WO GT81 + FF + DSLR - tube extender needed?

[Zadoq]

We've recently acquired a William Optics GT 81 for DSO imaging. The weather hasn't been great recently, so we haven't been able to give it a good shot.

However, we managed to set it up between clouds but then came across a small issue.

While setting up and going through the polar alignment process, we were unable to properly focus on Polaris.

Our initial setup was just using the GT81 and a 40mm Plossl eyepiece. It definitely felt that extending the drawtube was the right thing to do as although seriously blurred, the image was slowly getting less blurred.

As we ran out of drawtube extension, we added a star diagonal between the OTA and the eyepiece. The focus was again slowly improving as we extended the drawtube, but then we ran out of drawtube. I then unscrewed the star diagonal and then gently let it slide out of the OTA to see if we could reach focus if it was possible to extend the drawtube a little bit further.

It was indeed possible and focus was reach about 1 or 2cm further.

We used a 1.25" star diagonal that was fitted in the 2" to 1.25" adapter, because that's the only type of star diagonal we have. Was the issue that the 1.25" is shorter than the 2" and if we had used a 2" star diagonal, we would have reached focus?

Just before packing up, we decided to try to see if we would reach focus using our 1100D DSLR. The setup was GT81 + Hotech SCA field flattener + T-ring adaptor + T-ring + Canon EOS 1100D

We managed to reach focus, but the drawtube was quite extended: 7.3cm out of 8cm for Polaris

Our worry here is that we might run out of drawtube extension for other objects.

Should we be using a 2" tube extender between the OTA and the field flattener to avoid runnind out of drawtube length while focusing?

I guess we could find out the hard way and order one if / when we get that issue, but when the sky finally clears, we'd prefer to be imaging rather than waiting for a part, so that's why we're asking here

Thanks

Nico & Carly

[lensman57]

Hi,

If you have managed to reach focus with the flattner and the DSLR Polaris then you will reach focus for other DSO objects. As for observing  another EP of about 30mm or less and you should come to focus. I have the same situation while using a flattner as against a reducer/flattner with my ZS71.

Regards,

A.G

[Zadoq]

Thanks.

We have indeed not had any focusing problem when the camera is attached

Regards

Nico & Carly

[brantuk]

When you take out the diagonal you have to add a fair bit to attain the focusing length. Take a measurement from the objective to the other end of the scope's focal length to find out roughly where you need to be, and if an extender is needed. Some flateners/reducers have the ideal spacing for a DSLR. Hth

[goldblade1983]

We've recently acquired a William Optics GT 81 for DSO imaging. The weather hasn't been great recently, so we haven't been able to give it a good shot.

However, we managed to set it up between clouds but then came across a small issue.

While setting up and going through the polar alignment process, we were unable to properly focus on Polaris.

Our initial setup was just using the GT81 and a 40mm Plossl eyepiece. It definitely felt that extending the drawtube was the right thing to do as although seriously blurred, the image was slowly getting less blurred.

As we ran out of drawtube extension, we added a star diagonal between the OTA and the eyepiece. The focus was again slowly improving as we extended the drawtube, but then we ran out of drawtube. I then unscrewed the star diagonal and then gently let it slide out of the OTA to see if we could reach focus if it was possible to extend the drawtube a little bit further.

It was indeed possible and focus was reach about 1 or 2cm further.

We used a 1.25" star diagonal that was fitted in the 2" to 1.25" adapter, because that's the only type of star diagonal we have. Was the issue that the 1.25" is shorter than the 2" and if we had used a 2" star diagonal, we would have reached focus?

Just before packing up, we decided to try to see if we would reach focus using our 1100D DSLR. The setup was GT81 + Hotech SCA field flattener + T-ring adaptor + T-ring + Canon EOS 1100D

We managed to reach focus, but the drawtube was quite extended: 7.3cm out of 8cm for Polaris

Our worry here is that we might run out of drawtube extension for other objects.

Should we be using a 2" tube extender between the OTA and the field flattener to avoid runnind out of drawtube length while focusing?

I guess we could find out the hard way and order one if / when we get that issue, but when the sky finally clears, we'd prefer to be imaging rather than waiting for a part, so that's why we're asking here

Thanks

Nico & Carly

Hi zadoq I'm having exactly same issue with my gt81. Did you ever figure it why the focusing for your eyepiece is so far along the draw tube?

Cheers

[Zadoq]

Hi zadoq I'm having exactly same issue with my gt81. Did you ever figure it why the focusing for your eyepiece is so far along the draw tube?

Cheers

Hi Goldblade,

No, we never solved the issue regarding using an eyepiece because we're using the telescope only when it's attached to a camera, so it now works well enough for us.

Sorry I couldn't help more...

Nico