Dimming Laser


Barry Frazier <iteslap85@...>
 

I tried to dim the laser that is too bright for seeing the fringes in the camera’s live view (Nikon D300). This is a coated mirror. I used a LED dimmer switch between the laser and the 5W USB power adapter. The laser appeared to dim, but there was no change in intensity in the camera’s LCD screen.

Is there a way to dim the laser or reduce its power so I can see the fringes in the camera. It may be the Nikon D300 is too old because it doesn’t reflect the changes to the aperture or speed in the LCD. I also tried a neutral density filter, but that caused dark stripes to appear in the LCD image.

Thanks,

Barry


George Roberts (Boston)
 

The D300 is excellent.  Minimum ISO is 200.  Set it to 200.  Max shutter is 1/8000s.  Use that.  Aperture must be wide open.  Do not stop down the aperture.

ND filter sounds great.

I'm not sure what kind of dimmer you used but I've had good success with a simple potentiometer which is a variable resistor.  I use 1K which gives you a huge range of brightness and good control over the brightness.  This is the one I use:
https://www.digikey.com/en/products/detail/tt-electronics-bi/P160KN-0QD15B1K/2408886

You shouldn't need to dim though.  At ISO 200 and 1/8000s it will over expose but only a little and it will be okay.  Yes you will get some print-through where you can see the fringes in the final waveform in DFTFringe but just make sure you have around 100 fringes and turn up the gaussian enough and those print throughs will go away.  yes it's better to not overexpose but you will be fine.


Dale Eason
 

A led dimmer is probably a pulse modulator.  That is it produces a pulse wave to turn the led on a percentage of the time.  That will not work and is likely the reason you see strips when using the ND filter.  Like George said that camera is capable of a good exposure.  You need to put it into manual exposure mode however and then adjust shutter and ISO while making sure that the aperture is wide open.  
 
The correct way to dim a laser pointer is to put a variable resistor in series with the DC power supply.
Dale


Arjan
 

Barry,

I have made an adjustable current source to supply my laser, but this is a bare diode without any circuitry.
As Dale says, you should refrain from pulsing unles you use comparably long shutter times.
It is best to try to dim the laser in continuous operation, by current source or series resistor, and shorten shutter speed to 1msec or less.

Good luck!

On 16/10/2022 16:08, Barry Frazier wrote:
I tried to dim the laser that is too bright for seeing the fringes in the camera’s live view (Nikon D300). This is a coated mirror. I used a LED dimmer switch between the laser and the 5W USB power adapter. The laser appeared to dim, but there was no change in intensity in the camera’s LCD screen.

Is there a way to dim the laser or reduce its power so I can see the fringes in the camera. It may be the Nikon D300 is too old because it doesn’t reflect the changes to the aperture or speed in the LCD. I also tried a neutral density filter, but that caused dark stripes to appear in the LCD image.


Bruce Griffiths
 

The dark stripes are likely caused by multiple reflections within the ND filter. These would likely be less of an issue with an AR coated absorptive ND filter. With an uncoated absorptive ND=1.5 (31x) filter the resultant interferogram distortion will be less than a few thousandths of a wave and below one thousandth of a wave with a good AR coating on both sides of the filter.
With a reflective ND filter the perturbation will be much larger.

The presence of dark stripes also indicates your laser is likely operating in single mode and has a much longer coherence length than the filter thickness.

Bruce

On 17/10/2022 03:08 Barry Frazier <iteslap85@...> wrote:


I tried to dim the laser that is too bright for seeing the fringes in the camera’s live view (Nikon D300). This is a coated mirror. I used a LED dimmer switch between the laser and the 5W USB power adapter. The laser appeared to dim, but there was no change in intensity in the camera’s LCD screen.

Is there a way to dim the laser or reduce its power so I can see the fringes in the camera. It may be the Nikon D300 is too old because it doesn’t reflect the changes to the aperture or speed in the LCD. I also tried a neutral density filter, but that caused dark stripes to appear in the LCD image.

Thanks,

Barry


Bruce Griffiths
 

Thats another possibility if the laser is pulsed when using rolling shutter.
Operating a laser diode just above threshold can result in relatively unstable output and a high rate of mode hopping.

Bruce

On 17/10/2022 06:12 Dale Eason <doeason@...> wrote:


A led dimmer is probably a pulse modulator.  That is it produces a pulse wave to turn the led on a percentage of the time.  That will not work and is likely the reason you see strips when using the ND filter.  Like George said that camera is capable of a good exposure.  You need to put it into manual exposure mode however and then adjust shutter and ISO while making sure that the aperture is wide open.  
 
The correct way to dim a laser pointer is to put a variable resistor in series with the DC power supply.
Dale


Mike Chibnik
 

Hi Barry
I have one of George’s bath kits and the laser has a potentiometer in series with the power lead to the laser. I use its control knob to vary the current to the laser to set its intensity output to the right level. An added benefit of this control is that it can be set to minimum output so possible damaging on off power transients are eliminated. I would not use pulse modulation to control brightness like you would with simple LEDs.
Mike


George Roberts (Boston)
 

I don't usually sell the potentiometer as I hate getting the soldering iron in there and well, it's even more work on top of all the stuff I'm doing already.  However, I do offer it in kit form for people who are handy with a soldering iron.  It's just not on the website.  Email me if you really need it but I don't think you will need it with the D300 if you use ISO 200 and 1/8000th shutter speed.


Franz
 

Maybe the interferogram covers only a small area in the frame, because the focal length of the (photographic) lens is short, or maybe because the mirror has a high f-number?
In that case I can imagine that the interferogram is too bright for ISO 200 and 1/8000th.
Maybe try a longer focal length lens, or zoom in!
Also, if the interferogram is small in the frame, this could cause the camera's metering system to overexpose the interferogram ( in live view) if the camera is set to weighted average or something like that.
You could try spot metering, or whatever the D300's live view mode offers.


Barry Frazier <iteslap85@...>
 

Thanks everyone for your advice. I’ve ordered the potentiometer from George’s link. I’ll see how that works.

The 8 inch ƒ8 mirror I’m testing was really good using Ronchi and Foucault tests. This igram I managed to get shows a lot of slope errors (yellow) that I guess means the figure is transitioning too steeply and that light is being thrown out of the airy disk, but overall it’s within an 1/8 wave?



Regards,

Barry


Franz
 

Barry,
it is within 1/4 wave (+-1/8). The figure looks smooth, but not good enough to get the slope error to below 1 arcsecond everywhere. But not bad at all.
It looks a bit overcorrected. This could just be a thermal issue.


Barry Frazier <iteslap85@...>
 

I don’t believe it’s possible to determine what the slope error is using Ronchi or Foucault. This is why I’m turning to the Bath interferometer to correct these errors in the figure. These errors I believe are detracting from seeing more planetary detail.

On Oct 19, 2022, at 18:40, Franz <franz.hagemann@...> wrote:

not good enough to get the slope error to below 1 arcsecond everywhere


Dale Eason
 

Slope errors usually show up as roughness in Ronchi and Foucault.  In the past I have said it is hard to evaluate surface smoothness with the Bath and DFTFringe because of the high spatial frequency noise almost always present.  So the problem is then to try and separate that noise from true surface detail.  That depends on the skill of the tester and the quality of his equipment and test environment.

So now you have to decide.

I would expect to see some sort of detail corresponding to that slope error in the actual foucault or ronchi image.  If you know how to take good ronchi or Foucault images or how to visually inspect with those instruments and don't see any artifact then I would doubt the IF analysis.  


Dale Eason
 

On Wed, Oct 19, 2022 at 10:44 PM, Barry Frazier <iteslap85@...> wrote:
I don’t believe it’s possible to determine what the slope error is using Ronchi or Foucault
You may not know the exact magnitude but the rule of thumb is if trying to make the ideal mirror with good surface smoothness then you must not see a hit of shadows of small features.  If have seen the Bath show 1/60 wave features on the surface that just show up on a good Foucault test.  Once those go away then the odds are you have a truly smooth surface.