Starlight Xpress OAG sanity check

[david_taurus83]

I picked up one of these yesterday to replace my ZWO oag. Reason being, I have ordered an Esprit 100 and the required backspacing for the flattener is 55mm. With my Atik 460, SX FW and ZWO oag, the measurements were 13mm, 29mm and 16.5mm=58.5mm. Too much. So I've swapped for an SX OAG head (13mm) to achieve 13mm, 29mm and 13mm=55mm. Perfect. But for the life of me, I can't figure out how to focus a guide camera. Am I right in saying, with the main camera in focus at infinity, the distance from the prism to main sensor should equal the distance from prism to guide sensor? If so, working at 55mm spacing, the distance from centre of prism to main sensor is 44mm. However, the length of the prism stalk is 50mm. The prism is 8mm so the distance from top of stalk to centre of prism is 46mm. So with the main camera in focus, my guide sensor needs to be 2mm lower than the stalk to achieve the same focus? Tell me I'm missing something obvious please? I have tried with my Redcat this afternoon on some distant trees but the Redcat only has 4mm of focus travel (59mm backspacing).

[newbie alert]

You need to measure the distance from the prism to imaging sensor, and the same distance from the prism to the guide camera sensor, taking into account the backfocus distance ..

Slew to a large starfield and fine tune.. 

 

[david_taurus83]

18 minutes ago, newbie alert said:

You need to measure the distance from the prism to imaging sensor, and the same distance from the prism to the guide camera sensor, taking into account the backfocus distance ..

Slew to a large starfield and fine tune.. 

 

I have done. The distance from prism to main camera is 44mm. The distance from prism to top of stalk is 46mm. Its not possible to achieve focus with any guide camera. I'm really struggling to understand SX's design with this. Granted, I'm using an ASI120 but even with a Lodestar, how do you get the sensor lower than the stalk? I need another guide camera and have considered a Lodestar but I'm not spending over £400 if its not going to work either. 

[newbie alert]

18 hours ago, david_taurus83 said:

I have done. The distance from prism to main camera is 44mm. The distance from prism to top of stalk is 46mm. Its not possible to achieve focus with any guide camera. I'm really struggling to understand SX's design with this. Granted, I'm using an ASI120 but even with a Lodestar, how do you get the sensor lower than the stalk? I need another guide camera and have considered a Lodestar but I'm not spending over £400 if its not going to work either. 

Looks like you're measuring from the bottom of the prism and can't work not where the guide sensor will be,  does it slide inside or is there a c or cs type thread on the top.. lots of people use a sx pay and lodestar configuration,  so it must be possible 

[david_taurus83]

1 hour ago, newbie alert said:

Looks like you're measuring from the bottom of the prism and can't work not where the guide sensor will be,  does it slide inside or is there a c or cs type thread on the top.. lots of people use a sx pay and lodestar configuration,  so it must be possible 

It's 50mm from bottom of prism to top of stalk. The prism is 8mm so half that and the distance from centre of prism to top of stalk is 46mm. That is as close as a guide chip can get. To stay within the bounds of 55mm total backspacing for the Esprit flattener, the Atik 460 needs to be screwed directly to the filterwheel which when you add it up, makes it 44mm from Atik 460 to centre of prism! I can't move the Atik back say 10mm or so and do the same with the guide chip as that would put me at 65mm back spacing! 

I agree, people must use it successfully with a Lodestar etc but there's no way I can see it working at 55mm backspacing which seems a bizarre scenario seeing as 55mm is the de facto length for most flatteners/reducers. In fact, if I'm correct, the Trius cameras from SX have 17mm of spacing so add this to FW and OAG and total amount is 59mm? So not compatible with an Esprit it would seem? 

 

Edited November 13, 2021 by david_taurus83
Auto correct spelling faux pas

[newbie alert]

I see what you mean.. I'm pretty tight on my sx fw /oag/ sct/460 setup but thought most of that was with my oag

With my esprit 80 I have far more room but I'm not using a OAG..  I'm using a separate guidescope setup

Not so sure that there's much to gain from oag at 550fl..

[david_taurus83]

6 minutes ago, newbie alert said:

Not so sure that there's much to gain from oag at 550fl..

I like to take half hour subs!

[newbie alert]

If the guider is mounted solid it shouldn't matter. With a sct I can understand it with mirror flop etc.. 

[david_taurus83]

Believe me, I have tried. Tried fixed guidescopes (no rings) on top and on the focuser. Always get image drift. OAG just works and image doesn't budge.

[Laurin Dave]

It may be that its just not designed to work with the Atik which has 13 mm bf ..  SX cameras have 16.5 or thereabouts.. There is an 8mm adapter on the back of the Esprit100 flattener but I’m not sure that even with that removed there’s enough distance to get a custom adapter in whilst moving the camera back the 3.5 mm required to get the OAG focused..  I certainly couldn’t find a way to get my Sx46, SX usb fw and OAG on my Esprit100..  the newer SX filter wheel with integral OAG may work as it’s designed for 55mm bf with SX cameras .. 

[newbie alert]

2 hours ago, Laurin Dave said:

It may be that its just not designed to work with the Atik which has 13 mm bf ..  SX cameras have 16.5 or thereabouts.. There is an 8mm adapter on the back of the Esprit100 flattener but I’m not sure that even with that removed there’s enough distance to get a custom adapter in whilst moving the camera back the 3.5 mm required to get the OAG focused..  I certainly couldn’t find a way to get my Sx46, SX usb fw and OAG on my Esprit100..  the newer SX filter wheel with integral OAG may work as it’s designed for 55mm bf with SX cameras .. 

I'm sure it can be done as I do know people that use oag with bigger fracs but they use atik FW with 460/ 383 camera's with lodestar as a guidecam.. it's backfocus is 12.5mm to incorporate into that figure..

[david_taurus83]

4 minutes ago, newbie alert said:

I'm sure it can be done as I do know people that use oag with bigger fracs but they use atik FW with 460/ 383 camera's with lodestar as a guidecam.. it's backfocus is 12.5mm to incorporate into that figure..

The SX oag can only be used with the SX fw as it bolts directly to the faceplate. It's not screwed on. If others are using an Atik fw they will be using a different oag as well. 

[david_taurus83]

2 hours ago, Laurin Dave said:

It may be that its just not designed to work with the Atik which has 13 mm bf ..  SX cameras have 16.5 or thereabouts.. There is an 8mm adapter on the back of the Esprit100 flattener but I’m not sure that even with that removed there’s enough distance to get a custom adapter in whilst moving the camera back the 3.5 mm required to get the OAG focused..  I certainly couldn’t find a way to get my Sx46, SX usb fw and OAG on my Esprit100..  the newer SX filter wheel with integral OAG may work as it’s designed for 55mm bf with SX cameras .. 

It's a shame really as it's a quality piece of kit.

Someone is kindly lending me a slim TS OAG unit to try so will see if I can get that to work.

[newbie alert]

1 hour ago, david_taurus83 said:

The SX oag can only be used with the SX fw as it bolts directly to the faceplate. It's not screwed on. If others are using an Atik fw they will be using a different oag as well. 

Yes they are,presume a atik one