Always test your assumptions

[Ryderscope]

** EDIT: Before entering here please note that I included an update in post #6 below which confirmed my original calculations were correct. My response to David in post # 4 was therefore not correct. Still worth pondering though as to why my measured results do not match the theory. With that disclaimer, please read on :)

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As we know, it is important when designing adapter spacing to get the set back distance correct and to take into account the optical thickness of the filters that are in the imaging train. I would have thought by now, after many years of imaging, that I would have this process down pat having quite successfully designed many adapters which have worked well. However, this seems to not be the case. Let me explain.

I've just spent a couple of months and many late nights trying to dial in the correct spacing and tilt for the Astromechanics adapter that I'm using to pair a Canon EF200mm f2.8 USM lens to my ASI6200 MM Pro CMOS camera. See here for pictures of this configuration (https://www.astrobin.com/lhhbk2/B/). My filters are the Astronomik 50mm LRGBHaOIISII set which have an optical thickness of 1mm. In my (questionable) wisdom I simply added 1mm to the set back distance and adjusted the spacing in the Astromechanics adapter for an additional 1mm. I then spent several nights trying to work out why I couldn't get a flat field until last night in a moment of monumental epiphany I realised that I had this back the front and I should have been subtracting 1mm from the adapter spacing. A quick reconfiguration of the adapter and, voila!, a nice flat field emerges from the depths. I don't know how many times that I have correctly and quite successfully configured similar set ups without any problems, but not this time. I just goes to prove that we should always test our underlying assumptions when we are designing our set up and particularly when problem solving issues with our configuration.

For those that may be interested, here are details of what I am doing.

The image train is:
Canon EF200mm f2.8 prime lens -> Astromechanics adapter -> ZWO 2" EFW -> ASI6200 MM Pro.

The Canon EOS lens requires a set back distance of 44mm. The sums to work this out with my image train are:
Astromechanics adapter: 10.5mm; Optical thickness of filter: 1.0mm; Mechanical thickness of filter wheel: 20mm; Sensor set back spacing in ASI6200: 12.5mm.
The above adds up to 44.0mm and all is good.

Last night I managed to execute a successful test run on a widefield image of the Orion/M42/Horsehead region. Link here to images in my staging area on Astrobin (https://astrob.in/rb5wuh/0/). The image posted here is a singe Ha sub, 20 minute exposure, Gain 100. Only processing was a quick stretch, move black point and apply a high dynamic range process in Pixinsight. It is apparent that the exposure time is good and some shorter subs will be required to improve detail in the core of M42 (no surprises there!).

The lens has been stepped down to approximately f5.4 using a step down ring on the front of the lens. This helps to remove the diffraction spikes that come from the lens iris though there is obvious vignetting which I am sure will be dealt with in post processing and with flat frames. Pixel peepers will see some small evidence of star distortion around the edges of the frame but once again this can be dealt with in post processing or simply cropped out.

I am pleased with the way that the Astromechanics adapter is working as it provides direct access to aperture control and the lens focusing mechanism. I'm driving it all with SGP Pro which is working a treat. Link to Astromechanics web site here. (https://www.astromechanics.org/index.html)

So the thought for the day is to always test your assumptions.

In closing I do apologise for the fact that now I have got this working the clouds will probably roll back in again.

Clear skies,
Rodney

[AdamJL]

Great stuff, Rodney. Glad you got it all sorted out. Thank you for the write up as it will help me when I switch to mono this month.
It’s a wonderful adapter as well. Really well engineered.

[sunslayr]

That's interesting, just to clarify you originally you had

Adapter: 12.5mm +
Filter wheel: 20mm +
ASI6200: 12.5mm +
= 45mm - 1.00mm filter = 44mm

and that didn't work, but

Adapter: 10.5mm +
Filter wheel: 20mm +
ASI6200: 12.5mm +
= 43mm - 1.00mm filter = 42mm

did work? I'm a bit confused :lol:

[Ryderscope]

Very good. Enjoy the journey with your new set up Adam :)



Edit: A quick edit to this post after the event David to confirm that your sums were correct. I shall leave my response below for posterity :D

The default set back distance out of the box for the Astromechanics adapter is 11.5mm. This doesn’t make any allowance for the filter. As noted, I had incorrectly set this at 12.5mm rather than adjusting it down to 10.5mm.

The inclusion of a filter in the light path moves the focal point away from the OTA which, if not accounted for, will place the focal point behind the sensor (or further into the camera if you like). In other words, it adds optical distance into the equation which needs to be treated the same as a mechanical distance. Your example shows it subtracting which is incorrect. This is the correct sum:
Adapter: 10.5mm +
Filter wheel: 20mm +
ASI6200: 12.5mm +
= 43mm
+ 1.00mm filter = 44mm

Usually our goal is to find the correct length for an adapter that we need to place into our imaging train with a view to obtaining the correct set back distance. The information that is on hand as the starting point is the set back distance that we are trying to achieve, the mechanical distance of the existing components and the optical thickness of our filters. If we approach the problem this way we have:

Target set back distance = 44mm

Components in image train:
* Filter = 1.0 mm
* Filter wheel = 20.0 mm
* Camera set back = 12.5 mm
These components add up to 33.5mm. To work out the correct length for my adapter then I subtract this from my target set back distance thus:
44.0 mm - 33.5 mm = 10.5 mm

[Tilt]

Add me to the confused list.

Here I was thinking with no filter, the Canon back focus is 44mm, adding a 3mm filter which produces an optical thickness of 1mm will push the focus point back to 45mm. Therefore adding a 1mm spacer ring when using filters is correct? I'm confused with adjusting the adapter down from 11.5mm to 10.5mm, I figured you'd just add the 1mm spacer and be done?

[Ryderscope]

Ok, I must apologise. My original calculations were correct and the adapter size should be 12.5mm. My bad and sorry for the confusion :ashamed:. Your logic is perfectly correct which begs the question then as to why I had to wind back the adapter spacing to 10.5mm before I could get a flat field :question:.

To demonstrate the results, see attached images showing a section of the test subs taken and a copy of the output from the eccentricity measurement script in Pixinsight. Shown in the test images is a section of the test frame that had an open cluster that was useful to gauge the flatness across the field. The image in the left pane is with the spacing at 10.5mm and the image on the right pane is with the spacing at 12.5mm. There is a significant and obvious improvement on the image on the left.

Also shown in the attachment is the output from the FWHMeccentricity script in Pixinsight. A measurement of 0.45 is a typical guide of an acceptable level of eccentricity in an image. Anything below that is a bonus. Again, the right hand pane (12.5mm spacing) shows a classic radial pattern emanating away from the centre of the image. The eccentricity map on the left hand pane (10.5mm spacing) shows an acceptable amount of eccentricity across the field.

Why is it so then that the practical implementation does not match the theory? The only obvious answer is that the optical thickness for my filters does not match specifications and/or the actual set back distance for the Canon lens does not match specifications. I did measure the adapter width very carefully with digital calipers so I am confident that I had that correct.

Thanks for everyone's input so far and it is good to confirm my original calculations. I would appreciate further input if anyone has a good theory to offer as to why the reality does not match the theory in this case.

CS,
Rodney

[Tilt]

Canon must be giving us the run around if they say back focus is 44mm.

Or maybe :question:

As you mentioned, something else is going on. It could be the optical thickness is not exactly 1mm, the adaptor or bayonet on the lens itself are all contributing to spacing errors. Either one of them, or all of them by a tiny amount. The focus point might be razor thin? However as you've been doing, trial and error is in order and your findings are helpful.

[Joshua Bunn]

When the metal backfocus is quoted as 44mm, is it referring to where the image comes to focus behind the mounting surface of the lens OR is it referring to the position of the senor inside the camera body (44mm below the camera mounting surface?) If its the later, this could explain it, because there are filters inside a camera that will increase the length of the optical path, which may not be the case so much with your ASI camera. just some thoughts ...


Joshua

[Ryderscope]

This is a very good point that you are raising here Joshua. The physical back spacing on the Canon EOS series is definitely 44mm as measured from where the lens joins the camera body and the plimsoll line on the camera body (I've checked my 60Da and 80D to be sure!). However, it is correct that there are filters in this light path which means that the actual focal point is altered by the inclusion of the filters I do wonder about the Astronomik 50mm round filters as well as they are 1mm thick and have a specified optical thickness of 1mm as noted on the Astronomik web site. And in addition there is the possible variation in specification of the set back distance in the ASI6200 camera.

The good thing is that I have a configuration that is now working. Alas, the inevitable has happened and the Central West of NSW is now clouded out and raining <sigh>