Camera telephoto vs refractor

[acefisher]

I'm just getting into astronomy having spent most of my "spare time" on wildlife photography. I have the usual beginners setup  (I would guess) of  a skywatcher P150 Newtonian on an EQ3 mount. Actually I bought this at least 5 or so years ago and its been in my shed since then. I'm obviously on a steep learning curve but have started to get some results photographing  DSO's. Getting to the point, I have a selection of long telephoto prime lenses (canon 300mm F4 L, Canon 400mm F4 DO, Sigma 500mm F4.5). What is the difference between lenses created for photography and 400mm/500mm refractors? Would a "proper" telescope refractor give me anything above my telephoto lenses? A subsidiary question is the EQ3 up to DSO photography? I've set it up with a guide scope and polar aligned and then drift aligned and the results with exposures over 30 seconds have been unusable.

When I attached my camera and 400mmDO to a Star adventurer mount and roughly polar aligned it  I got less star trailing than with the EQ3 even at 60 seconds. Any advice would be most helpful.

[CraigT82]

Generally on two or three optical elements in a refractor, compared to 12-15 for typical telephoto lens? On a thousand pound frac a lot more money has gone into the glass than a thousand pound lens, especially if it' got image stabilisation and all the gubbins. 

Saying that though some seriously good astrophotography has and is being done with telephoto lenses.... check out astrobin for images taken with lenses. 

To be honest if I were you I'd carry on with your lenses and at this point invest your money into a decent, guidable mount.  

[acefisher]

Thanks for the help, I’m sure you’re right, thinking about it a lot of the cost of a camera lens is the aperture control, image stabilisation, weather proofing etc..I always buy my camera gear second hand new prices are unbelievable Canon have made ten grand a sort of entry level price for their L series telephotos. That would buy a hell of a mount and scope. 

[CraigT82]

Yeah some of the top end lenses are eye wateringly expensive, and don't get me wrong they are obviously fast, well made and optically excellent but when it comes to astrophotography there are better tools for the job. However the Canon 300mm f4 L is a particularly sharp lens and if you do some searching online tool find plenty of imagers using them, and seeing as you already have one you might as well put it to good use! 

As you probably know by know the mount is arguably the single most important item in an astrophotography rig... putting a high quality lens on a wobbly mount won't get you very far

 

[happy-kat]

From reading posts both the eq3-2 and the star adventurer are both able to be guided. Check out the imaging using an eq3-2 thread to see what members are imaging using that mount or even the star adventurer thread. Sounds like you already own all the gear to be imaging perhaps read up on how you could perfect and fine tune what you are doing to get the most from what you already own. 

[ollypenrice]

For good results I go for the HEQ5, autoguided.

Another issue with lenses is that stars are very exacting targets. It is hard for optics to get them right at fast lens F ratios but if you stop them down with a diaphragm you'll get multi bladed diffractions spikes. You can make a front aperture mask and leave a lens wide open, though.

Olly

[bobro]

14 hours ago, acefisher said:

I've set it up with a guide scope

Do you mean the mount is guided (motors, guiding software, guide camera etc) or the small scope is being used as a finder or polar alignment scope (unguided mount)?

Without guiding 30 sec will be a fairly typical max exposure length before star trailing starts with a P150 on an unguided EQ3. 

[Z3roCool]

I have a Canon 400 5.6 on an NEQ6 mount and get 2.5min exposures and pretty good results. I really want a scope but holding off until I get my skills up and bank account  Not guiding yet either but got the kit for it so will delve into that shortly. Here is a shot of  Orion with it.

 

[acefisher]

On ‎21‎/‎03‎/‎2018 at 10:04, bobro said:

Do you mean the mount is guided (motors, guiding software, guide camera etc) or the small scope is being used as a finder or polar alignment scope (unguided mount)?

Without guiding 30 sec will be a fairly typical max exposure length before star trailing starts with a P150 on an unguided EQ3. 

I had it set up with a guide scope mounted on the main scope barrel linked into PHD on the laptop. BUT I've since found out I made a few mistakes in the set up, not least after a few images the guide scope had dewed up and I wasn't getting any guiding!

[acefisher]

On ‎21‎/‎03‎/‎2018 at 10:43, Z3roCool said:

I have a Canon 400 5.6 on an NEQ6 mount and get 2.5min exposures and pretty good results. I really want a scope but holding off until I get my skills up and bank account  Not guiding yet either but got the kit for it so will delve into that shortly. Here is a shot of  Orion with it.

 

 

Great shot, I used to have the 400 5.6 its a fantastic lens, I part exchanged mine for a second hand 400 f4 DO and sometimes wish I hadn't . I'm going to take the advice on here and keep practicing with the setup I have and try and get the best out of the kit, when I hit a wall I might then look at buying new stuff starting with a better mount. It could take some time we've had two weeks of solid cloud and two more forecast.