SCT and DSLR

[adavis]

Hey all,

First time Astrophotography poster, so be kind :rofl:

I just purchased a Celestron Nextstar Evo 8 SCT and am very happy with the visual observing that it has given my family.

The plan is to do simple astophotography, yes I know that I need an equatorial mount to do it properly, just dimming my toes here :-P

I have a T adapter that fits my Canon 6D and the telescope, however Im having a focus issue that I cant out, hoping someone in here can point me in the right direction.

When using the DSLR and trying to focus, I can go from blurry to not so blurry, and back to blurry again, without transitioning through an in focus area.
It gets close to in focus, but never really gets there.

All I can think of is that the T adapter is not long enough as it is shorter that the ones that Ive seen on youtube vidoes for this telescope.

Anyone have any ideas?

[Merlin66]

Andrew,
Sounds like you’ve got the correct T thread adaptor.....
At f10 and a long focal length focus can be challenging.
Practice during the day on a distant object.

[adavis]

Hey thanks for the fast reply!

Yep I tried that too, but it never gets close enough to be in focus.
Ive seem moon photos from this model of telescope with similar body and they are pin sharp, which there is just no way that I can get anywhere close.

[bojan]

You need Bahtinov mask.
https://en.wikipedia.org/wiki/Bahtinov_mask

[bojan]

Pin sharp...

Sharpness of the image depends very much on visibility. No wind, especially no jetstreams - they destroy the sharpness.
Also, those images are usually stack of several thousends on individual images (taken as movie) and processed in PIPP, AutoStakert! and/or Registax.

[adavis]

Yup gotcha, but Ive seen video even in not good viewing conditions, and it was substantially sharp, yeah pin sharp isnt a good description, I know :P

Ill have a look at a Bahtinov mask, thanks.

[turbo_pascale]

How long is the T-adapter?
This is what I have on my 10" SCT.

In a lot of cases, you need to be able to move things back further in the image train, rather than just be straight on the back of the scope.

What is the order you have things in?

[adavis]

Yeah that is what I thought, mine is about half that length (not in front of it at the moment unfortunately)

It fits into the mount that the star diagonal fits in to (without the star diagonal of course) so maybe that length is the same.

Im going to print off a Bahtinov mask this weekend and give it a go, but im not confident, just by looking at the image in live view.

It's not my first DSLR night imaging rodeo, Ive done a load of nightscape and Milky Way photography, but only with DSLR lenses, nothing like this before, however I would think that I know how to focus a lens!
I was wondering if I am just missing something stupid, like the wrong T adapter.....

[Merlin66]

The standard SCT to T adaptor is 55mm this combined with the 55mm for a DSLR with T ring gives the design spacing of 110mm for the SCT.
There must be some other issue.....
You say the visual results are good, so I assume the collimation and optics are good.

[adavis]

Yup, visual are really good, no complaints at all.
I'll have a longer look over the weekend and try the Bahtinov mask.
If I still have no luck I will take some video to show what's happening.

[Outcast]

This...

Focus on a bright star every time before slewing to target.
If doing lunar or planetary, video is the only way, stack in autostakkert, sharpen using wavelet in Registax... both these programs are free

Single shot on planets is a lottery due to seeing conditions, 30 seconds of video will give you 500 plus frames, then look at quality graph in autostakkert & decide on percentage to stack.

You'll be amazed at the difference

[adavis]

thanks!

[Outcast]

Andrew,

These were taken through a Meade 8"SCT on an AZ/Alt fork mount.

Both were taken using a ZWO camera in video mode; approx 30 secs of video each. However, I stress that focus was achieved on a bright star using a bhatinov mask first. I find I can use the 'zoom' function in live view at x5 or x10 on the rear display of the camera (I use a 70d for DS work) to do my focusing & that works perfectly well.

If you're using a DSLR you may have to convert the video files to .AVI or preferably to .SER to stack in Autostakkert...

You can use a program called PIPP to convert the MP4 video from your DSLR to AVI, can't recall if it will convert to SER.. this program is free too...

[adavis]

Great thanks for sharing, that is the kind of images that I was hoping for.

[adavis]

OK, I bought a new T adapter (2 inch as opposed to the 1.25 that I had) and a Bahtinov mask.

Sad to say, only a slight improvement :shrug:

I can get the centre spike in the middle of the x no problems, but the resulting image is still not sharp.
Also, visually, the same issue as before, you focus from blurry one side of focus, to blurry the other side of focus, but it never gets sharp in the middle of the two, just less blurry.

Imaging train is now, Telescope, direct connect T adapter, Canon T ring, DSLR

This is the T adapter that i have:
https://www.bintel.com.au/product/celestron-sct-t-adapter/?v=6cc98ba2045f

I dont have any shots or video, but I am about to get that this weekend, so I will upload them when available.

[bojan]

Hmmm

Are you talking about blurred single shot or stack?
Because single shot will look really dissapointing... but only after stacking of, say, 1000 frames and applying Wavelets, the images of planets become as shown by Carlton.

[adavis]

Hey Bojan,

yes I am talking single shot, but not even taking the shot, whatever it is that I am pointed at is just not sharp.
Ill give the processing a try when Im next at home on a clear night and post up some examples and try the video/multiple exposures.

thanks for all your patience, undoubtedly this is something that i am doing wrong or expecting too much.:D

[bojan]

Yes and no :)
You are probably getting adequate result, considering it was sinle shot at actuall visibility... which will have detrimental effect if not good.

Also, do not forget camera may shake due to mirror/shutter action...


Take couple of minutes of video, remove bits athat shake too much, apply PIPP, Autostakkert! and Registax6...

You will be pleasantly surprised

[Outcast]

If you shoot in live view mode, the mirror is already out of the way & will reduce shake to some extent.

If you do not have a remote control for your camera, use the self timer set on 10 second delay, this will allow the camera & mount to settle after you press the shutter... not sure if timer function is available on video setting..

If you use video & timer function is not available in this setting then just delete the first 5 - 10 seconds of video to remove any vibration artefacts... otherwise, let Autostakkert do it's thing & select only the best 10 - 20% of frames for stacking...

To be honest, you are on a hiding to nothing trying to achieve sharp images with only single shots... video & processing as described is really the only reliable way.

Next time you have a visual observing session, look at the moon at high magnification... you will see that it does not remain in sharp focus all of the time... this is due to atmospherics (and can be other things such as tube currents, thermals off rooftops, etc...)... now imagine trying to time your single snapshot with when it is perfectly sharp... pretty much impossible...

Video on the other hand captures all of it, in focus, out of focus & everything in between. When you stack only the top 10 - 20% (less percentage if you have a large quantity of frames) you get nothing but, the sharpest images.. then in Registax wavelets.. you sharpen what you have even further & then, and only then will you have sharp planetary or lunar images...

Edit: You have to trust the Bhatinov mask focusing.. if you get that right, your images will be in focus at the point that atmospherics allows... what you will be seeing in the back of the camera will be exactly the same (minus colour) that you get at the eyepiece.. that is the focus flutters in & out due to atmospheric conditions, commonly referred to as seeing conditions...

[adavis]

Will do, thanks

[adavis]

Thanks for the explanation, it does make sense.

Just to be clear, I am not expecting a single shot to be sharp or the only process, I do know about stacking and using video, and expect to do this.
What I am doing primarily so far is basically using the DSLR with OLED screen to be a viewfinder.

I'll give all the suggestions a good go this weekend if the seeing is good.

cheers
Andrew

[Outcast]

Likewise Andrew, what you see on the OLED screen will represent viewing conditions, that is why what you see won't look very 'in focus' although, it may 'flutter' into focus for brief moments. The other way to see this is to look at the moon at high magnification.. the worse the seeing conditions the worse the flutter (infocus to out of focus) will be. Seeing conditions will allow brief (read very brief) moments of in focus...

In short, you may see literally micro seconds of infocus image through your display but, that will largely depend on your sampling rate & refresh rate, your eyes (via the brain) will produce sharper 'moments' than your video screen as your brain operates at a far faster refresh rate & also interpolates data in a manner your sensor & video screen cannot..

Trust your bhatinov mask focus, it is spot on, assuming you have the central spike directly intersecting the centre of the crossed spikes. Essentially, you have to ignore what you are seeing on your screen/monitor, capture the video, process it & then, and only then will you achieve your sharp planetary images..

Please trust me on this one... I have learnt it the hard way... I know what you want to achieve & as I & others have stated, there is only one way... video capture/stacking/wavelet sharpening...

[adavis]

Yep, no problems, I do 100% trust what everyone is telling me.
What is confusing me is that on the same night, in the same minutes, I can see beautifully sharp images through the eyepiece, but swap to the DSLR and refocus with Bahtinov, for a soft image.

Give me the weekend to follow everyones suggestions and I'll report back.

[Outcast]

Yep, that is indeed quite normal... our brain's ability to capture that sharpness as we view is far superior to what a video screen can replicate...

[bojan]

Exactly.

We have built-in image enhancement software in our brains.. which is still better than anything available so far...