Resolved SBIG 16803 - image train rigidity

Discussion in 'STX and STXL' started by Niall MacNeill, Jul 29, 2019.

  1. Niall MacNeill

    Niall MacNeill Standard User

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    Continued...

    New_Fan_ON_HIP_5119205_07_20201x1_20_11C300_000secsRedLight00000002_FWHM.jpg
    Star Roundness Trial 1: New Fan ON....2nd repeat, different area of the sky

    New Fan ONNGC 612105 07 20201x1-20.17C300.000secsRedLight00000003.jpg New_Fan_ONNGC_612105_07_20201x1_20_17C300_000secsRedLight00000003_eccentricity.jpg New_Fan_ONNGC_612105_07_20201x1_20_17C300_000secsRedLight00000003_FWHM.jpg


    Star Roundness Trial 2: New Fan OFF

    New Fan OFFHIP 5119205 07 20201x1-20.11C300.000secsRedLight00000001.jpg New_Fan_OFFHIP_5119205_07_20201x1_20_11C300_000secsRedLight00000001_eccentricity.jpg New_Fan_OFFHIP_5119205_07_20201x1_20_11C300_000secsRedLight00000001_FWHM.jpg

    Star Roundness Trial 2: New Fan OFF......repeat, a different area of the sky

    New Fan ON NGC 612105 07 20201x1-20.17C300.000secsRedLight00000001.jpg

    New_Fan_OFF_NGC_612105_07_20201x1_20_17C300_000secsRedLight00000002_eccentricity.jpg New_Fan_OFF_NGC_612105_07_20201x1_20_17C300_000secsRedLight00000002_FWHM.jpg



    Discussion:

    The Eccentricity figures are fairly well backed up by a visual observation of the stars. There is some evidence of field curvature and this is to be expected given that Celestron only guarantee star shape for an image circle diameter of 40mm, where the SBIG chip has a 52mm diagonal. With the new Fan ON, the average Eccentricity is in the range 0.51-0.58. This is a large improvement on the Eccentricity figures reported wth the old Fan, which were in the range 0.65 - 0.85 and were at the worse end of the scale in recent trials, suggesting a degradation of performance.
    The stars at the centre of the images where the Fan is ON show a small but perceptible amount of elongation.

    The Eccentricity for the Fan OFF trials were in the range 0.40 - 0.47. The stars at the centre of the images where the Fan is OFF are virtually perfectly round.

    Conclusion

    The previous fan arrangement , whether the fan itself and/or the vibration insulating foam, resulted in severe vibration causing substantial elongation of the stars in images captured with the SBIG 16803. Fan OFF trials showed this elongation was virtually completely eliminated.

    The new Fan and/ or insulating foam arrangement gives a substantially improved performance but it is by no means perfect and there is a demonstrable difference to the fan OFF images in terms of star roundness.
     
  2. Niall MacNeill

    Niall MacNeill Standard User

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    SBIG 16803 Fan images, before and after replacement

    Previous Arrangement:

    The insulating foam pieces do not cover the entirety of the side of the fan.

    IMG_2679.JPG

    The foam on one of the edges was quite badly damaged and looks like the adhesive side had been ripped back on itself during insertion

    IMG_2680.JPG


    New Arrangement:

    Foam covers the entire side of the

    IMG_2684.JPG

    Foam carefully inserted and snug in the holder.

    IMG_2688.JPG
     

    Attached Files:

  3. Niall MacNeill

    Niall MacNeill Standard User

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    A comparison of magnified images of the stars at the centre of the FOV for Fan On vs Fan OFF

    Fan ON left, Fan OFF right

    Screen Shot 2020-05-09 at 3.18.24 pm.png Screen Shot 2020-05-09 at 3.27.40 pm.png

    These two images clearly show that star elongation due to fan vibration is still occurring with the new fan and replacement vibration dampening foam, although the amplitude of the vibration has clearly been significantly reduced. The Fan OFF images have almost perfectly round stars.

    These figures and those derived previously suggest that with the old fan/ vibration dampening foam configuration, that the amplitude of the vibration was generally such that the stars were being elongated by 60% - 100%, dependent on the FWHM and that the more recent trials were on the worse end of this range.

    For the new fan/ vibration dampening foam configuration, the amplitude of the vibration is generally such that the stars were being elongated by 15% - 23%.

    With the fan off the 9%-13%. The centre of the image is more like 5%-10%

    Bearing in mind these are average figures and include an amount of field curvature from the corners of the images, and that the Eccentricities at the centre of the images are less, this broadly suggests the new Fan is responsible for a star elongation of 5-10%.

    Discussion
    The amount of elongation seen in the images when the fan is ON, gives a perceptible elongation to the stars and of course will reduce the resolution of any nebulae or other non-stellar features in the image.

    Is there a a specification for performance in this area? Is this performance comparable with that from other SBIG 16803 cameras in use around the world? What about for SBIG 16803 cameras hooked up to C14 Edge HD OTAs? Is there any interaction, which was mentioned as a possibility by Doug, between the camera and the OTA?

    How can I move forward from here to achieve an acceptable performance in terms of star roundness?

    Do I need to look to water cooling as an option where the fan which is cooling the circulating water is physically remote from the camera and particularly the sensor?
     
  4. Doug

    Doug Staff Member

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    A small amount of vibration is inevitable when using a fan, but usually it is insignificant. A very large factor is the rigidity of what it is attached to. If there's slop in the mount, or anything in the telescope structure that has a low mechanical resonance frequency, then the effect of a tiny vibration can be amplified immensely.

    I had a situation with my previous telescope where I had a counterweight attached at the front of the instrument (required to shift the center of gravity so the instrument would not contact the dome). I had horrible image problems, which were in fact induced by the fan, but that wasn't the core issue. I realized this because the problem appeared when I added the counterweight. The solution was to change the mounting of the counterweight to make it much stiffer, and the problem completely disappeared. It turned out I had inadvertently made a mechanical resonator!

    Yes if all else fails water cooling is an option. Then you can turn off the fan completely. Of course there are other issues, like making sure the hoses don't get kinked, making sure you have anti-freeze if you experience cold conditions, and cleaning out the system periodically to prevent organic junk from clogging the tubes.
     
  5. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug,

    Would you say what I am seeing is abnormally bad? The Mount is a Paramount MX+, so pretty much a top end Mount. I am not aware of any issues with its performance. I doubt the Mount is at issue. The Camera is bolted to the adaptor which is relatively short and screwed to the baffle lock nut. I doubt there is a more rigid imaging train. That leaves the OTA.

    Are there option for an ultra low vibration fan to replace the existing one? What do you recommend for the water cooled option?

    Thanks,
    Niall
     
  6. Doug

    Doug Staff Member

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    I don't have a specific recommendation on the fan. We do try to select fans that have lower vibration characteristics, but every fan in existence produces some level of vibration. From tests we have done, sleeve bearing fans tend to vibrate a bit less, but they have much shorter lifetimes before the bearing fails. Also any contaminants on the blades can affect the weight distribution resulting in more vibration.

    As for water cooling, it is pretty simple. Please see page 12 of the user manual. That might be your best option.
     
  7. Niall MacNeill

    Niall MacNeill Standard User

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    Guys, I still do not have a solution to producing images without star elongation due to the vibration of the fan. I understood you were going to send me a rig for water cooling to allow me to achieve the necessary cooling without having the fan mounted to the camera directly. I have been dealing with my supplier who tells me that he still doesn't know when this rig is going to be shipped. I have sent a PM to Colin and received no response.
    This forum post is marked resolved, which as far as I am concerned it is not.
    Can I please have some feedback as to where things are at, as after 1 year I still have not produced a satisfactory image from this camera.
    If you are unable to help me with the water cooled option, could I at least ask that you support me with the identification of a suitable ultra low vibration fan to replace the standard one.
     
  8. Colin Haig

    Colin Haig Staff Member

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    Niall, again, due to COVID-19, obtaining components is taking longer. I am expecting an answer today on when we will have parts; we have provided cost information to your dealer.
    Without you mirror locked down, any vibration is going to be amplified.
     
  9. Doug

    Doug Staff Member

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    Things are definitely improving as lockdowns are easing, but it's taking a while for suppliers to clear out their backlogs, and courier companies are still impacted.

    Supposedly the water pump is out for delivery this morning. Hopefully in a few days we'll have everything we need for the water cooling kit. Thanks for your patience.
     
  10. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks to you both for keeping me apprised. I understand the difficult situation with the pandemic. I am also going to buy a 12V ultra low noise/ vibration 60mm fan by Noctua as a fall back. It is only 45AUD. It only has half the volume throughput of the existing fan but should work OK during the cooler months. You may recall Andrei Ioda said he solved his problem with such a fan. The liquid cooling as a supplementary system will allow me to achieve temperature in the summer months.
    There are a number of forum posts referring to deep sky imaging with C14s where the focus is done at the primary mirror, mainly because of the lack of back focus space. There is some discussion that mirror flop can occasionally cause the loss of sub where the auto-guiding can't keep up with the shift. Clearly that means the mirror can't be locked. I haven't come across anything that suggests fan vibration is an issue when the mirror remains unlocked. Besides that, my trials with the mirror locked, showed no benefit to star elongation at the centra of the FOV when the fan was operating. All I did get was some field curvature (concentrically elongated stars in the corner of the image) as the mirror locks clearly put an amount of pressure on the mirror inducing some tilt and a loss of collimation.
     
  11. Colin Haig

    Colin Haig Staff Member

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    Pump arrived today; hopefully the rest of the kit comes together.
     
  12. Niall MacNeill

    Niall MacNeill Standard User

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    Gentlemen, I bought a 12V Noctua fan of the same dimensions as the one that came with the camera (60mm x 25mm). It is reputedly very low noise and vibration and only draws 1.5W compared to the 5W for the existing model. Of course I don't expect the cooling to be as good, but hopefully enough to get me going at least in the winter. However the new fan has a number of different 3 pin plugs and three wires the go to the fan. Unfortunately, they are different to the connector for the fan that comes with the camera. Can you give me any advice on connecting the fan to the camera?
     
  13. Doug

    Doug Staff Member

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    If there is a red and a black wire, those go where the current fan's red and black wires go. Third wire should not be connected.

    You will probably have to cut and splice the wires - use solder and appropriate insulation of course.

    Camera modifications are at your own risk.
     
  14. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug. So I can just cut the wire from the original fan with the connector that attaches to the camera, then attach the red and black wires to the red and black wires on the new fan?
     
  15. Doug

    Doug Staff Member

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    Yes.

    Again, please be aware that any damage to the camera resulting from your modifications is not covered by warranty.
     
  16. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug,

    I can assure you that I would rather not have to do this. But I don't have a camera that I can use at the moment and I think you'll agree the risk is low. I am quietly confident this will allow me to produce acceptable images in terms of star elongation or hopefully the lack of it, at least in the winter months. It is just a lower powered fan of the same dimensions. If the power draw is any measure of the potential for vibration, then the result will be satisfactory. The fan design is, from what I understand, amongst the best out there for low noise and vibration. I am hopeful that this option will get me by until a liquid cooled option is ready. :)
     

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