Takahashi Adapters for FSQ106

I have been going crazy trying to understand what adapters to use with the FSQ. It only has about an inch (29mm) of focuser travel, so you always have to use different adapters for each camera combination you attach. What a pain.

Of course, Takahashi does not mark their adapters with any identification to let you know what the part is. In fact, they don’t even mark the telescope OTA with the model! I think I have an EDX-III, but it is hard to know.

Their System Charts are a joke. They are not up to date – the chart in the EDX-III manual is apparently not correct. Is any of this on line at Takahashi or Texas Nautical? Of course not. You work with a pdf that is a copy of a copy. There are at least 3, perhaps more, numbering schemes for each adapter. You cannot look up parts by their System Chart number (i.e., #30). You cannot typically look up parts by their Part Number (i.e., TKP 0100) in the Texas Nautical web site. Some of the System Chart parts have associated Part Numbers, most do not. Often these numbers are incorrect.

When you find parts online, there are no specifications for them – thread sizes, lengths, nothing. Thus, you cannot figure out back focus requirements.

Threads are listed by the diameter of the hole or shaft. Of course, you can’t exactly measure the thread size because they interleave when assembled. For example, the male 90mm thread measures 92mm; the threads make it oversize from the 90mm spec. The measured values are slightly off from the spec. Generally you can sort of match them up, but there apparently are 54mm and 56mm parts. I think the thread count per inch is always the same – nowhere do I see any specification of the thread pitch.

So, some measurements:

• The FSQ106 back focus distance is 178 mm. This measured from the camera chip to the back of the OTA casting (not the back of the focuser). See the back focus calculations below.
• After the focuser housing there is either a 4mm ring (for the EDX3) or a 10mm ring (for the EDX-IV). Apparently I have an EDX-III. Oddly, Fred at Texas Nautical uses System Charts for the EDX-IV and EDX-V for the EDX-III, since the EDX-III charts are obsolete or wrong.
• The STF8300 has a 57mm back focus distance.
• The ST-i has a 13mm back focus distance.
• The focuser has a maximum travel distance of 29 mm, roughly 1 inch. You want the focuser to be positioned about 1/2 way in, or 15mm.
• The 4mm ring at the end of the focuser has a width of 92mm.
• A T-thread is 41mm diameter.

Some Part numbers:

STF8300 Adapters

The diagram shows the adapters I use to connect the STF8300 (with OAG and filter wheel). This works.

Back Focus Calculation

Using the above measurements,

178 = 57 – 15 + 4 + 28 + 27.7 + 15 + 57 = 173.7  approx right

Planned Adapters for the ST-i

Based on the diagram below, I have ordered a part from PreciseParts.com to correctly space the ST-i. I need to use the FSQ/ST-i to guide while the STF8300 is in for repair.

Back Focus Calculation

Assume ST-i goes in an inch = 25 mm. ST-i has backfocus of 13mm. The extender from Precise Parts would be 50mm, giving a calculation of

178 mm = 57 + 4 + 28 + 27.7 + 50 + 25 – 12 = 178.7