Double Cluster and a Focus Problem

[Knight of Clear Skies]

After having some success with my Canon 1100D/50mm lens/fixed tripod combo I had another test run this week, trying to go a little deeper. This time I targeted the Double Cluster, opening up the aperture to f2.2, taking nearly a hundred 3.2 second subs and adding some bias files to the darks I took the previous session. Unfortunately I ran into a problem, looking at my subs the focus is fine on the first one but gets progressively worse with each exposure. I suspect the action of the shutters or perhaps the wind is enough to shake the focus ring out of position. It's very progressive (every subsequent frame is given a lower score by DSS), so the shutters could be causing it. 

The 1100D doesn't appear to have a focus lock option, does anyone know if there is a way around this please? Could software control lock it even though the camera doesn't have that option? If I can't lock focus I'll just have to refocus frequently. I'm thinking of buying a tracking mount, so if it is the shutters this will be less of a problem if I can take fewer longers subs. I don't think the lens is defective as such, I have to shake the camera body pretty hard to get the focus ring to move at all, but perhaps it isn't as tight as it should be. Any help would be appreciated.

Here's the result of stacking 17 or so subs. Pretty poor, but at least it is recognisable.

[Daniel-K]

did you lock the focuser? what scope are you using

[Knight of Clear Skies]

No scope, it's just my DSLR with a 50mm lens sitting on a fixed tripod.

[Daniel-K]

are you using a shutter release cable? focusing with a lens can be a real pain with out BYE. http://www.backyardeos.com/ i use to use this this way you can monitor your image and your focus

[glowingturnip]

I'm not really experienced in this area, but I believe that modern DSLR's aren't particularly designed with the focus ring in mind, they're designed to be used with autofocus.  That's why the focus ring always feels a little sloppy (at least on mine), and I think even the weight of the lens alone is enough to move the lens out of focus over time.

I've seen suggestions of masking tape over the focus ring to hold it in place.  I dread to think how sticky your lens will get though !

[frugal]

Have you checked to see if your lens is misting up? As the lens hood on my wide angle lens is not very long the front of the lens mists up easily. It is not such an issue with the 300 mm lens as the lens hood is a lot longer.

Sent from my SM-N9005 using Tapatalk

[Langy]

As suggested masking tape or maybe a bit of blue tack could help to lock it in place.

I have to watch this on my 300mm lens as it is so easy to adjust.

[Knight of Clear Skies]

I'm not really experienced in this area, but I believe that modern DSLR's aren't particularly designed with the focus ring in mind, they're designed to be used with autofocus.  That's why the focus ring always feels a little sloppy (at least on mine), and I think even the weight of the lens alone is enough to move the lens out of focus over time.

I've seen suggestions of masking tape over the focus ring to hold it in place.  I dread to think how sticky your lens will get though !

That might do the trick thanks, although I can imagine applying the tape or blue tack without moving the ring could be a bit of a nightmare.

Have you checked to see if your lens is misting up? As the lens hood on my wide angle lens is not very long the front of the lens mists up easily. It is not such an issue with the 300 mm lens as the lens hood is a lot longer.

I don't think so, it didn't feel like fogging-up weather and my car was mist free when I went back to it. But I didn't actually check, so that is a possibility thanks.

are you using a shutter release cable? focusing with a lens can be a real pain with out BYE. http://www.backyardeos.com/ i use to use this this way you can monitor your image and your focus

Yes, I need to look at BYE or APT next, but I'd also like to be able to image with as little kit as possible if I don't have my laptop with me. Focussing with the 50mm on liveview isn't too bad as I can see very faint stars, it's much more tricky with the kit lens.

[frugal]

I don't think so, it didn't feel like fogging-up weather and my car was mist free when I went back to it. But I didn't actually check, so that is a possibility thanks.

Remember that your camera is generating a small amount of heat and it is the difference between the two sides of the lens that can cause the condensation. I have been out in what I think are dry conditions, where the car has been fine but the front of the lens keeps fogging up. At one point I put a pause between frames in Backyard EOS just so I could wipe the lens clear

Sent from my SM-N9005 using Tapatalk

[Knight of Clear Skies]

Thanks for the tip. Thinking again, given how hard I had to shake the camera to get the focus ring to slip that's a real possibility. I don't have a hood for my 50mm, might that help?

[frugal]

Thanks for the tip. Thinking again, given how hard I had to shake the camera to get the focus ring to slip that's a real possibility. I don't have a hood for my 50mm, might that help?

A lens hood will certainly help. The only problem I found with my 18-135mm lens is that the lens hood needs to go out at a wide angle so that it does not appear in the field of view, and a short wide hood will be less effective than a long narrow hood (I almost never get dew on my 300mm zoom lens which has a longer lens hood).

I ended up getting a dew heater to put around the bottom of the dew shield a couple of weeks ago, but unfortunately it has been so cloudy that I have not had a chance to play with it yet ;(

[ollypenrice]

Telescope Service make these in three sizes.

They are on the infernal side, being spindly and sloppy of thread, but they do work and allow both a finer focus and a more durable one. My brief foray into lens imaging was greatly enhanced by its use.

Olly

[Knight of Clear Skies]

It looks a bit like a torture device for lenses that have been bad. Could be handy thanks.

[mcinek82]

I'm not really experienced in this area, but I believe that modern DSLR's aren't particularly designed with the focus ring in mind, they're designed to be used with autofocus.  That's why the focus ring always feels a little sloppy (at least on mine), and I think even the weight of the lens alone is enough to move the lens out of focus over time.

I've seen suggestions of masking tape over the focus ring to hold it in place.  I dread to think how sticky your lens will get though !

When it comes to focus, sometimes a proper software can help a lot. Eg. in digiCamControl (for Nikon) you can send commands to the lens to shift focus. That's a great improvement as you don't touch the setup and don't cause any vibrations. I suppose BYE has similar function - you could try it.

But I'm not sure if it helps anything with shifting focus during imaging (though the experience itself is pretty strange).