DSLR decides I have dark skies then changes its mind/Issues with finderguider flexture

[JR1987]

Ok so last night some impromptu clear skies appeared (totally contradicting the weather report, as per usual yet in a good way for once!)

So i rushed out and set up hoping to try and get more subs to add to my sunday session on the horsehead with my newly modded dslr. 

I should point out at this point that on sunday I discovered I could keep the histogram low enough (around 25%) on 300second exposures which I was really happy about, And wondered why I hadnt been able to before (however usually I was at iso1600 and this time I was going for 800 so I put it down to that) Got a number of subs but had to throw most away due to guiding errors.. and I also noticed that the nebulosity was still quite faint in the individual subs on the camera screen, even though the histogram was at a comfortable level.

So anyway I go out last night, go for 300seconds again, and boom its totally blown out. Even though the scope is exactly the same, camera is the same, same settings, exactly the same target, scope is in exact same spot that it was, the same time (around 22:00). 

Makes no sense to me! Its like my camera was imaging on a lower ISO even though it said 800. 

But then I tried to go down to ISO 400 to see if that would shift it down and yes it did. 300 seconds and the histogram peak is pretty much where it should be AND funnily enough there is SO much more data in the 400 subs from last night, you can clearly see the horsehead nebula in the single subs on the camera.. Which has made me question if I should be imaging at 400 anyway.

Has anyone else had this kind of issue before? Ive included pictures below as proof! 

Also as a smaller question I keep having ever so slight guiding issues that I think is down to flexture in my finderguider.. The adaptor from modern astronomy doesnt quite fit tightly enough on the finder and has a little bit of wiggle room. has anyone got any remedies to make this a little more secure?

[D4N]

Only times I have had issues like that are when I have made a mistake such as using the wrong filter or leaving the bahnitov mask in place.

/Dan

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[ArmyAirForce]

Probably down to sky conditions. I shot this video with a camera lens. At about 16 seconds, the sky goes from pink, to purple/blue, as high very thin cloud cleared, reducing the amount of scattered light pollution.

https://www.youtube.com/watch?v=6VXfdXYLFyw

[JR1987]

Probably down to sky conditions. I shot this video with a camera lens. At about 16 seconds, the sky goes from pink, to purple/blue, as high very thin cloud cleared, reducing the amount of scattered light pollution.

https://www.youtube.com/watch?v=6VXfdXYLFyw

Wow! that drop at 17 seconds. Clearly that was bedtime. I would put it down to light pollution if it wasnt within 2 minutes difference in hour. I always use my IDAS LP filter. thats the only one I use. 

[MarsG76]

I too find that exposing at ISO400 tends to reveal more nebulosity. I put that down to being able to expose the subs longer before the sky glow becomes a problem and so by the longer physical exposure capturing more of the actual nebula photos.

As far as changing sky glow in your same time same ISO exposures is generally normal at urban locations at the 22:30 time. You would find that the sky glow would start to go down again after midnight as people go to sleep and a substantial number of lights gets turned off.

[Uranium235]

From night to night, the sky conditions have a big impact on the quality of your subs. If there is more high cloud/mist, then that will reflect more LP and therefore send your histogram right. Also, it depends on where you are pointing. If you are pointing straight up, you are less likely to be affected by changeable conditions as you have less atmosphere to look through.

If your finderguider has a bit of wiggle in it, you could try using some PTFE tape (the stuff plumbers use) on the threads - that will pack it out a bit. Also dont forget to vecro up all your cables to remove any cable strain/dragging (you want nothing pulling on the guide cam).

[JR1987]

If your finderguider has a bit of wiggle in it, you could try using some PTFE tape (the stuff plumbers use) on the threads - that will pack it out a bit. Also dont forget to vecro up all your cables to remove any cable strain/dragging (you want nothing pulling on the guide cam).

Well It just so happens I have some PTFE on the van. Why didn't I think of that!? Thanks for the good idea! 

I expect I will do some tests at the same exposure and Iso to see what happens with regards to the different nights, see if I get more changes..

however until then I think Ill give imaging at 400 a go. When I think about it, the ISO is only making the camera amplify the signal more.. its not actually making the sensor any more sensitive. So surley the same amount of photons are falling on the sensor, so in theory we can just 'amplify' them with the histogram stretch.. without so much noise.