Relatively inexpensive lens for Canon for widefield

[Outcast]

Currently I have my Canon EOS1100d piggy back mounted on a 80mm ST refractor whilst I work on developing my EQ mount skills & refine my polar alignments.

It's ostensibly there to add weight to the rig so I can achieve balance in RA, otherwise my poor little ST is far too light for the behemoth Celestron counterweights.

Since it's there, I've been using it for test shots to see how my PA's and tracking are going...

It's currently got a 55 - 250mm kit zoom lense attached which is a bit heavy for the way I have the camera mounted (single stud up the tripod thread, lens not supported).

Since I've decided not to purchase the Nikon d7000 I mentioned in another thread, I was thinking of maybe finding a relatively inexpensive prime lens for widefield shots.. happy to look secondhand & it doesn't have to be a canon brand lense, just a reasonable quality one for crop sensor... but, would like to keep it under $200 if possible.

Any suggestions on focal length, brands.. Reasons...?

Cheers

[raymo]

A great lens for widefield is the Canon "nifty fifty" 50mm f/1.8. Get a good used one on ebay around $100-120 last time I looked. Yongnuo do a cheap clone of it about $75-85 new, surprisingly good value for money, also on ebay. I mount my 55-250 on my little nano tracker,
which is in turn mounted on the tripod via the standard 1/4" thread although I rarely use for astro as it is too slow.
raymo

[assbutt94]

Canons 50mm f1.8 is a great lens. 120-130 ish new, should be cheap 2nd hand. Small and light even with the lens hood. I dont think ive used mine for astro stuff though.
It was my first lens not in a kit.

[Outcast]

Thanks guys

I had been thinking about an 85mm f1.8 but, a quick look online shows they aren't exactly in my budget, even second hand.

I will have a look for a 50mm f1.8; I understand on a crop sensor they give you the 35mm equivalent of 85mm in true focal length.. is that correct?

Cheers

[Ukastronomer]

he has a 55 so not really wide angle

check this out

https://www.ephotozine.com/article/top-10-best-budget-wide-angle-landscape-lenses-2018-32535

SAMYANG

[Outcast]

Thanks Jeremy,

I also have the 18 - 55mm kit lens.. I might have been a bit off in my statement about widefield..

I was thinking of something midway between the 55 & 250mm & substantially faster.. Not widefield in the sense of terrestrial photo's I know but, I was thinking of widefield in the sense of wider than my 80mm ST Refractor & definitely wider than my 8" SCT. Not sure if that makes sense & happy to be advised differently if I'm headed down the wrong path.

I also have a little Sony RX100 which is a cracker of a camera for true widefield, 24 - 135mm Carl Zeiss zoom & 10x digital zoom on top of that, 24Mp, light weight & reasonably fast at f1.8 to f4.. It is my goto camera for terrestrial shooting now as it fits in my pocket & the resolution allows me to use the digital zoom to frame shots then go back to optical zoom for the shot & then bring out the feature I want in post processing with pixellating the shot. It really meets my modest needs & avoids me carrying a DSLR body & a bag of lenses.

Cheers

[raymo]

Hi Jeremy, there are several factors to consider here.
Firstly, budget of around $200; hard to find a decent prime lens shorter than 50mm for that price, even used.
The 55-250 is about twice the length of the 50mm, and more than twice the weight[which Carlton remarked upon.]
For AP a lens is not usually used wide open because of optical aberrations,
usually at least one stop down, and often more. The 55-250 is wide open
at 55mm at f/4, so would normally be used at f/5.6 or f/6.3, which is glacial
compared to the 50mm used at f2.8 or f/3.5.[it is actually acceptable at f/2.5].
The 50mm is also slightly sharper than the 55-250.
raymo

[Camelopardalis]

Another vote for the thrifty 50....I picked one up last year during a sale for $125 new. Works well with the 1100D, although you want to stop it down to >f/3.2 to somewhat tame the coma/astigmatism...although you can’t get rid of it entirely. In the bang for buck stakes, it’s hard to beat.

Apparently the 40mm is a teeny bit sharper and better controlled in the corners, but I’ve not used one.

[Camelopardalis]

It’s one of those quirks of the photography world, crop factor and all that jazz. Since photographers are used to a certain FOV from a given focal length with a full frame sensor, they use that as the yardstick, even though far more cameras are sold with different sized sensors than full frame...

The reality is that the FOV is dependent on the sensor size and focal length of the lens/scope, as follows:

FOV = (sensor size / focal length) X 57.3

This is more handy, because once you have more than one camera / sensor / lens / scope you can calculate the FOV quickly and easily for any combination without trying to figure out what a crop factor means :lol:

Similarly, you can get your theoretical resolution by substituting the sensor size for pixel size, and then multiplying the answer by 3600 (since there are 3600” per degree).

Just remember to keep your units constant...5.2 microns, for example, is 0.0052mm.

So for the thrifty 50 and 1100D:

FOV = ( 22.2 / 50 ) X 57.3 = 25.44 degrees

and resolution = ( 0.0052 / 50 ) X 57.3 X 3600 = 21.45”/pixel

[raymo]

The 50mm f/1.8 in question will give the F.O.V. of an 80mm, but not
the magnification.
raymo

[Capt_buscemi]

Hey that's an awesome little summary, thanks for that post!



Been slowly setting up a rig with a couple of Samyang lenses, the venerable 14mm/f2.8 for nightscapes and the 135mm/f2.0, I've been wondering exactly how to get FOV calculations. In the case of the 135mm I'll use it with SGP and now I can input this data to do platesolving.

[xelasnave]

Hi Calton
Just go with what you have and look for a sweet spot...it may work well at fl 80mm..whatever..but that is the first best step.
What you need is an appropriate counter weight so you can mount the camera on the mount just by itself.
Piggy back works but not so good..I found I could not piggy back run the main scope as well..the vibration seemed to great for my liking...
I made a great counter weight for my BWM (Bogan Widefield Mount☺) using a toilet roll center with sinkers set in resin. ..its easy to do..set up the camera on the mount and hang a bag of sinkers to get the weight then make a mold..toilet roll or similar with a central tube for the shaft drop in the sinkers and cover it with resin...paint it black and it looks flash..I added a nut so as to have a fastner but that did not work out ( in could but some resin got on the thread) ...but I hold it in position with some blue tack which works great.
Put your savings aside for the one day I will go narrow band☺.
Alex

[Merlin66]

Mark,
Have a look at CCDCalc - it allows you to input your lens/ telescope data as well as the camera (pixel size and sensor size)
It then shows the FOV and compares it with various images.....
http://www.newastro.com/book_new/camera_app.html