Seagulls, focus, plate solving - some help required please

[powerlord]

Last night I attempted to use my C8 with my asi224. I'd previously used it with my 6d + .67 reducer and shot dumbell nebula and got nice clean results. So I figured OTA was collimated good, and I was ready to try it for another target.

Then plan was to get it setup at the start of night, try dumbell, and maybe Ring Nebula, but really just pre-cursors so it was all mostly focused and PAed so I could try saturn in the morning.

After 3 hours, I gave up and went to bed... here's what went wrong.

Equipment: my eq5/eqstarpro, asiair, 120mm/50mm guidescope, asi224 direct on C8. zwo eaf, macbook air.

Firstly, not having anything in daylight I can see at a decent distance away I knew I'd have fun focusing, and unless it's in focus, I can't PA, plate solve, etc.

So I used my macbook air direct on the 120mm to get decent focus on the guidecam, then tried the asi224.

As I'd bought the standard celestron t-ring adapter here (https://www.celestron.com/products/t-adapter-for-schmidt-cassegrain-telescopes) I naively assumed that it was setup for a back focus of ~55mm of a DSLR, and therefore would need another 42mm or so of tube with the ASI224 (12.5mm back focus).

Hunting back and forward with the EAF I couldn't see anything, but with such a narrow fov - was it because I was out of focus or nothing to see - even pointed at Arcturus, I couldn't be sure I WAS on it or not... cue about  an hour trying to focus.. finally finding a bit of a light gradient and hopeing I was seeing a defocused star, repeating, trying to move towards it again, losing it, etc... argg....

Finally I got close enough and hit end stop.. could it be I'm not at correct back focus.. cue removing 10mm and trying again... then another 12mm.. Finally I tried just the adapter above... and finally, finally I got somewhere, and managed to eventually focus by reducing the size of the edge of the curved gradient I was seeing, then moving towards it till I eventually could see the star itself and focus on it.

That confuses me, as it means I can't see how I can possibly get focus with my DSLR - but then again the focus range of the C8 is massive which is part of what makes it a pain to focus.

Anyway, on the mac things looked ok now, so I put it back to HOME position and went back to the asiair.

Now the fun continued. My stars look like seagulls. All of them - not just the edges. polaris is in the middle and looks like attached. I should have checked this on bright arcturus, but it had taken me the best part of an hour to find the b4gger, so I was where I was - seeing some stars near polaris, or polaris.

Undeterred, I tried autofocus anyway, and it actually worked! by which I mean it made the seagulls as crisp as they would get.

So thats the first issue.. I could swear I read about stars looking like seagulls somewhere but search isn't finding anything. why ??

secondly, I figured, well lets put that aside, can we plate solve. well..no. asiair says my fov is 0.15 degree and below 0.2 it says plate solving may not complete.

And they were right, it didn't. try and try again I couldn't get a plate solve so PA wasn't going to work either.

So that's the second issue: assuming its not just my stars being seagulls and plate solving doesn't work with an asi224+C8 is this just a mad combination or are people working with this sort of thing ? I mean it seems like what would be a common enough setup for planetary observation - but without plate solving 'goto' isn't really possible with the asiair - not with such a narrow fov anyway as you'll never find the target.

Do people setup with a larger sensor (my asi1600 say), plate solve, PA, goto saturn and then swap over to something like the asi224 ?

Anyway, it was now 01.30 in the morning, so I packed it in and brought everything back inside.

that was an efficient use of the one clear(ish) night in 2 weeks. 🙄

 

[Cosmic Geoff]

If you want to find an object with a C8 + ASI224 you really need to centre it in an eyepiece and then switch to the camera (which is what a flip mirror assembly is for), or have an accurately setup optical finder.  

I have also found out that with this combination a live stack often fails to work because of 'not enough stars'.

I have no idea why your stars look like seagulls but it looks like something was seriously awry.

[powerlord]

Yeh, for that reason I have setup my 50mm 240FL finder to be as accurate as I can. So centred in the asi120 should be centred in the C8. Though must be a fraction off as wasn't helping last night.

weird. I mean it can't be the C8 - it took this pic fine a few weeks ago with a 6d and .67 reducer. Unless the .67 reducer/corrector is doing one heck of a lot of correcting...

[michael8554]

When you switched to the ASI224, if you were still using the FR then yes, you had to make up the FR Back Focus to 55mm with the extra spacers.

Removing some of that spacing gave you Coma, which is what caused those "seagulls".

And that 42mm should have got you very close to the focus position you had with the DSLR.

Swapping the camera for a parfocal eyepiece would help to find a star to focus on, but difficult if your connections are all screwthread.

If you have a Guidescope, then add shims to tilt it to make the FOV concentric with the ASI224 FOV.

Then a star central in the wider guidescope FOV should be central on the ASI224.

Plate Solving would most likely reject stars with that much Coma.

M27 would have filled your FOV, not a very pleasing framing.

M57 would have been better.

Michael

[AstroMuni]

3 hours ago, powerlord said:

Do people setup with a larger sensor (my asi1600 say), plate solve, PA, goto saturn and then swap over to something like the asi224 ?

I dont. I have an f/5 scope so fov is a double of yours. I dont have a guidescope and use a telrad to do initial adjustments. My mount is a Go-to so makes it a bit easier.
I have made a small mark on the focuser where I get focus with the ASI224 so its a good starting point.  After that its a matter of getting the best focus for that day with gentle nudges on the focuser knobs. A homemade Bahtinov mask also helps.

All platesolving is done post getting the good focus.

[powerlord]

thanks, the targets were really just to test it ready for saturn in the morning.

When I used it with the 6d before it was with the FR. Last night it was direct. no FR. Something I've not tried with the 6d. But  as you say, I'd have thought I needed that 42mm.

I think I'll need to find a daylight target far away and do some tests.

I though coma was more blurring/comet trail kinda thing on edges ? Here, with nothing between the asi224 and the C8 mirrors, surely if its focused it must be at the right back focus ?

 

[powerlord]

10 minutes ago, AstroMuni said:

 have made a small mark on the focuser where I get focus with the ASI224 so its a good starting point. 

yes, good idea. I've now marked a line at far end stop on ota and a mark on knob. So for a given configuration I can record the focus position. For example, the celestron t-ring + asi224 was 10 turns in/clockwise and then 8oclock.

stu

[powerlord]

Er... I may have found the issue.

Did think it was funny that when I eventually got focus it seemed fine on Arcturus, yet later on Vega it was seagulls...

er.. after getting focus on arcturus, I only then turned on the asiair to start PA, etc.

the asiair that has my dew heater plugged into it.

well.. you say.. so what...

can you spot what's wrong with this picture:

 

I will get my coat....

 

what the hell i was thinking when i put that on, I have no idea. muppet.

 

[rickwayne]

No, I can't.

[powerlord]

Sorry, maybe should have explained.. I've got the fecking dew shield wrapped around the wrong end... heating the mirror up instead of the glass at the front of the scope.

Hence... initially focused nicely, then dew shield switched on.. and mirror starts getting heated.. warping all over the place - enter the seagulls.

I had to remove the vixen plate to slip the dew shield under it, and during that seem to have not realised I'd then tighened and adjusted it on the wrong end.

I'd say - make sure you don't make the same mistake....but frankly it's too stupid a mistake to be common I think.

stu

[michael8554]

"Happiness, is a warm mirror, yeah yeah yeah "

Michael

[powerlord]

So, an update - it wasn't the dew heater... though I imagine that was not helping matters. It was badly out of collimation.

I finally got some bobs knobs today, and roughly calibrated it in daytime against a few BBs stuck on some flock. It was waaaaaay out. I'm sure its not perfect, and I might tweak on a star, but you can clearly see the difference just focusing on a ball bearing now 20m away. Before I was focusing on trees a mile away and just figured the poor picture was seeing conditions - now I see it was collimation. amazed I got M51 as good as I got from it, but now should at least be good enough to try Jupiter - just in time for the crap weather to come in!

[powerlord]

So.. it was still terrible. I'm keeping this thread updated as I suspect it may help others.

I think I was sold a pup that has been taken apart many times. After lots of hunting around a reading, I see that the 'corrector' has to be in a certain orientation. Its confusing and surprised me, as I'd really just thought it was a bit of glass holding the secondary in place - there is no obvious 'lensing' going on...but apparently they correct astigmatism.. which is exactly what I have.

apparently there should be a 'serial number' which 'should be at 3 oclock' or a mark lining up to 3 oclock. well.. initially had no luck seeing serial number or marks, but then I took it apart again later in sunlight, and after lots of viewing at different angles I now see the outside covered in old, removed sharpie lines. all around the ege! That made me realise it had been taken apart loads of times.. then finally I noticed the serial number - I'd been expecting some professional etching, not a hand written scrawl on the edge I'd just initially thought was a scratch!

So - I've stuck it back together with that serial at 3oclock, and tonight I wil attempt to collimate. I've read the standard way, I've also cut out a mask to try the duncan method. Really hoping, fingers crossed this time I can get it done. I'm gonna use the 1200D live view on a 7" HDMI field monitor to do it.

Sacrifice a viginal chicken for me please, as I've about had it with this thing.

stu

 

[powerlord]

So , made a bit of progress last night. Now that I THINK the corrector is in roughly the right position I can actually focus stars to something approximating a point.

Good enough to finally use for the planets for first time.

Posted results in planet section:

 

But, still not right. Here's the pattern that I see when I'm de-focused for example. So though rings are in the middle, there's this big 'flame' sticking through all the rings. What in the name of squirrels is that ??

 

[Avocette]

Not squirrels, just the Schlieren effect in play, due to the air currents inside the optical tube which have not reached thermal equilibrium.