Nov 3

ISS – Zwo ASI 174MM vs. 224MC testing (at 4800mm focal length)

Currently I have been using my well tested ASI 224MC colour camera which I really love. To me International Space Station imaging is about colours as well as details, although I know it is not as beneficial as a monochrome camera for details. Mono cameras tend to deliver more finer details, no question about it. But I am emotionally attached to colour imaging and I rather give up some of the details for colours…

But I have been given a mono 174MM camera for testingย and this morning I finally had some clear period, a break in clouds – I do not want to complain too much, but October was bad. Super mega bad in terms of weather, I probably had a couple of days without cloud cover, rain and wind all the time. So I had to be patient and wait for my opportunity.

Before I show you the result from this morning, allow me to begin with some research. My good friend Philip Smith recommended me a FOV calculator website, so I did look up my current configuration with Jupiter and compared it with the 174MM setup. This site will show you, how large or small an object will look with certain setups.

This is what I found.

One thing was immediately clear, to achieve similar magnification with the 174MM camera, I had to apply a larger focal extender, in my case a TeleVue 4x powermate. It means I am using 4800mm of focal length and I had some worries about it. This is not something I have not tried before, but the result was somewhat questionable. In July 2018 I gave a try to a 4x powermate on my 10″ dobson, using my beloved 224MC camera.
I had two opportunities, first was disastrous and the second was somewhat better – see the result here. At that time I only had a laptop with usb 2.0 and I recorded so little amount of frames at that magnification that I gave up experimenting completely.

Things are different with 174MM camera for a couple of reasons.

  • pixel size difference – 174MM has bigger pixels
  • sensor size difference –ย 174MM has larger sensor


Taking all this into consideration I applied almost exactly the same camera settings today for exposure (0.777ms) and gain (241) compared to 224MC camera settings. I think the video frames were a bit underexposed, I was maybe a bit too conservative with the approach. Also sunrise was only 30 minutes away and the sky was quite bright already at 6:25am.





This is the best I could get out of the frames. Single frames did not look too good after post processing (maybe because they were too underexposed), so instead I stacked 13 consecutive frames which improved the overall result. Next time I definitely have to apply more gain for sure and I also hope I will not be as close to sunrise as today. Based on my previous experience brighter sky does not help at all.



Skywatcher 250/1200 Flextube dobson telescope
Zwo ASI174MM camera
TeleVue 4x powermate
Baader IR-Pass filter (685 Nm)




  1. Abdulkareem

    Hi Szabolcs,
    I have similar camera, just received, I plan to use it with TV powermate 4x, on 5 inches refractor 950mm focal length, so the focal length will be 3800 @ f30 ( this what I have now).
    For imaging ISS. Kindly, what is your suggested set up for ASI174mm on term of exposure, gain, and offset.
    The pass will occur one hour after the sun set.
    The mount I have is equipped with automatic satellite tracking.
    First trial will be today.
    Appreciate your response and feedback.

  2. Szabolcs Nagy

    Hey Abdul,

    Thanks for writing! Hope you’ll be successful today ๐Ÿ˜‰
    In general I would suggest keeping your expo time at 1ms or below, probably the best to start with 1ms. Gain is something you need to experiment, I can’t give you any value because it is totally down to your setup. Point your scope at one of the brightest stars and the star should be half dim (if that makes sense). I don’t know anything about offset, I only do expo and gain settings.

    What mount do you have if you don’t mind me asking, which tracks satellites? Looking forward to your first images!
    Good luck! ๐Ÿ˜‰

  3. Abdulkareem

    Hi Szabolcs,
    Thanks for advice.
    The mount, which I will track with is 10micron HPS. I already done some satellite tracking, after uploading the latest TLE, so, no itโ€™s the time to image.
    As you know, making trials and practicing setting for ISS will consume time, due to limited pass intervals and/or weather condition.
    Thanks again
    Very much appreciated

  4. Szabolcs Nagy

    Hey Abdul,

    I do know what you mean, figuring out the correct camera settings is a painfully long process and I do believe most of the people loosimg interest there. Consistency and enthusiasm is key…

    Micron is commonly used mount for ISS and satellite imaging, I do not have one sadly but would love to give that a try one day or just even seeing one in action.

    I wish you best of luck and please promise to share your better photos on my website’s Guest photos! ๐Ÿ˜‰

  5. Martin

    Hi I have the 250 flextube goto. Really struggling with focus with the asi 224 . Any tips please chap .

  6. Szabolcs Nagy

    Hi Martin! I always use Bahtinov mask for precise focusing, it just works and is very simple to use. I would highly recommend you to buy one for the 250 tube, it worth every penny ๐Ÿ™‚ Good luck and let me know how it went. Kind regards, Szabi

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