Celestron C8 SCT with F/6.3 reducer - Guiding issues, help!

[chris_g]

Hello all,

Finally, the C8 is capable of taking images longer than 15 seconds. Using an Orion ST80 and an ASI290. The image consistently shifts up and slightly right.

Vignette, Ok this tube has a long narrow baffle, or so my research tells me why I have some nasty vignette with my APS-C sensor and the reducer. Ok flats and cropping.

If guiding were so bad as to have the center point change basically with each frame, would I not have some truly ugly star trails on a 3-minute sub?

Help! Guide log attached

First Shot

last shot taken an hour later

[chris_g]

Two more examples, this time 300 second subs. Stars dont look the greatest but there's no trails, but the image is again moved up and right. They are about 45 minutes apart. What am I doing wrong?

First sub

Last sub

[JayTee]

The way your stars are displaying shows that you are not at the optimal spacing for the focal reducer. Your stars at the edge of image show that they are radial streaks around the center point of the image.

Optimal spacing behind the Celestron FR is 105 mm. So you need to measure the distance from the ASI290s sensor to the edge of the flange (where you attach the nose piece), subtract this distance from 105 and now you know how big the spacer is you need to put in between the FR and the sensor.

[chris_g]

Hi @JayTee That's my Canon T3i connected to the C8 not the 290, sorry shuld have specified equipment better. It's on a 50mm extension tube to the T-Ring, 55mm to the sensor, 105mm. I even used a micrometer to make sure the extension tube was 50mm. I had a filter drawer in the image train earlier but took that out because I thought it was causing the vignette. If I'm at 105 now which way to I go?

Edit:

When I do put the 290 on the OAG, 105mm to it from the prism?! I am going to need a LOT of extension tubes!!!!

[chris_g]

I know you're right, I just thought I measured it correctly. Am I going to need those extension tubes for use in the OAG as well to get it to come to focus?

Thanks,
Chris

[chris_g]

Teach me to measure, flange to the top of the WO thread adapter is 60mm. not 55mm. I have no idea why I did that. But I still think it's wrong, would 5mm make that much of a difference? I'm thinking I should have measured from the bottom of the thread adapter, not the top making it even further out. Getting it in that close will be a challenge because of the camera body and the EAF.

Correct me i I am wrong, when I solve an image and it reports the focal length, it should report 1280 or there abouts for the .63 flattener. It was reporting 1225. That would make the flattener operating at F/6 confirming that I don't know how to measure. When I had the filter drawer in the image train, the solving was reporting 1286, so I had it right then. I'll go back and take a look at the pictures when it was installed.

Lessons learned, I post these in hopes that some will find hem amusing and learn from my mistakes. I know I do, both! I will get good data from this tube.

@JayTee, does that explain the image consistently shifting up and to the right with what looks like decent guiding? I'll go back and check those settings too. It didn't calibrate correctly the first time so I reran it again and it all seemed good, including the graph, for the most part.

[DEnc]

Regarding the image shift, I imagine you’re dithering with PHD2, right? Random dither?

Perhaps you’ve stopped the imaging sequences at similar points in the overall dither pattern, resulting in similar-looking displacements when comparing first and last images. Here’s how that would work:--

I posted an observation about the random dither function to the good folks at the forum Open PHD Guiding. I noticed the pattern of the dither displacements was fixed, changing primarily in the length of the displacements, but keeping the overall pattern. They confirmed that the dither sequence repeats with each new session--the C++ pseudo-random number generator used in the code to generate the random dither displacements produces the same sequence of random numbers with each new instance.

Here are plots of the X,Y coordinates of the lock position on my guide camera in sessions where I used different dither parameters. The red crosshair is the first lock position. The fixed pattern is very evident, in spite of the change in dither parameters. When dithering with the same parameters, the patterns in the two sessions overlap.

[SkyHiker]

That image shift is caused by polar misalignment. Everything gets worse with longer focal length, with your 80mm you might have noticed nothing.

Apparently, if the prescribed FR distance does not work you have to play with it to see what works best. The stretched star patterns in the edges look very symmetric so there's no collimation problem or skewed focuser tube.

[chris_g]

That image shift is caused by polar misalignment. Everything gets worse with longer focal length, with your 80mm you might have noticed nothing.

Apparently, if the prescribed FR distance does not work you have to play with it to see what works best. The stretched star patterns in the edges look very symmetric so there's no collimation problem or skewed focuser tube.

Thanks for that, I was using APT's polar alignment routine and got it to within about two arc minutes on both axis. I will endeavor to get it closer. I did see a slight shift with the 80mm but didn't give it that much thought because it wasn't that dramatic. APT also has a function that lets you recenter on the object during the imaging session, so I was using that every so often. Better to fix the problem instead of hiding it. I am hoping to get the OAG working, I believe I've got it positioned correctly now and I've since added another extension tube to the 290.

The FR issue was me and all me, it should work with 50mm to the T-Ring, I somehow got 60mm stuck in my head when I took the filter drawer out of it to see if that was causing some of the vignette issue.

[chris_g]

@DEnc, thanks for the info on dithering, while I know random generators aren't random and depend on the seed given, this a is a consistent shift up and to the right with every image. @SkyHiker's explanation makes more sense. I also remember reading that longer focal length is less forgiving with guiding, so the I hope to have the OAG functioning next time out.

[JayTee]

I also want to throw in the possibility that you may have "mirror flop" it is much more likely in the C11 and larger but it can still occur in a C8.

See this link
https://www.cloudynights.com/topic/5826 ... -in-a-sct/