ARGH. Double diff spikes caused by meridian flip?

[BrendanC]

Hi all,

I'm having the darnedest time with my rig of late. So many issues, and now, after I thought I'd fixed them all, I get this:

It's from my shoot last night. It only appears when I stack images before and after the meridian flip. If I stack just images before or just after - even stacking with images from the previous night, matching before with before and after with after - I just get nice, single, crisp spikes. Stacking before flip with after flip gives me... this.

I always thought the only thing that determines the angle of spikes is the OTA's rotation within the clamps, and I most certainly did not change this last night. Everything is solid.

I've read that it could be cone error?

Any takers? I'm fixing to chuck this game in sometime soon if I cannot get just one decent night's shoot in months...

Thanks
Brendan

[ONIKKINEN]

If you have a lot of cone error, you get this. Also, the closer you are pointing to the celestial pole the stronger the effect becomes. My setup has enough cone error to cause a roughly 2 degree rotation change pre/post meridian when imaging M81 for example, but the difference in diffraction spikes is not really at all noticeable in that case so you probably have some serious amount. I think severe polar alignment issues will also cause "effective cone error" like this, but it would have to be pretty bad and you would notice it in guiding performance.

Since its a newtonian you could be out of collimation, which will introduce cone error as the light path is not exactly central through the mirrors then. Has this ever occured before? If not, my bet is something was knocked out of alignment. Or something too simple to think of, like the scope not sitting well in the mount clamp and something like that.

[BrendanC]

Thanks. Perhaps I have a lot of cone error, but this has never, ever happened before. Having said which, I don't think I've ever imaged such a high object pre- and post-flip before either. Guiding was great and I'd polar aligned only a couple of nights previously, and I don't tend to need to keep doing it every time, going by experience. Collimation seemed fine. Perhaps the OTA wasn't seated right?

I did recently replace the secondary vane with one of those Backyard Universe spiders. Could this have anything to do with it? I really don't know any more. I've had repeated weird, random issues for about a year now and I'm pretty much running out of steam with astro. This feels like the last straw, to be honest. Thanks anyway.

Edited August 21, 2023 by BrendanC

[ONIKKINEN]

The backyard universe spider assembly is that CNC machined solid piece right? I dont see how that could do this since it pretty much guarantees a central (to the tube) secondary.

How high is the target? Cant figure out from the example image what it is. Anyhwere within i would say 10-15 degrees of Polaris will be tricky if there is cone error and personallh i think its too much trouble to shoot there. My setup wont really even plate solve and go-to properly too close to the pole.

Wish i could offer better advice for the last part, but i have had repeated weird issues from night #1 onwards more than 3 years ago 😐. I have accepted that this is how it goes, for better or for worse.

[BrendanC]

You know, this sentence did actually make me LOL, albeit in a rueful, resigned kind of way...

4 minutes ago, ONIKKINEN said:

Wish i could offer better advice for the last part, but i have had repeated weird issues from night #1 onwards more than 3 years ago 😐. I have accepted that this is how it goes, for better or for worse.

So the Dark Shark hits around 68 degrees alt, which I guess is pretty high. I've also had problems with meridian flips during this shoot, which I'm wondering might also be down to plate solve errors exacerbated as you get nearer Polaris (still waiting for someone to help with that issue too).

I mean, the workaround is to process the nebula without stars, which I would do anyway, and then just use one set of stars rather than both. But, I don't like workarounds, and I don't like knowing something's wrong with my kit. Even more, I don't like knowing there's something wrong with my kit that I don't know about. Even so, as you rightly say, there's always something wrong. My recent set of issues has covered tilt, collimation, calibration, non-functioning camera, non-functioning focuser, non-functioning mount, spider vanes, pinched optics... the list goes on. Then, you get one chance every few weeks to see whether you've fixed it, and another totally ridiculous, random problem pops up.

I've been through so many barriers and jumped so many hurdles this year, I don't feel I've progressed at all. I stayed up till 2am last night to babysit the supposedly automated meridian flip (which still didn't work, you can read about it here if you like https://stargazerslounge.com/topic/412762-calling-apt-astrophotography-tool-gurus-meridian-flip-failed-need-help/ )which, given I've been doing automated flips for nearly three years with no problem, as well as stacking images before and after flips with no problem is just, as I say, random and ridiculous. 

Thanks for the help again, and the wry philosophical observation.

 

[BrendanC]

Just bumping this one in case anyone else has any input? Opinion is divided as to whether this is cone error, poor polar alignment, the new Backyard Universe spider I recently had installed, or something else entirely...

[GalaxyGael]

It sounds like the effect of cone error, and even though it never happened before, make sure your dovetail bar is attached evenely then run a test for cone error. Something near the south meridian is good and see if it centers either side of a manual flip.

But, if thats not the issue and your new CNC spider is definitely collinear for each pair of vanes, the only thing I can think of is that either side of meridian you have a slight tube rotation within the rings, which will also cause double spikes. Have you check the OTA rings to ensure they have not loosened from thermal (hot, cold, hot, cold) working. I have had screws loosened due to the hot and cold over many weeks.

[BrendanC]

Good call on the ring screws, I never considered that. They seem OK but I'll make absolutely sure they're tight (I was sure before!). Apart from that, well, other people have mentioned cone error too, so that's the leading candidate so far. Looks like I'll have to learn another arcane mount-related technique...

 

[GalaxyGael]

Good thing about cone error is that it can be fixed easily, and the vixen plate would need to be (to your eyes) wildly angled or placed with such a big lateral preference to one side of the OTA centre to see such a pronounced effect. It looks worse on brighter stars as the spikes extend further so any gap is magnified further away from the bright star core.

What i used to do was use a calipers to make sure a losmandy plate was centered when attached to the rings, since some screw holes have some play in them.

OTA rotation will do the same, useful to sanity check the ring tightness.

Two last small things: (1) when dealing with issues, the following happened to me. I had one saddle bolt tightened when balancing, making it easy to slide the OTA and nip it up to get the right balance, then tighten the second screw. One night, I forgot to tighten the second saddle screw and had star issues as the OTa was tilted a bit in the saddle. you may not have such a simple error, but just in case...:)

(2) when you installed the CNC spider vane, is it now perfectly parallel with the outer rim of the OTA? i.e. that the entire CNC unit  are not tilted a little.

Edited August 23, 2023 by GalaxyGael

[BrendanC]

I thought I would see quite a wild cone error, as you say, if this were the case. But it really doesn't look bad to my eye. The calipers idea is a good one, and in fact I just used my digital calipers to confirm that the spider is absolutely spot on, so it's not that.

I'll definitely check out the screws as you say. I also intend to polar align with the NEQ6's PA scope, then verify with SharpCap, just to make sure SharpCap isn't throwing me a curve ball, as well as try ConeSharp to address any cone error. I'll get there eventually, I hope, but I really could do without these never-ending issues...

Edited August 23, 2023 by BrendanC