lenses that reach infinity

[mindburner]

hi basically i'm after a DSLR that I can piggyback on my 127mm scope with it's OSC.

I don't want to have to focus it, just put to infinity on say a 50mm lens and take exposures using an interval timer.

I have a Canon 1100d and 1:8 50mm, which needs focused.

I also have a Pentax K-3 and :2:8 35mm macro lens which I have not tried yet.

Anything else people have had success with?

cheers

[MartinFransson]

I don't think you can do it like that. All lenses require careful focusing. It's easiest to do it with live view zoomed in or with a program like BackYardEOS.

Sent from my phone using Tapatalk

[Alien 13]

I had a selection of M42 lenses and adapters and by trail and error and some minor skimming of the inside edge of the adapter got them all to focus at the end stop.

Alan

[ronin]

Most camera lens will focus at and beyond infinity, just ignore the little mark that is likely there to signify infinity, it is likely not accurate enough for point stars. Also I would not expect the edge stars to be pin sharp.

Your problem is that you will have to manually focus a camera lens and then keep it at the position/setting.

The remaining camera functions will need to be manually over ridden as well.

All you are doing is what people do with wide field (Milky Way) images when they put a camera on a tripod or mount.

[kalasinman]

Even with astro work the very wide angle lenses will be tolerant and give a large DOF.

[davefrance]

I use my Canon 18-55mm kit lens set at 18mm for wide field - milky way that sort of stuff. Set on manual focus I turn to its far setting which I know is past infinity and then just ease back a little. I know then the stars will be focussed (well acceptably so). The greater the FL of the lens the more difficulty you will have getting an acceptable image without using live view or taking some test shots first.  

[Stub Mandrel]

For lenses of 50mm or less, focus just before the infinity mark, or stop down (on any lens) and put infinity near the to end of the DOF range.

Longer or unusual lenses can be dealt with by trial and error: (1) a fixed aperture mirror lens that (by trial and error) is at infinity with the line aligned at the 'short' end of the infinity symbol and (2) a wide angle adaptor which used with a 28mm lens means it  focuses infinity at about the 1.2meter mark (but with a lot of latitude) (3) a 135mm lens that , again, focuses just short of the infinity mark.

I suspect eth two long lenses could both be dealt with by skimming the M42 adaptors like Alan has.

[mindburner]

many thanks ll for such great information. I will have a go this evening with my Canon 1100D and the 50mm lens, Get a rough focus with live view and just blast away at F4

[ollypenrice]

When you focus pick a moderately bright star, not a very bright one, and zoom in to pixel scale to focus it. Put the star at the intersection of the one third lines (imaginary lines parallel with the chip edges and a third of the way to the opposite side.) This will make a big difference. I'm afraid there is no way, in astrophotography, of wriggling out of the need to focus!!!

lly

[Alien 13]

As Olly said, with camera lenses on a very bright star you will find that getting good focus difficult because of "star bloat" each colour will have a slightly different focus point what you will see though is that dimmer stars in the FOV will pop into view when best focus is obtained.

The result is that the faint stuff will have good focus and some of the very bright stars will be bloated but that is reasonably easy to fix in software, this issue will be less obvious with better quality lenses.

Alan

[perfrej]

In the old days, say the 60's and 70's and partly into the 80's, many high quality camera lenses were adjusted at the factory to have focus at infinity at the hard stop. I have a few Nikon lenses that are like that. In order for it to work, your DSLR must be as perfect as the old SLRs were, i.e. maintain a very tight tolerance for the chip distance. Somehow I think this is a feature not really cared for today as autofocus DSLRs need lenses that go beyond infinity anyway for the AF to work, and hence the chip distance tolerance is of less importance.

/per

[Macavity]

In the absence of reflector diffraction spikes, I've been trying a FLO Bahtinov mask. 

Notably with an Altair Astro ED66. It works... does seem easier than just eyeballing?

In passing, I notice they do make small Bahtinov masks with camera filter threads.

http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/bahtinov-masks-bahtinov-masken.html

I am experimenting with increasing my field of view with camera lenses. Of course, it

may well be a bit "different" for uz Video Astronomomers. Or maybe not so much? 

(A latest, more upmarket, VIDEO camera lens is shimmed to make infinity "midway"!)

I sacrificed a UV filter - There are a whole host of mask types one might try to make?

Maybe even *create* artistic star spikes. lol. Here's an imaginative use of hair combs:

http://www.petealbrecht.com/blog/blog.htm   (See entry for Nov 22 2009)