Struggling with Astrophotography

[Herzy]

I've had my telescope for years, however, I just recently started astrophotography! The details of my equipment are described below.

Scope: NexStar 90SLT Computerized

Camera: Nikon D3200

So my problem is focusing and just terrible pictures coming out. (I am taking 30 second exposure pictures). 

                Focusing

Alright, a big problem is that I can never properly find any deep sky objects because the live-view just simply can't see anything unless it's really bright! My actual scope reveals hundreds of stars in the eyepiece, but as soon as I replace it with the camera I see nothing, and therefor I can't focus the camera! The only decent picture I've gotten is m42, because I can actually see the stars in the live view.

                 Bad Pictures

For the few objects that are bright enough to focus, it seems like I can never get a good picture without star trailing! There is no noticeable wind, and I haven't touched the telescope, and the telescope is tracking the sky, but I still get Rubbish pictures with trailing!

[gnomus]

Regarding focusiing, do a search for 'Bahtinov Mask'.

[steppenwolf]

37 minutes ago, Herzy said:

Scope: NexStar 90SLT Computerized

Unfortunately, this mount is not suitable for long exposure imaging because it is an altazimuth mount. This type of mount tracks objects by making discrete horizontal and vertical movements whereas celestial objects actually move in a smooth arc. This is one reason for you getting trailed stars. An equatorial mount is required for long exposure astrophotography.

The key to focus is to find a bright star and focus on that using the Liveview screen and then moving to the object that you want to photograph. The use of a Bahtinov mask makes this task easier. Swapping from eyepiece to camera is very unlikely to work for you.

[Chris]

For deep sky imaging you need a optically fast scope such as an f/5 reflector or an f/6 refractor plus an equatorial mount. You're using a very slow f12 scope on an alt/az mount which will be much more suited to short exposure planetary and lunar imaging. 

For planetary lunar imaging you basically film the object at say 30 frames a second, and load the film into Registax (or similar) which picks the best frames and stacks them. Your slow long focal length scope is best suited to getting up close to planets.

You'll struggle with DSO imaging with your current setup because in alt az mode you'll get field rotation, plus the mount isn't accurate enough besides that, and your optics take much longer to collect light compared to fast scopes. 

Finally, your Mak will struggle with long exposures due to its focal length. If you hold your arm out in front of you, you can keep your finger quite still, but if you hold a pool que out in front of you at arms length the end of the que will wobble like mad. This is the difference in tracking between short and long focal length scopes, and your Mak has quite a long focal length. 

You need something like a 130pds on an EQ mount for DSO's

 

 

 

 

  

[JemC]

Hi Herzy

I am still learning myself and am in no way an expert, with regards your equipment,

Nexstar 90SLT generally come on a ALT/AZ mount, you really need a Equatorial mount with tracking function for Astrophotography,
you also need to make sure that you have polar aligned the mount as best you can, this will help to minimise star trailing

Pretty sure more experienced members will be along soon and be able to give you more info

Regards
James

 

[JemC]

they got there while i was typing

[Herzy]

Thanks for all the replies! I didn't mention in the original post but I do have a equatorial mount that came with the Celestron 127eq, will that suffice? i can buy one of those motors that turns it for you! 

One problem though, and the reason I bought a new scope, the mount is REALLY unstable... Everything bounces about in the eyepeice from even the slightest touch! 

[Herzy]

35 minutes ago, Chris Lock said:

For deep sky imaging you need a optically fast scope such as an f/5 reflector or an f/6 refractor plus an equatorial mount. You're using a very slow f12 scope on an alt/az mount which will be much more suited to short exposure planetary and lunar imaging. 

For planetary lunar imaging you basically film the object at say 30 frames a second, and load the film into Registax (or similar) which picks the best frames and stacks them. Your slow long focal length scope is best suited to getting up close to planets.

You'll struggle with DSO imaging with your current setup because in alt az mode you'll get field rotation, plus the mount isn't accurate enough besides that, and your optics take much longer to collect light compared to fast scopes. 

Finally, your Mak will struggle with long exposures due to its focal length. If you hold your arm out in front of you, you can keep your finger quite still, but if you hold a pool que out in front of you at arms length the end of the que will wobble like mad. This is the difference in tracking between short and long focal length scopes, and your Mak has quite a long focal length. 

You need something like a 130pds on an EQ mount for DSO's

 

 

 

 

  

What exactly is the difference between an f/12 vs an f/5?

[Ouroboros]

Hello - I'd be tempted to have a crack at some astrophotography even if your scope isn't ideal for deep sky objects. But how about starting with an easy target like the moon?  It'll be useful focusing experience. You can take single shots at first and then play with video.   

As for deep sky objects, slew to a bright star, focus on that and then slew back to a deep sky object. You might get a half decent shot of the bright nebula  in Orion even with short exposures.  

While you're practising like this you can maybe consider upgrading to a telescope with an equatorial mount. 

[johnrt]

The f ratio of a telescope is the ratio of the focal length of the telescope / the aperture. For example a 400mm focal length scope with 80mm aperture is an f/5 scope. The higher the f/number the "brighter" the image delivered to the camera chip.

Reducing 1 f/stop halves the light delivered to the camera chip, and therefore doubles the amount of exposure time required by the camera to collect the same amount of photons. The difference between an f/5 and an f/12 scope is that the f/12 scope is about 2.5 f/stops slower than the f/5 scope, so needs over 4 times as long exposure time to achieve the same amount of light hitting the camera chip.

[Chris]

What he said

[Tinker1947]

As mentioned a EQ Mount, and it really needs to be a GoTo, as you have found out you can't see the DSO's so something like a HEQ5 set-up nicely will sort out the GoTo, a 130P-DS would be a nice fast scope to get you going.....

[ollypenrice]

These are good answers. All I can add is that there are lots of manufacturers out there who offer products which have all the right 'words' in place - like 'motorized equatorial mount' but whose products just don't work well enough to work for astrophotography. The mount has the almost impossible task of trying to track the sky with an error of, ideally, less than half a pixel during one exposure. Now pixels aren't very big, so you don't just need a motorized equatorial mount, you need a good one.

Although Steppenwolf above won't publicize it on a thread like this, he has written an excellent intro to AP called Making Every Photon Count. It's available from FLO, the forum sponsor.

By the way, your title 'Struggling with Astrophotography' would probably describe every astrophotographer in the world...  It certainly describes me.

Olly

[cfpendock]

10 hours ago, johnrt said:

The higher the f/number the "brighter" the image delivered to the camera chip.

 

I don't understand this.  I always believed that a lower f number gives a brighter image.  For example, f 3.5 is a lower number than f 10.  But I know that imaging at f 10 needs more time than imaging at f 3.5.  Therefore surely the image is brighter at f 3.5?

Chris

[swag72]

As Olly has said, you really should start with the book 'Making Every Photon Count' - Read it once, twice and thrice ...... then sit down and think about what you have read and what you think you may need. If you don't understand what you need and why, then read it again!!! A little tongue in cheek perhaps, but this really is something of an imagers bible and is so easily and clearly laid out that it is easy to read and understand.

Of course there's always lots of us imagers about to help, but you can't beat a nice book I reckon

[Pompey Monkey]

18 minutes ago, cfpendock said:

I don't understand this.  I always believed that a lower f number gives a brighter image.  For example, f 3.5 is a lower number than f 10.  But I know that imaging at f 10 needs more time than imaging at f 3.5.  Therefore surely the image is brighter at f 3.5?

Chris

You are correct and I suspect that's just a typo. The rest of the post makes sense. ?

[cfpendock]

Thanks Paul.  Although the same post goes on to say "Reducing 1 f/stop halves the light delivered to the camera chip, and therefore doubles the amount of exposure time required by the camera to collect the same amount of photons."  Increase, surely?

Chris

[Andy Mitchell]

I know this pain, I started with a similar Skywatcher 90mm Maksutov on an EQ1 and used my existing standard DSLR (Pentax KX). As people have said seeing anything but the brightest stars in live view is a non-starter. Some things that I found helped:

* Use tethered shooting where you control your camera from your laptop. This is great as you can take a longer pic and view it immediately on a large laptop screen in high res. Much easier to see the diffraction spikes from a Bhatinov mask. You still want to be pointing at a bright star mind. The other good thing is when you are focused and pointing at the right thing you can tell the software to take a ton of pictures, and go and get a cup of coffee.

* Find your target using the finder, so you don't have to refocus for the camera. If your finder isn't helping consider a different type. My scope came with a red dot finder, if you are somewhere dark and hence can see lots of stars you can use a star map and star hop. If you have lots of light pollution, consider something like a 7x50 finder as you will see a lot more stars.

* Plastic clothes peg on the focus knob to do the final focus. Having a long bit sticking out to the side makes it easier to do the super tiny moves you need to get the focus spot on.

I don't know your mount, but as its computer controlled I presume there is some sort of alignment process where you tell it where Polaris is and a few other stars? Presumably if you do this you can use goto and get near where you want and it will have a good go of keeping the centre of the frame "still". Alt-Az mounts do suffer from field rotation but if your on short exposure I wouldn't expect to see much. Probably worth checking how well you are aligning to know you are getting the best out of the kit.

Ultimately if you want to do deep sky photography you will probably want a faster scope as others have said and a better mount.

 

 

[johnrt]

2 hours ago, cfpendock said:

Thanks Paul.  Although the same post goes on to say "Reducing 1 f/stop halves the light delivered to the camera chip, and therefore doubles the amount of exposure time required by the camera to collect the same amount of photons."  Increase, surely?

Chris

 

Chris, I use the word reducing as that's how my brain thinks of moving from say f/2.8 to f/4, reducing the amount of light and reducing the aperture of the lens / ota. My brain thinks of light transmission not numerical value. Hope that makes sense. Perhaps stopping down is a better more understood phrase.

My brain also thinks of say f/2.8 as a higher f/stop than f/32 for example, for the same reason as above, I appreciate this might be confusing and most people will call it lower, but lets settle for *WIDER*?

[dph1nm]

The SLT mount is fine for 30sec exposures at 660mm FL, as I use it at that. You do lose some exposures, but, hey, that's life. You can do any Messier object quite happily. However at 1250mm FL the mount is going to struggle. One way round this is to bin up your images 2x2 or 4x4 before you stack them, but this is a bit of a faff. You could try superpixel mode in Deep Sky Stacker, as this effectively halves the resolution.

One good thing is that the trailing with the SLT mount tends to be in random directions, so when you stack lots of shots together stars tend to come out round (if a little fuzzy)!

NigelM

[dph1nm]

14 hours ago, johnrt said:

Reducing 1 f/stop halves the light delivered to the camera chip, and therefore doubles the amount of exposure time required by the camera to collect the same amount of photons. The difference between an f/5 and an f/12 scope is that the f/12 scope is about 2.5 f/stops slower than the f/5 scope, so needs over 4 times as long exposure time to achieve the same amount of light hitting the camera chip.

Remember that, unlike in the terrestrial photography world, changing the f-stop does not automatically reduce the aperture. Providing that patch of sky fits inside the FOV, all telescopes of the same aperture will collect the same amount of light from a given patch of sky, irrespective of f ratio.

NigelM