ZWO ASI1600MM-cool + Samyang 135mm F2.0 – daytime test

As the skies are cloudy and my new telescope mount is still not fixed onto the telescope pier – because I still need to work on the adapter for it, I thought I would try the ZWO ASI1600MM-cool with the Samyang 135mm F2.0 in daylight to answer a few further questions I had.

The setup for this test is as shown below. Now the flange focal distance of an EOS lens is 44.0mm. The thickness of the imaging train below is: Xagyl 1.25″ motorized filter wheel 19mm, Geoptik EOS-T2 adapter 19mm, ASI1600MM-cool 6.5mm, which works out to be 44.5mm, which means because of that extra 1/2 mm it should not be able to achieve infinity focus. But I wanted to test this out to confirm that this is true.

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Question 1: Will using the Geoptik EOS-T2 adapter, Xagyl 1.25″ motorized filter wheel with the ZWO ASI1600MM-cool at its shortest back focus distance of 6.5mm (with the black T2 mount removed) allow for infinity focus to be achieved?

Answer: No. It does not. See image below.

Question 2:  Does using the Samyang 135mm F2.0 with 1.25″ filters result in vignetting when used on the ASI1600MM-cool at its shortest back focus distance?

Answer: No. It does not. See image below.

Image below was taken to answer both of these questions. As you can see, I am unable to achieve infinity focus but not by much. Also, you can see that the corners do not show significant vignetting, which is great as it means that I should be able to use my Samyang 135mm F2.0 lens with my 1.25″ filters and not worry about vignetting.

Image taken with Samyang 135mm F2.0 wide open with 1.25" Astronomik CLS-CCD filter in Xagyl filter wheel
Image taken with Samyang 135mm F2.0 wide open with 1.25″ Astronomik CLS-CCD filter in Xagyl filter wheel

What did I find out from this test?

If I wish to use this combination of lens, filter wheel and camera for imaging, I will need to get a thinner Canon EOS-T2 adapter than the Geoptik EOS-T2 adapter in order to achieve infinity focus. But it would also mean that the changing of filters can be automated and a whole lot easier.

Back focus of 6.5mm with detachable adapter removed - leaves male T2 threads
Back focus of 6.5mm with detachable adapter removed – leaves male T2 threads

Another important finding from today:

One thing to be aware of is that the red mounting plate with its Male T2 threads can come unscrewed as there are no screws fixing it in place. So if some force is needed to remove the camera from certain accessories while mounted via the male threads, as happened with me when taking it off the Xagyl filter wheel, then the mounting plate can come loose.

The mounting plate loosened
The mounting plate loosened – when using the black T2 mounting adapter, this is unlikely to come off
Mounting plate removed. The rubber seal which keeps the sensor chamber air tight.
Mounting plate removed. The rubber seal which keeps the sensor chamber air tight.
Replacement desiccant pellets with foam stickers
Replacement desiccant pellets with foam stickers

If this does happen, do be aware that the desiccant pellets can become dislodged. So I would advise caution when unscrewing the camera, for if the mounting plate does come loose, you may need to check that the desiccant pellets have not moved from their storage area before tightening the mounting plate back. Otherwise you could damage the desiccant pellets, which is what happened to me – so 2 of the desiccant pellets had to be replaced.

Desiccant pellets placed in its allocated slots
Desiccant pellets placed in its allocated slots

I have not had any such issue with my ZWO ASI178MM-cool though. I have brought this issue to Sam’s (of ZWO) attention, and hopefully now that I have posted about it here, people can be aware of the potential problem and take the necessary precautions to avoid damaging the desiccant pellets.

Boon