25mm is really short, just over an inch. Is your camera screwed directly into the front cell adapter?
The filter drawer may have to be the only thing between the camera a scope. Celestron says there is a clear filter there already that should be removed when using other filters.
"Observing programs that require a filter usually require
more than one filter, so the filters are mounted in a filter
rotating wheel. A small motor turns the wheel so that
different filters can be inserted into the light path without
manual intervention. All too often though, the filter wheel
is considerably larger than the camera body itself, and
when placed at the focus of a RASA, the filter wheel
blocks a considerable amount of light. There is currently
no easy solution to the filter-wheel dilemma, although
custom solutions do exist. For example, filters can be
mounted in a sliding drawer and manually inserted into
the light path."
Obviously it can be done. They note that an optimum camera had around 15mm back focus.
They also say that the RASA creates a 43.3mm circle of light, so a sensor diagonal of this dimension is optimum.
"For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s
back focus, also known as its flange focal distance, is the distance between the camera sensor
and where the camera body mechanically connects to a camera lens or telescope. Therefore,
cameras with sensors deeply recessed within their body having more than 29 mm of back focus
cannot be used with the RASA 8 as the sensor cannot be placed at the proper location."
Back focus with included camera adapter 25mm (.98")
Back focus from top of threaded collar 29mm (1.14")
The ASI1600 has 6.5mm of back focus, so you theoretically have 22.5mm for the filter drawer and adapters or a M42 to M42 extension tube. ZWO filter drawer is 21mm.
Before you get the filter drawer this or this might do the trick if it's M42 threads. The ZWO extension is in stock. I think this will thread onto the collar and the camera onto it.
If you want to be sure to nail-it, I would get this one, also in stock.
Edited by audioengr, 16 September 2020 - 04:44 PM.