I have a whole new level of respect for imagers using SCTs and OAGs!!

[oymd]

Ok, so here's my problem:

I setup my imaging train as such:

C8--> reducer--> 50mm Celestron T2 extension--> OVL OAG (15mm width)---> 10mm T2 extension---> 10mm T2 extension---> ZWO 11mm Spacer ---> 294MC Pro sensor recessed at 6.5mm

My train is very very close to 105mm

I get good stars, and I platesolved and Astrometry shows that my pixel scale is 0.745 arcsec/pixel, so my focal length appears to be bang on 1280mm.

The problem is that I CANNOT push my 290MM mini far enough inside to get into focus.

I can see very unfocused stars in PHD2 looping, but the guide camera is bottomed out against the OAG's shaft that brings in light from the prism.

The only solutions I have are:

1- Moving the OAG farther away from the sensor: NOT POSSIBLE, AS THE CELESTRON T2 CONNECTION IS FIXED AT 50MM

2- Adding a 5mm T2 extension on the camera side, but this led to poor stars and my FL changed when I platesolved!

I ended up swapping one of the 10mm extensions with a 15mm extension, and I could finally focus my guide camera on the OAG.

Am I missing something?

Mu current setup puts the OAG at about 43mm from the sensor!! That is just too far for the OAG, but anything nearer, and I cannot focus the guide camera.#

Currently the guide camera is bottomed out completely against the OAG shaft in my helical focuser.

I am using the OVL OAG, and attached to it is the ZWO non rotating helical focuser.

 

P.S. Another crazy solution would be to chop off 5mm from the OAG's shaft that is holding the prism!!??

Or find a SCT T2 adaptor that fits to the reducer, but is smaller than 50mm??

 

Thanks

[vlaiv]

1 hour ago, oymd said:

Or find a SCT T2 adaptor that fits to the reducer, but is smaller than 50mm??

https://www.astroshop.eu/other-adapters/omegon-sct-adapter-ultrashort-t2-male-2-/p,33228

You now have 50mm - OAG - 10 mm, why not simply change it for 30mm - OAG -30 mm combination (or something similar)

[Padraic M]

It sounds strange that you can't reach focus with that setup. Can you double check your measurements?

Let's take this as the distance from the prism centre line to the imaging sensor:
OVL OAG (13mm/2 = 6.5)---> 10mm T2 extension---> 10mm T2 extension---> ZWO 11mm Spacer ---> 294MC Pro sensor recessed at 6.5mm = 44mm.

The distance from the prism centre line to the guide cam must be the same to achieve focus. I don't have schematics for the OVL OAG, but the ZWO OAG has a 35mm stalk. I also can't find the size of the OVL prism so I'm assuming 10x10mm. So the equivalent measurement is:
OAG stalk length 35mm - half the prism depth 5mm + ASI290mm Mini sensor inset distance 8.5mm = 38.5mm.

This should give you 5.5mm to spare. One obvious question - you don't happen to have the ASI290mm 1.25" extension fitted? Worth asking! You could unscrew the protective window, that would save a few millimeters but would be a risk to the sensor. I wouldn't chop anything off the stalk!!!

Reference pictures here:

[newbie alert]

Sounds odd as Im using a SCT and 290 and my focus point is much further out, mind you my spacing is at 113.30 from the first of the threads and plate solved at 1280mm 

[oymd]

14 hours ago, Padraic M said:

It sounds strange that you can't reach focus with that setup. Can you double check your measurements?

Let's take this as the distance from the prism centre line to the imaging sensor:
OVL OAG (13mm/2 = 6.5)---> 10mm T2 extension---> 10mm T2 extension---> ZWO 11mm Spacer ---> 294MC Pro sensor recessed at 6.5mm = 44mm.

The distance from the prism centre line to the guide cam must be the same to achieve focus. I don't have schematics for the OVL OAG, but the ZWO OAG has a 35mm stalk. I also can't find the size of the OVL prism so I'm assuming 10x10mm. So the equivalent measurement is:
OAG stalk length 35mm - half the prism depth 5mm + ASI290mm Mini sensor inset distance 8.5mm = 38.5mm.

This should give you 5.5mm to spare. One obvious question - you don't happen to have the ASI290mm 1.25" extension fitted? Worth asking! You could unscrew the protective window, that would save a few millimeters but would be a risk to the sensor. I wouldn't chop anything off the stalk!!!

Reference pictures here:

The OVL's OAG stalk is 45mm. That might be the problem?

Its this one:

https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

[oymd]

12 hours ago, newbie alert said:

Sounds odd as Im using a SCT and 290 and my focus point is much further out, mind you my spacing is at 113.30 from the first of the threads and plate solved at 1280mm 

113mm!!

And you plate solved at 1280mm??

Guess those 0.63 reducers are not very consistent??

But, here's a question though....I think I read somewhere that the FL of SCTs changes with change of focus?

Maybe the variation is due to movement of the primary mirror?

[oymd]

17 hours ago, Padraic M said:

It sounds strange that you can't reach focus with that setup. Can you double check your measurements?

Let's take this as the distance from the prism centre line to the imaging sensor:
OVL OAG (13mm/2 = 6.5)---> 10mm T2 extension---> 10mm T2 extension---> ZWO 11mm Spacer ---> 294MC Pro sensor recessed at 6.5mm = 44mm.

The distance from the prism centre line to the guide cam must be the same to achieve focus. I don't have schematics for the OVL OAG, but the ZWO OAG has a 35mm stalk. I also can't find the size of the OVL prism so I'm assuming 10x10mm. So the equivalent measurement is:
OAG stalk length 35mm - half the prism depth 5mm + ASI290mm Mini sensor inset distance 8.5mm = 38.5mm.

This should give you 5.5mm to spare. One obvious question - you don't happen to have the ASI290mm 1.25" extension fitted? Worth asking! You could unscrew the protective window, that would save a few millimeters but would be a risk to the sensor. I wouldn't chop anything off the stalk!!!

Reference pictures here:

 

15 hours ago, newbie alert said:

Sounds odd as Im using a SCT and 290 and my focus point is much further out, mind you my spacing is at 113.30 from the first of the threads and plate solved at 1280mm 

Here's a paragraph from the review of the OAG on FLO's website:

The Turret is 45mm long, which might be the problem?

The 90° prism was of high quality and no aberrations were visible when examining a daylight scene through it. The prism is held firmly in its socket at the end of the adjustable turret by a single tiny grub screw. There is no tilt adjustment for the prism but the 45mm long turret allows for plenty of adjustment in and out of the casing and this is secured in position by a single knurled bolt on its short side. I would have preferred to have had a second bolt working at right angles to the first to stop any chance of the turret ‘rocking’ in its channel but this is just in my personal wish-list as none of the commercially available OAGs have such a second bolt. I made up a tiny shim using kitchen foil to act as a buffer on one side of the turret and this ensured that there was no unwanted movement of the turret in any direction. The light port through the turret is 5.6mm in diameter which suited my 4.9mm x 3.7mm sized guide camera well but would cause vignetting with larger sensors – not a disaster but a shame as it could rob larger sensors of an increased guide star choice.

 

 

[newbie alert]

7 hours ago, oymd said:

113mm!!

And you plate solved at 1280mm??

Guess those 0.63 reducers are not very consistent??

But, here's a question though....I think I read somewhere that the FL of SCTs changes with change of focus?

Maybe the variation is due to movement of the primary mirror?

 

7 hours ago, oymd said:

113mm!!

And you plate solved at 1280mm??

Guess those 0.63 reducers are not very consistent??

But, here's a question though....I think I read somewhere that the FL of SCTs changes with change of focus?

Maybe the variation is due to movement of the primary mirror?

Yeah the focal length will change with focus, slightly but I'm sure you can't plate solve an image with unfocused stars, not with astap anyway.. it will also slightly change and I mean slightly on position of where the scope is pointing... Higher up on the zenith and gravity has more of an effect. . This is mirror flop,which SCT are well known for..

There's a few different Celestron reducers, the made in Japan version, the Chinese one plus a few other brands, I'm sure they differ from brands and also batch to batch..

[newbie alert]

5 hours ago, oymd said:

 

Here's a paragraph from the review of the OAG on FLO's website:

The Turret is 45mm long, which might be the problem?

The 90° prism was of high quality and no aberrations were visible when examining a daylight scene through it. The prism is held firmly in its socket at the end of the adjustable turret by a single tiny grub screw. There is no tilt adjustment for the prism but the 45mm long turret allows for plenty of adjustment in and out of the casing and this is secured in position by a single knurled bolt on its short side. I would have preferred to have had a second bolt working at right angles to the first to stop any chance of the turret ‘rocking’ in its channel but this is just in my personal wish-list as none of the commercially available OAGs have such a second bolt. I made up a tiny shim using kitchen foil to act as a buffer on one side of the turret and this ensured that there was no unwanted movement of the turret in any direction. The light port through the turret is 5.6mm in diameter which suited my 4.9mm x 3.7mm sized guide camera well but would cause vignetting with larger sensors – not a disaster but a shame as it could rob larger sensors of an increased guide star choice.

 

 

Which oag is this?

[oymd]

5 hours ago, newbie alert said:

Which oag is this?

OVL OAG:

 

https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

[Frogliza]

On 20/04/2022 at 17:24, oymd said:

OVL OAG:

 

https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

Hey oymd, this is an old thread but I wanted to ask if you get good stars to the edges of your FOV? I'm using a ZWO ASI294mc and NexStar 6SE and having a similar problem to your original post.

Did 105mm of backfocus work? 

[oymd]

10 hours ago, Frogliza said:

Hey oymd, this is an old thread but I wanted to ask if you get good stars to the edges of your FOV? I'm using a ZWO ASI294mc and NexStar 6SE and having a similar problem to your original post.

Did 105mm of backfocus work? 

Hi

105 worked, but overall results were not great. Edges are not good, but was told that’s the best regular non edge SCTs can offer. 
I abandoned the endeavour all together and got back to my refractors.