Hi,
I am having trouble with the optical assembly on the high resolution microscope. The print was reasonable successful and I have bought the correct spec lens on AliExpress (50mm focal length, 12.7mm diameter achromatic lens) It looks correct too.
The objective lens is from amazon and is listed as:
Objectives Lens,40X 185 Achromatic Microscope Objective, 40X Biological Microscope Magnifier Achromatic Objectives Lens,20mm Standard RMS Thread
185 achromatic microscope objective, if I understand the literature correctly, corresponds to a 45mm parfocal distance. I cannot find a single lens on amazon which actually directly specifies the parfocal distance - but I believe it is pretty normal. The actual length of the lens is 37.8mm, which suggests it is not a 35mm lens.
The focus point, however, is almost touching the lens. If I remove the optical assembly and place a prepared slide right next to the objective, I can get a reasonably clear image - but there is insufficient adjustment in the microscope to compensate for this.
The lens was very cheap - just £8.44 including delivery, so its possible its simply the wrong thing… or was mis-sold. But I would be interested in feedback from the helpful experts here before I try out a different (more expensive) lens.
I have attached a photo of the optical mount without the objectvie lens attached, showing the mounted PMMA lens.
BTW: I am using all standard parts, pi camera v2.1 (with lens removed).
If 185 means tube length it is possible that objective is infinity corrected. Did you print optics_picamera_2_rms_f50d13 or optics_picamera_2_rms_infinity_f50d13 for optic module?
Is the 37.5mm the whole length, including the threaded part?
A x40 objective will have a very short working distance, less than 1mm as you have found. This means that the lens from the shoulder of the thread to the lens top is just a little bit shorter than the par-focal length. That comes to a bit over 44mm for a 45mm par-focal objective. It sounds as though you may have a 35mm par-focal lens. The lens is described as ‘achromatic’ but not ‘plan’. It will have reasonable performance in the centre of the field of view, but will be very poor around the edges. There are low cost objectives from dedicated microscopy suppliers (Amscope, Motic etc) that specify better what they are, and are not very expensive. This is an important part of the image forming system.
If you have kept the lens from the Pi camera, I would strongly recommend printing the lens spacer and assembling the low cost optics. The resolution/magnification is similar to a 20x objective, and the image quality is remarkable. It will give you a reliable way to test out using the microscope, before moving on to the RMS optics.
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Hi,
First of all thankyou again for the super helpful replies.
I managed to find some more time to work on the microscope and confirmed I did have a 35mm parfocal length objective. I ordered the correct part and managed to get the microscope fully working, albeit with a bit of dust on the CCD.
Its amazing how well it works and I look forward to exploring science with my son!
I will also asseble the low-cost optics out of curiosity.
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Great that it is working
We have seen a few cases of this recently. I believe the key number here is the 185mm. Which I believe specifies the conjugate distance. This is from the object itself, to where the image is formed.
The image is formed 10mm inside the tube. So the conjugate distances is
tube_length - 10 + parfocal_distance
For a standard 160mm tube length, and a 45mm parfocal distance the conjugate distance is 195mm.
We should probably make a 185 version (i.e.) a 35mm parfocal length version of the optics module. But we should add markings so that it is easy to identify. We should also update the lens description to be clear that the standard RMS optics module is for a conjugate distance of 195mm.
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It is probably worth putting the conjugate distance on the specification. I was purchasing a lot of kit from Amazon, because of the shorter delivery time and on there, it seems that few places will quote parfocal distance. I did eventually find one, but it was only specified in an image.
I expect making a different optics module does indeed sound like a great idea, as I think the 35mm kit does appear to be cheaper!
I have a merge request in to add the information:
For the modified STL, it’s a little more work as I don’t have a 35mm objective to test with. There is always a bit of a trade off between supporting every option and making sure every supported option is well tested. We may need to find a good way to support options but mark them clearly as untested/experimental.
That is awesome, thankyou.
I am more than happy to print and test with my spare 35mm objective!
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I have just changed the openscad parameter for the parfocal length. This does make the Optics module somewhat taller so you may need to stand the lens tool on something so that it has the height to reach the lens gripper.
rms_optics_module_35mm-parfocal.stl (473.7 KB)
Thanks, I will try printing that tomorrow.
Hi, I managed to get that print working without much problem.
Here is an image of a budgie feather…
That image would suggest that better focus was possible… maybe its just the sample. I can try a few other things if needed. I am away for a couple of weeks now, so it might be a problem!
Good to see it is working. It is always hard to tell what is a design issue on our part, vs. sample depth, vs. objective quality.
Do you at least get move out of focus either side of this position? This solves the initial issue of focus. Is the focus/imaging significantly different on your other objective?
If you have any prepared microscope slides that would be great. But we probably should follow up here by buying a 35mm parfocal length objective here and putting it through its paces.
