STF CCD, FW8 and OAG 8300 Backfocus Validation

Do you have a microfocuser on the back of the EdgeHD ? Why not just try racking the focuser in or out a bit?

 

Starting at the top, yes, the backfocus requirement is 57.5 mm for STF-8300 + FW8-8300 + 2mm thick filters + OAG-8300 + STF Adapter Plate.
Do you have the focal reducer?

If the Celestron Autofocus thing you are referring to is this:
https://www.celestron.com/products/focus-motor
The Celestron Focus motor moves the primary mirror - which moves the position of the image focal plane - which changes the spot size and affects collimation.
The reason there are mirror locks is to prevent problems caused by the primary mirror shifting.

I meant whether you had a micro-focuser like this:
https://www.optecinc.com/astronomy/catalog/tcf/19740.htm

Baader Planetarium makes an excellent device called a Vari-Lock that allows for a significant range of adjustment, and they also have spacer rings in various thicknesses.
https://www.baader-planetarium.com/...20-29mm-with-spanner-tool-(t-2-part-25y).html
https://www.baader-planetarium.com/...29-46mm-with-spanner-tool-(t-2-part-25v).html
They also have short extension tubes:
https://www.baader-planetarium.com/en/baader-t-2--7.5-mm-extension-tube-(t-2-part-25c).html
https://www.baader-planetarium.com/en/baader-t-2--15-mm-extension-tube-(t-2-part-25a).html

Our friends at Innovation Foresight have a detailed article on the backfocus issue with the Celestron EdgeHD scopes:
https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/

You could send over a blob FITS light frame and I could take a look.

 

Thanks for the images.
Here's a few things, in no particular order.

Recommendations / Questions:
1. MaxIm Setup
In MaxIm, File... Settings. Site and Optics tab.
Set up the optical parameters of your telescope and the guider.
The guider has a 0.7x focal reducer, so if your scope has a focal length of 3910mm (guess), the guider focal length is 2737mm.
You'll need this correct for guiding and other purposes.
View... FITS Header shows the values for everything captured in the image. The optical parameters were all nonsense (0) so we know this isn't right.

2. Celestron focus driver
How is the Celestron 94155-A Focus Motor connected, and what exact software and versions are you using to drive the Celestron focuser?
https://www.celestron.com/products/focus-motor
a) Focus Motor -> USB cable -> Computer
Focuser_USB_ASCOM_Setup.exe
Celestron ASCOM Driver 1.5.0

b) Focus Motor -> Celestron Mount (CGX-L?) -> Hand Controller -> USB cable -> Computer
CPWI 2.3.5

In the FITS header, I see nonsense values for 2 of the three entries:
FOCUSPOS = 9656 /Focuser position in steps
FOCUSSSZ = -NAN(IND) /Focuser step size in microns
FOCUSTEM = -NAN(IND) /Focuser temperature in deg C

NAN = Not a number.
This could be an issue with the Celestron software, or something on our end with how we interpret it.
So, if you can let me know exactly what software and ASCOM driver, then maybe we can reproduce it.

This may have an impact on the autofocus routine in MaxIm.

3. Pick-off position
It would be a good idea to shoot a flat field image (exposure peak maybe 45000 or so) so we can see if the shadow of the pick off mirror is hitting the main camera. You can do this in twilight / with a white t-shirt over the scope if you don't have a handy way to do so.

The main camera image "blob" isn't a nice round donut. So either you were shooting through trees, or the pick off is in the way (or there is some other obstruction).
A simple check would be to look down the nose toward the sensor, and take a 10s image, and eyeball whether you think the pick off is occluding the sensor.
Also, the orientation of the OAG can matter - eg if the pick off is centred along the long edge of the sensor, it is less likely to block the main chip.

4. Jupiter Check ?
Jupiter is so darn bright - you gotta wear shades
Usually you need very short exposures - like 0.01 second, not 0.5 second.
I don't think this is Jupiter.
Plus, the low end of the exposure speed for the STF-8300 is 0.1 second (you might get lucky below this, but it's what we spec).

Right now J has an apparent diameter of about 44" (arcseconds). (It ranges from 30" to 50").

Your system resolution:
Main camera: 5.4um pixels (binned 1x1) / 3910mm focal length x 206 = 0.28 arcseconds per pixel.
So, the image size should be around 44/0.28 = 157 pixels.

Guider: 7.4um / 3910 x 0.7 x 206 = 0.28 arcseconds per pixel, so again, 157 pixels.

Measuring the two images,
Main blob = 72 pixels
Guider blob = 124 pixels

So whatever the cameras are seeing, that's not Jupiter. (Definitely not for the main, and I doubt it for the ST-I).
And whatever the ST-I is seeing, I think it is overexposed and so appears swollen and also isn't in tightest focus.
You can play with the Screen Stretch in the ST-I image (try 650, 2900) to see off-axis reflections and a donut, so it's not optimal.


We really need to get the main camera focused first, then move on to the ST-I.

5. Manual Focusing
Focus the main camera first.
Set it up to do continuous exposures, maybe 5-10 seconds long at first, binned 2x2 to start.
2x2 will reduce the download time, and 5 seconds should be enough to get some modest to faint stars to show up.
There should be small donuts all over the main camera field.
Drive the focus position until you see donuts, and they should converge to points or small circles.

You can then shorten the exposures, and continue to adjust the focus position.
Then when you have the smallest circles, switch to 1x1 binning, and continue the process.
Then record this focuser position.

You can also subframe - Expose tab - turn on Subrame, Use mouse, draw a box around a few stars to speed the process up.

Once you've found optimal focus, then switch to Camera 2, and slide the ST-I in/out of the OAG till it is focused nicely.


Let us know how it goes.

 

Tim, you're correct, not sure how I got the 7.4 / 2737 wrong. 0.557 arcsec per pixel is what I get as well for the guider.
Thanks for jumping in. Point remains that main camera has to be focused.

 

Nice chart. Thanks - that's probably the tidiest collection of info I've ever seen!

The OAG-8300 is blocking light to the main chip (unless something else is in the optical path from the light source).
You're right - looks like the OAG needs to be backed out of the field of the main chip.
The shadow is cast across much of the chip. (See orange arrow):

See Page 2 in the manual to see how to do the adjustment:
https://diffractionlimited.com/wp-content/uploads/2016/04/oag-8300_manual.pdf
The instructions mention where to position for various focal lengths - your setting will likely be different.

That black top edge that goes all the way across might be the semi-circular segment that holds the pickoff.
The StarChaser off-axis guiders don't have this segment - an improvement:
https://diffractionlimited.com/product/sbig-starchaser/
This might not improve with the OAG-8300.

The Flat STi.fit - this is an ST-I C colour camera, right?
Everything looks normal there. Mono cameras are better for guiding. Color you sometimes have to bin 2x2.

Jupiter - STF.fit
That looks better, but you're not quite focused yet.
The moons are great targets.
If you adjust the Screen Stretch histogram a bit, you'll see they are small donuts.

So tweak the focus a bit further to get those down as small as possible.

 

Darren, when you have a second, I'd like a bit of info.
Can you give me a screenshot of this DeviceHub screen?

 

Ok, so it's the Celestron software giving that Not a Number (NAN).

I'm not sure what the "flat part" is.

Light from telescope ----> \ -----> CCD
|
V
where the \ is the pick off, which sends light to the guide camera.

Does that help?

 

The good news is you got rid of the black edge.
@Doug - do you have any ideas? - the shadow is really prevalent.

Darren - what are you using for a light source for your flat?

 

That's a neat looking panel. (I found it on Amazon).
Is it evenly illuminated? Is the scope dead centre in the middle of it, and the panel is square to the front of the scope?
The "shadow" is about a 10% variation in brightness.
Here's another test: Rotate the camera in the focuser - eg 45 or 90 degrees roughly. Does the shadow stay in the same position? If it does, then it's still related to the camera hardware. If not, then it could be your panel.

 

Doesn't look like the shadow of the mirror to me. Looks more like a filter wheel in-between slots.