Tak FSQ-106ED: Metal back distance important?

[Mikkel Kroman]

Hi all

On Takahashi's website it says that the metal back distance is 178mm. Do I need to "hit" 178mm precisely? Or is it okay that it is not 178mm as long as I am able to reach focus with my ZEO EAF?

My stars seem elongated so that is why I ask. I use a ZWO ASI1600MM Pro.

Kind regards

Mikkel

[Mikkel Kroman]

Anyone?

[Adam J]

15 minutes ago, Mikkel Kroman said:

Anyone?

You say your stars are elongated, can you post a image, preferably unprocessed single sub.  In so far are i can tell that 178mm is simply the available back focus in the system, so just the amount of equipment that you can place infront of the sensor while still being able to achieve focus without needing to rake the focus tube in too far. As the scope is a flat field refactor it should not matter what the back focus is. 

Now if you are using the 0.73x reducer with the scope instead of using it a native focal length you will need to place the sensor 72.2mm behind the reducer to achieve a flat field. Also you would need to compensate for filter thickness. 

Does that help?

Adam

 

 

[Mikkel Kroman]

28 minutes ago, Adam J said:

You say your stars are elongated, can you post a image, preferably unprocessed single sub.  In so far are i can tell that 178mm is simply the available back focus in the system, so just the amount of equipment that you can place infront of the sensor while still being able to achieve focus without needing to rake the focus tube in too far. As the scope is a flat field refactor it should not matter what the back focus is. 

Now if you are using the 0.73x reducer with the scope instead of using it a native focal length you will need to place the sensor 72.2mm behind the reducer to achieve a flat field. Also you would need to compensate for filter thickness. 

Does that help?

Adam

 

 

Thank you. That was very helpful! I will try and upload an unprocessed sub.

[ollypenrice]

Can you confirm that you're using the reducer? If not there is nothing to do other than focus.

Olly

[Stuart1971]

2 hours ago, Mikkel Kroman said:

Anyone?

As @ollypenrice said, if you are not using a reducer, then all you need to do is fix your camera on the back and focus, and ignore anything else, the point at which it focuses is the point  at which it focuses, ignore any backfocus numbers you have seen they are just a for a guide as to where you sensor will be in relation to the back of the scope once focus is achieved, but this will vary…

Now if you are using a reducer, then critical backfocus will be needed between the reducer and your camera sensor, that’s a different story….👍🏻

[Mikkel Kroman]

19 hours ago, ollypenrice said:

Can you confirm that you're using the reducer? If not there is nothing to do other than focus.

Olly

I am not using a reducer

[ollypenrice]

23 minutes ago, Mikkel Kroman said:

I am not using a reducer

In that case there is simply nothing to do other than find focus. The 106 is a Petzval, meaning the flattening lens is fixed in the rear of the main tube. The need to respect a precise distance between flattener lens and chip arises when the flattener lens is fitted into the moving draw tube.

Unfortunately, Takahashi seem to be unable to produce well-collimated Q-series telescopes reliably. Mine was very good but I have seen several bad ones and had messages from a significant number of frustrated owners. One SGL member has been told that the instruments were not designed for use with modern small pixel sizes. If this is so, Takahashi should state it clearly.

However, your elongation may be from tilt in the chip. This is very common. You can find a discussion of how to test and fix it here:

I hope you get it sorted out.

Olly