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CMOS newbie questions (moving from DLSR)


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I'm taking delivery of my ZWO ASI294MC today and I have questions:

  1. Back focus... what is it?  I'm guessing its the required distance needed from the back of the telescope to achieve focus with the camera?  ZWO say most telescopes require 55mm, but is that a rough guide or does it have to be exact?  Will my FF/FR on my 80ED eat into that back focus?
  2. Can people recommend what I should do the test the camera is working correctly?
  3. How do I determine what length on sub I should use for flat frames?  I previously just used AV mode on my DSLR.
Edited by scitmon
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I'll kick off with 

No1:   Your definition looks right.  Remember it is focus from an object at infinity ie.  stars, planet Moon etc    Don't set everything up to be all beautifully focused on your neighbours apple tree !! 

          55mm comes from a typical backfocus  with a DSLR and T-mount . I think a lot of optical design came from the fact customers mostly owned DSLRS back in the day.  Then CCDs came along, now CMOS.   You will need to take the backfocus of your FF/FR and then start  planning a cunning use of spacers to recreate this value behind the FF/FR.

          Remember your camera (294) will have its own set distance between outer casing and chip surface...( usually a lot less than 55mm )  that needs taking away from your FF/FR back focus above.

eg. if the casing-chip distance is 6.5mm, and the FF/FR did have for example, 55mm, then you need to fashion  48.5mm.

         This whole procedure screams of precision and accuracy, but people get good results without going silly  and fussing over  hundredths of a mm. 

Get yourself a digital vernier.  I think really fast systems  ( F4 and less)  tend to be more critical of this dialled in distance.

Good luck with it all. It is worth the effort to get it right.

 

Sean.

 

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1 hour ago, scitmon said:
  1. Back focus... what is it?  I'm guessing its the required distance needed from the back of the telescope to achieve focus with the camera?  ZWO say most telescopes require 55mm, but is that a rough guide or does it have to be exact?  Will my FF/FR on my 80ED eat into that back focus?

The term is somewhat overloaded.

For a refractor and some reflectors, backfocus is how far behind your telescope the focal plane is. The more equipment you have behind your focuser, the closer in you have to move it to put your sensor at the focal plane. This is what people mean when they talk about "using up" backfocus or having enough room in the backfocus for some equipment.
If you're using a flattener/reducer then it will have its own backfocus distance which will be different (not necessarily smaller or bigger).
The important thing is just whether or not you're able to achieve focus.

Some SCTs focus in a slightly different way, rather than moving the sensor to the focal plane they move the primary mirror to move the focal plane on to the sensor. This means you're able to reach focus at a wide range of backfocus distances.
If you introduce a flattener/reducer (or if there's an internal one) then it will have an "optimal backfocus" listed. You need to put the sensor as close as possible to the distance to get the maximum performance out of the scope. This can mean a lot of playing around with extenders.

tl;dr For most scopes the focal length is fixed so you just need to make sure you can reach focus. For some SCTs you need to be really careful about where you put the sensor to get the best performance.

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I have the 294MC Pro, the first time I used it was with my 72ED and OVL flattener. I put on the required number of spacers (which come with the camera) to get the 55mm back focus and then attached it directly to the back of the flattener. I had no issues achieving focus, however I have had to add a nylon spacer (that also come with the camera) as the stars were ever so slightly elongated in the corners., apart from that no issues at all with both the 72ED and redcat. 

The 294MC Pro has a few quirks when it comes to calibration frames. Basically, don't use bias frames, use darks, flats and dark flats. Also try and make sure your flats (and corresponding dark flats) are over 3-4 seconds long as the camera shows a lot of inconsistencies in very short exposures. I use the APT flats aid for my flats with a target ADU of 25000, a double layer of white t-shirt and a white ipad screen and I don't have any issues. 

 

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With 294MC, as said above, the spacers came with the camera.

You need the 11mm screwed onto the front of the camera, then the 21mm and finally the 16.5mm. Screw this assembly directly onto the back of you flattener/reducer and you should be good to go. :D

If you want to do the maths: 11+21+16.5=48.5mm then add the 6.5mm from the front of the camera sensor to the front face of the camera casing to give you a total of 55mm. You also get another adaptor which screws into the 11mm spacer which will allow you to mount a 1.25" filter in there. It's an idea to use a UV/IR cut filter in here becasue the 294MC doesn't have a UV/IR cut filter built in and the stars can look a little bloated without one. 

As for testing the camera, connect it up to the scope and get out there! Failing that, you could start building your dark frame library. Just put the cover over the sensor (the camera doesn't need to be connected to the scope for this), connecting it to a laptop/ASIAir and cool to your required temp, I used -10° last season but I'm going to try -15°C for this coming season. Then set your gain (I stick with a gain of 120) and take about 20 frames for each exposure setting you're likely to use. 

The beauty of these types of camera over the DSLR is that you can build your darks library before you start and then just reuse the masters, unless you decide to change the gain or offset settings you're using. ;)

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22 minutes ago, Budgie1 said:

With 294MC, as said above, the spacers came with the camera.

You need the 11mm screwed onto the front of the camera, then the 21mm and finally the 16.5mm. Screw this assembly directly onto the back of you flattener/reducer and you should be good to go. :D

If you want to do the maths: 11+21+16.5=48.5mm then add the 6.5mm from the front of the camera sensor to the front face of the camera casing to give you a total of 55mm. You also get another adaptor which screws into the 11mm spacer which will allow you to mount a 1.25" filter in there. It's an idea to use a UV/IR cut filter in here becasue the 294MC doesn't have a UV/IR cut filter built in and the stars can look a little bloated without one. 

As for testing the camera, connect it up to the scope and get out there! Failing that, you could start building your dark frame library. Just put the cover over the sensor (the camera doesn't need to be connected to the scope for this), connecting it to a laptop/ASIAir and cool to your required temp, I used -10° last season but I'm going to try -15°C for this coming season. Then set your gain (I stick with a gain of 120) and take about 20 frames for each exposure setting you're likely to use. 

The beauty of these types of camera over the DSLR is that you can build your darks library before you start and then just reuse the masters, unless you decide to change the gain or offset settings you're using. ;)

Thanks thats sounds reassuring… on the other side of my ff/fr i’m using a t nose instead of screwing it directly onto the draw tube, will this introduce unwanted spacing or it it only measured from the ff/fr glass?  I screw my idas lp filter on the end of the nose too which should cut out the need for an additional IR filter?

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18 minutes ago, scitmon said:

Thanks thats sounds reassuring… on the other side of my ff/fr i’m using a t nose instead of screwing it directly onto the draw tube, will this introduce unwanted spacing or it it only measured from the ff/fr glass?  I screw my idas lp filter on the end of the nose too which should cut out the need for an additional IR filter?

Yep, the spacing is from the rear lens of the ff/fr, so that'll be fine. If you're already using the LP filter then you don;t need the UV/IR cut as well.

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