SCT, FR, OAG & DSLR focusing

[1CM69]

Hi

very new to guiding and using an OAG on my CPC9.25 SCT and need some help.

A few weeks back I managed to capture 10min subs of IC434 without any real issue. I had my CPC9.25 mounted on a wedge, did an EQ North Align, then Polar Align > Align Mount routine. I slewed to a target and set about getting both my guide camera, Lodestar X2 & my DSLR both into focus on my attached Celestron 93648 OAG.

My setup as follows:

SCT > OAG > DSLR via Baader 2958550 Protective T-Ring

The complete OAG thickness calculated from the manual is 66.8mm, comprising (SCT Adapter 25.3mm, OAG Body 29mm & Male M42 Camera Adapter 12.5mm)

I can't seem to find info on optical thickness of the Baader T-Ring but can only assume that when attached to my DSLR, gives the norm 55mm to sensor. If this is the case then I am already at 121.8mm.

To get focus in the guide cam I had to add a 1.25" nose piece extension and the original end of the guide cam is just visible above the top of the guide cam holder.

 

Now skip forward to last night, I thought I'd try getting wider FOV images of the same target, no problem or so I thought, I have a Celestron f6.3 94175 reducer, I'll just install that directly onto the back of my SCT and attach everything else exactly as above, behind it.

Well, getting my DSLR into focus was no big issue, just had to turn the focus knob a bit, the usual when using the FR but my issue was with my guide cam, the stars were huge doughnuts.

Through a process of elimination I figured that I had to move the guide cam in nearer to the prism, so I had to remove the nose piece extension and push the camera all the way in to the holder as well as winding the OAG guide cam focuser all the way down. 

Doing this I managed to get sharper results from the guide cam but these stars were no longer round, they were very oblique circles, pointed at the narrow ends.

After lots more fiddling and swearing  I came to the conclusion that I could not solve this issue on site so gave up and removed the reducer.

Later on I realised that it may be that I needed to adjust the position of the prism i.e. slide it in more to the light beam. As it happens I did not have a suitable allen key with me to try this.

My question/s is/are:

am I correct in thinking that I was possibly picking up stars right on the edge of the FR in the guide cam, the edge where all the aberration is?

just wondering why it was necessary that I had to reduce the distance to light source of the guide cam but the DSLR distance was fine staying put & only needed refocusing?

 

Thanks in advance

[Davey-T]

I get banana shaped stars I'm my SCT OAG but it doesn't bother PHD, it still guides OK.

Dave

[1CM69]

4 minutes ago, Davey-T said:

I get banana shaped stars I'm my SCT OAG but it doesn't bother PHD, it still guides OK.

Dave

Hi Dave,

is that only when using an FR with the OAG or all the time?

I have nice round stars without the FR attached, albeit very few.

Kirk

[Davey-T]

I've got the F/R fitted all the time, can't remember what it was like without it.

Dave

[1CM69]

16 minutes ago, Davey-T said:

I've got the F/R fitted all the time, can't remember what it was like without it.

Dave

OK, brilliant thanks Dave. 

Major headache averted. ?

[Jammy]

I get decent shape stars through my OAG with a similar setup.  I've found though that any stars near the edge of that FR start to look out of shape.  I think it's to do with the quality of the FR more than anything else.

I did note that you say the overall distance from the FR to your camera sensor is 121mm.  I've read that the optimum distance from the FR to the camera sensor should be between 105mm and 110mm.  I did ask FLO and they advised 105mm.  By being further away stars start to exaggerate comma.  You can be as close as 80mm but the focal reduction becomes less the closer you get.

However, for the Edge HD the distance is 120-something millimetres.  (126mm I think I read).

Also, I now have the OAG fixed as close to the DSLR body as possible to make focusing both cameras possible without having the stalk of the OAG pushed in too far where it can start to interfere with the light path to the scope.

When I get change I'll post you a photo of my setup.  It may or may not help you

[1CM69]

13 minutes ago, Jammy said:

I get decent shape stars through my OAG with a similar setup.  I've found though that any stars near the edge of that FR start to look out of shape.  I think it's to do with the quality of the FR more than anything else.

I did note that you say the overall distance from the FR to your camera sensor is 121mm.  I've read that the optimum distance from the FR to the camera sensor should be between 105mm and 110mm.  I did ask FLO and they advised 105mm.  By being further away stars start to exaggerate comma.  You can be as close as 80mm but the focal reduction becomes less the closer you get.

However, for the Edge HD the distance is 120-something millimetres.  (126mm I think I read).

Also, I now have the OAG fixed as close to the DSLR body as possible to make focusing both cameras possible without having the stalk of the OAG pushed in too far where it can start to interfere with the light path to the scope.

When I get change I'll post you a photo of my setup.  It may or may not help you

OK, thanks

[MarsG76]

On 11/30/2017 at 23:23, 1CM69 said:

am I correct in thinking that I was possibly picking up stars right on the edge of the FR in the guide cam, the edge where all the aberration is?

yes... I find that if I get oblong stars and tilt the prism to pick stars closer to the light path than the roundness in the OAG does come back... there is a pin type of scre at the front of the OAG that tilts the prism whe you turn it.

 

[MarsG76]

On 11/30/2017 at 23:23, 1CM69 said:

why it was necessary that I had to reduce the distance to light source of the guide cam but the DSLR distance was fine staying put & only needed refocusing?

The light patch is shorter or more compressed, so when you move the mirror into a position where the focal point is on your DSLR sensor, the shortened light path still applies in the OAG, so you need to move the guide cam closer into the OAG holder where the focal point is...