she, but to pair up with folks with the same camera brand, who are new to this. Anyway, Saturday evening we're on location and about 11pm a guy drives up and sets up his stuff right next to me - he just got off of work. Turns out that he just picked up his K70 the week before. He had read the manual twice and had a 28/f2.8 lens, and had shot the first 20 images of his life with a dslr. So, we started talking since we were the only two who shot Pentax. He shot several frames and they were beginner ok frames. By that time some folks left and we moved to a better location. I pulled out my gps astrotracer and mounted my 18-35/f1.8 lens on his body, calibrated the setup and he shot a wonderful milky way over picket post mountain image. He was ecstatic. We have been shooting together ever since.
I shoot with a K1, so I looked up "Astrophoto" User-Mode in the K70 manual. It really didn't provide any of the setting parameters, and he never uses it. That's as much as I know about it. However, having said that - this is how we shoot and it works quite well. You haven't said if you are using the gps astrotracer and if you are doing milky way with landscapes or deep sky. So, I'll assume no gps for this.
- Switch to "M"anual or "B"ulb mode - This way you will have complete control over the parameters you set. The light meter is useless at night, so the other modes will try to evaluate the scene based on no information, so just by pass them. The main difference between Manual and Bulb mode is that with Manual mode you can only set the shutter duration up to 30 seconds. With Bulb mode you can exceed 30 seconds - take it up to minutes if you wish.
- For framing, we crank the ISO up to something high 10,000 and above, aperture wide open, with a shutter speed of about 1 second. Take a look at the image on the rear screen and adjust as needed (be it landscape or just stars). If you don't see anything, increase the shutter speed, until you see something. You can also check focus here.
- For Focusing, (put the body in manual focusing) find something bright in the sky, enable LiveView and zoom in to 16x, then use the arrow keys <>^v to get the bright object into the rear monitor. Adjust the focus until it is nice and sharp. Take another frame to check everything out.
You should be ready to go now.
We usually start out at ISO 800 to ISO 1600, wide open aperture (f1.8, f2, f2.8) depending on the lens. With ISO 800 you will get a bit more color in the milky way. We have gone as low as ISO 400 with good results. ISO 800 seems to be optimal on the K70 (which kicks in the Accelerator Chip at ISO ~630 which will help with noise). For a shutter speed, you can use two different approaches:
- 200 Rule - Take the focal length you are shooting at 200/focal length = the shutter speed in seconds. 200/18 = 11 seconds
- I like to use this website ---- Night Sky Photography Shutter Speed Calculator – With the K70, the crop factor = 1.5, megapixel =24, focal length = (your lens), pixel tolerance = (I use anything between 4 and 10, the smaller - the sharper the stars will be). This will provide 3 shutter speeds, I just take the largest value. So for 1.5; 24; 18; 4 provides 11.3 seconds or just 11 seconds.
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Focusing alternative methods.
- During the day - focus on something say 1/2 mile away, and tape the focus ring on the lens down. Make sure that you are in manual focusing.
- Illuminate something far away with a flashlight, or place the flashlight on something and walk a far distance away and focus the camera.