[INSTRM-193] SM independent exposure control Created: 19/Sep/17  Updated: 14/Dec/17  Resolved: 14/Dec/17

Status: Done
Project: Instrument control development
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Story Priority: Normal
Reporter: fmadec Assignee: Unassigned
Resolution: Done Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Relates
relates to INSTRM-195 Design routine to expose on blue-red ... Done
Sprint: 2017-10A
Reviewers: fmadec

 Description   

If I remember well, some years ago, they was a request from IPMU to have a separate control of the two shutters of a Spectrograph Module.

I just wanted to clarify one point on the hardware configurations: the red shutters is placed just before the first dichroic, so when the Red shutter is closed, none of CUs see light (Blue, Red and NIR).

If the Blue shutter is open while Red shutter is closed, Blue CU will only scattered light due to white coating on the red shutter (done for the back illumination)

If the Blue shutter is close and Red Shutter is open, only Red and NIR CU will see light.

This has an impact on the exposure control, I don't what is the intend on that but I think it was important to point out this hardware limitation

 Comments   
Comment by naoyuki.tamura [ 01/Nov/17 ]

According to my e-mail records, we got an inquiry from Subaru about whether an exposure time can be set differently from arm to arm. But due to the shutter configuration, this is technically not possible. The reason and the limited flexibility are the following:

  • The open and close of the red shutter affects the exposures on ALL the cameras.
  • Meanwhile, the blue shutter can open and close the light path to the blue camera without any impact to the red and NIR arms, so the blue camera exposure time can be determined arbitrarily as long as the red shutter is open.
  • While the red shutter is open, (at least technically speaking and as far as I understand) the NIR camera exposure time can be determined by the timing of reset & read of the H4RG detector, so it can be shorter than the open shutter time.

At the meeting we had with Subaru in Aug 2016, we explained this situation, and they at least understood that the exposure time cannot be chosen arbitrarily from camera to camera due to the hardware (i.e. the shutters) configuration, so we shouldn't have to worry about impossible implementation.

Regarding the remaining flexibility:

  • The blue camera exposure time can be determined flexibly by the blue shutter as long as the red shutter is open. This means that the blue camera exposure time can be the same or shorter than the ones on the red and NIR cameras. This is reverse in fact to the usual demand where a longer exposure time is often desired at shorter/bluer wavebands until the noise is sky-limited as much as possible. Nevertheless there might be such a use case (perhaps not from the PFS survey but from open-use observation programs) needing independent control of blue camera exposure time (no matter how much shorter than red and NIR), so we should pursue implementing the independent control of blue shutter, given that it is not too hard (I believe it is not).
  • A use case requiring a shorter exposure time on the NIR camera may be expected more naturally. So I believe we should try implementing this functionality.
Comment by fmadec [ 14/Nov/17 ]

That's a very good summary Naoyuki. Thanks.
And I agree with that.

Comment by fmadec [ 14/Dec/17 ]

we agreed on that

Generated at Sat Feb 10 16:22:29 JST 2024 using Jira 8.3.4#803005-sha1:1f96e09b3c60279a408a2ae47be3c745f571388b.