9 replies to this topic

[mbgnjasb]

Hello. I recently upgraded my camera from Nikon D5300 to ZWO 2600 MC pro. It seems the extra weight of the new camera is causing my Orion 115mm triplet's 3 inch focuser to sag. I tested the star shapes for different orientations of the camera. The stars are always elongated (triangular shape) along the declination axis, so I am guessing gravity is playing a role here. I am wondering if a camera tilt adapted can correct for this misalignment. Any other possible solutions to this problem? My guiding hovers around (0.5" rms), so that is probably not causing any problems. I have been struggling with bad star shapes for a while now, so I would really appreciate any help.

 

Thank you so much.

[KTAZ]

No. In the event of a sloppy focuser, a tilt corrector can only correct for sag in, you guessed it, one position. As soon as the scope moves enough for gravity to pull the focuser tube another direction, your tilt will change.

 

Your options are to service your focuser and, hopefully, find a way to tighten it up, or get a better focuser. Tilt is an extremely touchy animal; it takes very little to make it better, or worse.

Edited by KTAZ, 17 September 2023 - 09:01 AM.

[arbit]

I wouldn't have thought just a 700g camera would do that to a 3 inch focuser.

Some thoughts:
How much is the focuser drawtube out? Can try using threaded extensions to reduce that.

Is everything threaded or are there any compression rings in the train?

Is the focuser itself solidly attached and square to the OTA?

Is the focuser tension okay?



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[Robert7980]

There are probably adjustment screws on the focuser to tighten it up. I’m not familiar with your scope, but all of my scopes and focusers have some sort of adjustment to remove excess play… I’d start by looking at that. Unfortunately though I think you stumbled on the reason there are crazy expensive focuser upgrades on the market, they aren’t just there to look pretty… I bought an import focuser for my RC that was about the cheapest I could find and it was still 400 bucks. It has zero play in it though, so there are some options that aren’t a $1000+. 
 

Hopefully adjusting some set screws will correct the issue for you. The 2600 is a bit of beast, even more so when it’s got OAGs and filter-wheels attached… I would think an Orion triplet should be able to easily handle it, so you’re probably ok.. A quick search of images looks like there are a number of brass set screws, not sure if that’s the model you have, but I’d be checking those for a loose one… 

 

I’ve resorted to Loctite 242 on most of those type adjustments as I keep finding loose ones… Adjusting them is a bit of an art, so you need to go by feel. Tight enough that there’s no play, but loose enough that everything still moves smoothly… 

 

Just a guess, but might be a possible easy solution… 

Edited by Robert7980, 17 September 2023 - 12:07 PM.

[mbgnjasb]

I just realized that I am being most likely stupid (sleep deprivation is really affecting me I guess). Below, are two 120s subs (cropped) captured at two different orientations of the camera last night (centered at  the star Alcyone in Pleiades).

 

 

 

The shapes rotate which means it is sensor tilt and not focuser sag, correct? I think these triangular shapes are really screwing with my perception. Below is another 180s sub from Bortle 2 sky taken a week ago

 

 

The setup takes really nice images but star shapes have been really wonky since I upgraded. Following are the pictures of my image train

 

 

 

Everything is screwed in. I have tightened the focuser tension but adjusting it really didn't make any difference to star shapes which is why I again suspect that it is sensor tilt.

 

Edited by mbgnjasb, 17 September 2023 - 04:57 PM.

[unimatrix0]

Just seeing your setup, I don't think you got a focuser sag. 

 

If you want to close out focuser sag or tilt, take a shorter exposure. 120 seconds leaves too much possibilities at play.  

 

Like flexture issues or guiding issues. 

 

Take 10 or 5 second subs and do your analysis on those.

 

To close out focuser sag, point at something to the Zenith, so your scope at 90 up, and your focuser is not bending any direction. 

[Robert7980]

There are countless things that affect star shape, it’s a real rabbit hole…

 

As frank said starting with ruling things out is good idea. I’d add taking some shots with 90 degree camera rotation to see if the pattern follows the rotation. Tilt and flex can come from anywhere and can change over the night, guiding, polar alignment and seeing play a part as well… Pretty much everything has to be solid for perfectly round stars, it’s very difficult and extremely expensive to achieve… So deciding what is good enough before hand is probably wise… 

 

Your images don’t look bad to me and post processing can fix minor issues easier than optical and mechanical tinkering… So goals and methods to reach those goals are important… I started down the road to perfection and quickly abandoned it when I realized what I was getting into, so I just live what what I get… I still attempt to get things as nice as I can, but I don’t pixel peep and get frustrated at all… It is what it is, as it where… 

[mbgnjasb]

So, I did another test last night. Instead of Polar aligning perfectly, which causes the Polaris to be out (1x flattener at 805 mm focal length) or at the very edge of my FOV (0.8x flattener), I turned off the mount and centered Polaris in my FOV. I took 1-2s exposures and still saw the triangular stars pointing upwards as before no matter the orientation of the camera. Guiding is clearly not the issue here. It seems that either something is wrong with the focuser or possibly pinched optics. Funny thing is that now matter how much I tightened my focuser locking screw or tension screw, this aberration doesn't go away. It is always there. Does that eliminate focuser sag being the issue? Any other possible issues?

 

Thank you for all you help.

Edited by mbgnjasb, 19 September 2023 - 03:06 PM.

[Oort Cloud]

No. In the event of a sloppy focuser, a tilt corrector can only correct for sag in, you guessed it, one position. As soon as the scope moves enough for gravity to pull the focuser tube another direction, your tilt will change.

Your options are to service your focuser and, hopefully, find a way to tighten it up, or get a better focuser. Tilt is an extremely touchy animal; it takes very little to make it better, or worse.

The easiest solution is to avoid fast optics. Spend your time adjusting tilt/focus/backspace, or spend your time collecting more photons. Either way, time is the currency of this hobby (once you've spent a bunch of actual currency, of course).

[Oort Cloud]

I just realized that I am being most likely stupid (sleep deprivation is really affecting me I guess). Below, are two 120s subs (cropped) captured at two different orientations of the camera last night (centered at the star Alcyone in Pleiades).





The shapes rotate which means it is sensor tilt and not focuser sag, correct? I think these triangular shapes are really screwing with my perception. Below is another 180s sub from Bortle 2 sky taken a week ago



The setup takes really nice images but star shapes have been really wonky since I upgraded. Following are the pictures of my image train






Everything is screwed in. I have tightened the focuser tension but adjusting it really didn't make any difference to star shapes which is why I again suspect that it is sensor tilt.

Sensor tilt will look the same regardless of rotation. When the aberration moves around the sensor during rotation, that's tilt in the optical system (basically everything _except_ the sensor).