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Tips for getting target onto sensor?


Adam Y
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So during my first proper session, my conclusion is that the most difficult part of this is getting the target onto the sensor!  Even if I have it lined up perfectly in the eyepiece and tracking is on, when I pop the camera in, the object is nowhere to be found!  The best I could manage was to slew the scope around while watching the histogram for some sort of spike to tell me i was near a bright thing, then hoping to narrow it down from there.  There has got to be an easier way, it took me almost half an hour to get Jupiter where I wanted it!

 

Any tips or tricks to share?

 

Also, an autofocuser is going on the wish list!!

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6 hours ago, Adam Y said:

So during my first proper session, my conclusion is that the most difficult part of this is getting the target onto the sensor!  Even if I have it lined up perfectly in the eyepiece and tracking is on, when I pop the camera in, the object is nowhere to be found!  The best I could manage was to slew the scope around while watching the histogram for some sort of spike to tell me i was near a bright thing, then hoping to narrow it down from there.  There has got to be an easier way, it took me almost half an hour to get Jupiter where I wanted it!

 

Any tips or tricks to share?

 

Also, an autofocuser is going on the wish list!!

It's tricky isn't it....:classic_rolleyes:. It's worth working out a parfocal eyepice to camera configuration with something like the Moon which is so big and bright that you can't miss it. I use a 40mm widefield eyepiece with my C14, which usually works well, but sometimes it can still be a problem - use a parfocal ring to get the camera and eyepice parfocal. Then you can use the eyepiece to centre the planet, so that when you swap it for the camera you have a better chance of it being on the sensor. Also make sure that you are not using the capture ROI when you start out; you want to have the maximum sensor available.

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I went through the same thing when I started.  I know how long this can take. I tried various types of finder but it still took ages. 

 

The way I do it now is to use plate solving.  I control the mount and camera from a laptop and I use three pieces of software - EQMOD, Astronomical Photography Tool (APT) and Cartes du Ciel (CDC).  There's other software which does the same thing.  They communicate between one another. The steps I take are like this -

 

Get the scope pointing at roughly the right position.

Take a picture

Use APT to plate solve which works out the exact coordinates of the centre of the image.

Synchronize the three pieces of software. In APT you just click on the synch button. This makes all the software know exactly where the scope is pointing.

Then using CDC I tell the scope to go to the target, Jupiter say. The scope will then move to the right position. 

Take another picture and usually the target is close to the centre of the field. If I want to get it more exact then I plate solve again and do it again.

 

I do this now even if I'm doing visual observing. Get the scope on the target with the camera and plate solving then replace the camera with an eyepiece. You will probably have to refocus.  It saves a lot of time.

 

 

 

 

 

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It's not easy with a long focal-length scope; and in my experience, swapping out the eyepiece and popping in your camera doesn't guarantee that your image will fall on the sensor. There can be slight shifts in alignment from various moving parts - even the thumbscrews holding the camera in place can move it enough to throw the image off. If it's out of focus, it's actually easier to find because the donut image covers much more of the sensor.

 

As an interim step to plate-solving, a finder scope with a (illuminated) reticule eyepiece can be good to center the target on the sensor. You can align the finder scope with the main scope, with the camera already in place, during daylight on a distant mountain, chimney or tree. Then at night, place the reticule exactly on your target, and the camera should see it too. Of course, then you can't use the same scope for observing and photographing but we all learn that lesson eventually!

 

In the longer term though, plate solving is the way to go. APT as mentioned above is great with DSLRs, as is NINA with astro cameras. It's a game changer, especially if you can't see your target by eye (faint nebula/galaxy).

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I've managed to find a process that works for me for now, though plate-solving is still something I'll research.  I do have an illuminated reticle EP and I have found that if I can get it lined up there, that it will almost certainly be in the frame in Firecapture.  IF:  a) tracking is dead-on so that it stays put, b) I immediately twist the focus knob in several turns so that it isn't too washed out when I pop the camera in a c) I have the gain up so that the still-unfocused object shows bright enough for me to finish the job.   I have also noticed I can give the back end of the scope a tiny nudge up or down which will often be enough to show me which direction to move in case it isn't in the frame.

 

Still not perfect, but getting there!  Now if I could just do something about the seeing!!

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Plate Solving is a bit complicated to set up, but it is one of those things that once you have it up and running, you wonder how you ever managed without it!

 

I use ASTAP in NINA. Goto target, 10 second capture, 10 second plate solve, slew to centre target. Simple as that!

 

Sorry, can't help with the seeing!

 

Regards

 

Graeme

 

 

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2 hours ago, Graeme said:

Plate Solving is a bit complicated to set up, but it is one of those things that once you have it up and running, you wonder how you ever managed without it!

 

I use ASTAP in NINA. Goto target, 10 second capture, 10 second plate solve, slew to centre target. Simple as that!

 

Sorry, can't help with the seeing!

 

Regards

 

Graeme

 

 

 

Yup, I have tried 3 times to get plate solving to work, each time with help from some-one who knows how it works and using different programmes.   Everything needed is loaded, but it doesn't work.  Or it Solves but doesn't centre.  Or terrible disconnection problems.  However it looks like it might have been an out of date EQDir cable that might have been causing some of the problems.  Have now bought a new cable and am waiting for the warmer weather before messing around outside trying to get it to work again. 

 

Carole      

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Good USB leads are crucial. I recently had a lengthy thread on the CPWI forum thinking there was a problem with either the new beta software upgrade or my CGX mount. After some fault finding it turned out to be the 5m USB!

 

 

Edited by Graeme
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5 hours ago, Graeme said:

I use ASTAP in NINA. Goto target, 10 second capture, 10 second plate solve, slew to centre target. Simple as that!

 

Can plate solving be used with an alt-az mount, or does it require Eq?  The captures you are referring to - are these with your imaging camera or a separate guide camera, with separate software to control the mount?  Trying to work out the connective tissue between taking the capture and plate solve, and those actions resulting in a control signal sent to the mount.   Looks like some research ahead of me in the coming weeks!  🙂

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Alt-Az or Eq doesn't matter as long as the mount is a goto that the software doing the image capturing can control. 

 

The plate solving is done using the main imaging camera using your image capturing and mount control software (Fire Capture, SharpCap, NINA, APT etc.)

 

Guiding is a completely separate can of worms! 😃

 

 

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Ahh so if I understand correctly, plate solving is simply a more accurate/reliable way of getting aligned but does not constitute part of a feedback path for guidance?  So use plate solving to get perfectly aligned then rely on the scope's native guidance features to (hopefully) keep things in place?  

 

Can of worms is a great way to put it!  (reminds me, anyone know about vermiculture? LOL)

 

 

 

 

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'Aligned' is a dangerous word in that it has multiple different meanings - polar aligned, star-aligned etc. but I think I know what you mean and yes you're correct.

 

Plate solving gets you on target within the constraints that you specify. I have my pointing tolerance set at 2 arcminutes (1/1800 of a degree). The software takes a capture through your main imaging camera, compares to a star database, and determines the coordinates of the centre of your photo (and the rotation angle). If this doesn't match the coordinates of your target within your defined tolerances, then it adjusts the mount by the calculated difference (the 'error distance') and tries again. It will stop either when it has centred your target within your tolerance, or it has failed to do so within a set number of attempts. Your capture software will manage the test images, the plate solving and the mount control.

 

It can be tricky to set up; it needs to know your scope focal length and your camera's pixel size, but once it's set up and working, it doesn't need any maintenance. It makes getting an imaging session started very straightforward.

 

Many of us go through entire imaging sessions without ever looking up (whether that's a good or a bad thing). I've certainly spent ages scratching my head over the laptop, changing software settings, rebooting etc. wondering why the images are all blank, only to look up and realise the sky has clouded over!

 

But the process you've defined above will work fine for you, while you decide if you want to get into a more automated approach. These techniques can be added incrementally as money, time and experience allow.

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Haha yes this happened to me the other evening!  Got Mars actually lined up and tracking nicely and spent way too much time playing with the settings in FireCapture, thinking I was doing something wrong.... look up... clouds.  😒

 

Thanks everyone!  Although my current method should see me through, I am always wanting to learn more.   

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Hi Adam, i am quite new to this too, bee at it since summer, in the last 3-4 session i have now figured out most of the bugs i was having especially with regards to getting a target in to the frame.

 

i found paying attention to getting making sure the tripod is set up correctly and that home is as accurate as i can make it fixed 99% of the issues i was having. The guiding software (ASCOM) presumes that when it thinks the scope is in the home position it is correct and so makes all is adjustments from home (so if youre start not in 'home' it makes the correct movement but the result is wrong because the start point was wrong).

 

i found a red-dot sight helped when using the hand controller to make sure i was actually pointing at what i thought i was pointing at.

 

i also use stellarium lots to control the mount now and get pointing at when i am looking for, there are tools in it to help you figure out what orientation to have the camera at etc, it also allows you to see whats in the area around what your trying to take a picture of so if you are off you can make corrections.

 

give me a message if you want more incoherent help from a nebbie who is up against the same problems as you.

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