Celestron 4SE focusing distance, anyone?

[jarl72]

Hello people,

I bought a Celestron 4SE mak a couple of weeks ago, and I'm already entertaining the idea of creating an accessory I "absolutely need" (of course!).

I'd like to be able to mount my Olympus E-PM1 to the rear port of the scope, but apparently there are not that many alternatives to do so. The thread on the rear port of the 4SE is supposed to be 1.375", 24 TPI (BTW: anybody knows if this is the same as the ETX 60/90 rear port thread?), not a standard SCT. From what I have gathered the way to mount the camera is to use a mak-to-SCT adapter, then an SCT to T adapter, and finally a T to micro 4/3rds adapter. The alternative is to use a Celestron 93635-A mak-to-t adapter,and then the T to micro 4/3rd adapter. Either way the solution is sub-optimal.

My T-to-m43, however, is made out of two pieces. The T threaded piece is held in place by means of three hex set screws (see picture), meaning that it should be possible to replace this part with a mak-threaded tube, and this would create my Mak-to-m43... which is what I'm after.

The question is: how long does this need threaded tube need to be? Or, in other words: how far behind the rear of the telescope should the camera sensor be located?

Thanks!

[jarl72]

Uhm... where did the answers go? 

[jarl72]

Je je... the answers went to the other thread (on another astronomy forum). Too many tabs open at the same time

For information for anyone who lands here: the diameter of the small ring in the picture happens to be very close to the diameter of the mak-to-SCT adapter... so I mounted it to the larger piece using the three set screws already on it (and one shim to take up some of the excessive play), and this allowed me to mount my E-PM1 to the 4SE. There are lots of wrinkles to be ironed out, but as a proof of concept it works The distance I got (~60mm) looks suspiciously short, though... I'll have to measure it again...

(Moderator: if you don't mind, could you please remove my previous post?)

[Big Si]

The length of an "official" celestron 4se widget is 57mm without the T-adapter.

Si

[jarl72]

I apologize for my ignorance but... could you please explain what a "widget" means in this context (I'm not being obnoxious... I honestly don't know )? Is the 57mm the distance where "official" Celestron devices (e.g. a camera's imaging plane) ends up being located when mounted on the scope?

After posting my question I realized that the focuser had a long travel from the point I was testing: it was close to one of the ends, and I had assumed the measurement I got was close to the maximum distance possible. In fact it's the opposite, and by adjusting the focuser I was able to get the moon projected in a piece of paper some ~50cm behind the OTA. The magnification changes a lot, of course, but at first sight it looks like it's equally sharp at any distance from the ~6cm I measured originally to the ~50cm I measured afterward. Intuitively, though, there's probably an "optimal" distance. Is that the 57mm you mentioned?

[Big Si]

The widget in question is this:

http://www.firstlightoptics.com/adaptors/celestron-maksutov-t-adapter.html

The length is top to bottom, excluding the floating ring that engages the threads on the back of the 4SE.  The distance to the focal plane will depend on the choice of camera. My Nikon (DX sensor) measures 100mm from the camera focal plane to the top of the attached adapter, the other Nikon (FX sensor) is 115mm.

Si

[jarl72]

LOL... I had positively no idea what you were talking about.

Thanks a lot for the measurements!