Finding explanations for visual 'issues'

[McEloff]

Hi,

I've now had my 250px dob for a few weeks and managed to get a few good sessions out of it, but there has been times when the viewing was not as good as I wanted, and I couldn't figure out why... so I was hoping I can be pointed to a resource to explain the reasons for it? 

How do I tell whether I am being influenced by collimation, focus, atmospheric conditions, dew or light pollution etc? I know this a very broad question, but was hoping there was a resource somewhere that I can look at that explains that if x happens, then it is likely because of y, if that makes sense?

As an example, when viewing Mars again this weekend I ended up with a 'cloudy' circle all round it, which I thought may be dew, but none was obviously visible on the secondary (and it went away with dso's). If it was light dew, it may have been amplified as I was pointing the scope over some houses, so maybe the light pollution had more of an effect than when pointing it straight up?

Fwiw, I'm in Bortle 5 skies, I let the scope cool down at least an hour and use ES 82's (6.7, 11 & 24) and I believe I am collimated (I do check with a cheshire). 

Thanks for any help!

[jetstream]

Very light high cloud can cause the haze around bright object and observing over buildings causes wobbly images- if you add both together the views could be challenging IMHO.

[kev100]

Hi there. You’re correct that any or all of the factors you mentioned could contribute to the issue you’re having. Collimating is probably the easiest thing to eliminate. Through the Cheshire, if you can see all the mirror clips equally, the cross hairs align and the centre spot is ... well ... centred, you’re collimated ... You could also check by looking at a bright star .... does it resolve to a pin point , with sharp and clearly defined diffraction spikes?

Dew can definitely be an issue, but if you can’t see it on the secondary (it never occurs on the primary in my experience), then have you checked the eyepieces themselves, being the only other place it could accumulate? As the issue doesn’t occur on dsos, though you don’t mention which, I’d say dew is unlikely to be the cause. Be careful not to breathe on the EPs ...

Stray light pollution/reflections could be the issue. Is your dob is a solid tube version (if not, do you use a light shroud?). Try a different/darker location.... (I know, easier said than done).

I’d say the issue is some combination of atmospheric seeing conditions, plus the fact that Mars is incredibly bright at the moment, and all that light is scattered somehow (you could try fitting the dust cover to the scope and pulling off the small aperture cover to restrict the light coming into the scope...)

I spent much of last night viewing Mars with my (same) scope and from a pretty dark site, and although it was bright and clear a lot of the time, it did shimmer and wobble around horribly at times, with stray light artifacts coming and going. My eyes were watering quite a bit due to the blustery winds  ...

Kev.

Edited September 21, 2020 by kev100

[Mark at Beaufort]

I have found that transparency has not been the best over the past week. To assess how good visual observing can be - look at Ursa Minor and count the number of stars.

[SiriusB]

"As an example, when viewing Mars again this weekend I ended up with a 'cloudy' circle all round it."

        That sounds like high haze, not very aesthetic but not necessarily bad for viewing planets. 

You can often get steadier seeing with some haze or even an Autumn mist,which sometimes give excellent seeing.

For faint fuzzies it's a bit of a kiljoy though.

[Ships and Stars]

Hello @McEloff, I live in NE Scotland in your neck of the woods and mainly view DSOs such as nebulae and galaxies. I've really struggled the past two weeks with conditions - there's been lots of haze and thin, high cloud that's not easily detectable at night and it's not something a normal weather forecast will clock - they will show clear skies. Well, clear enough you can see quite a few stars and think it's good, but in fact it's been really soupy. That kills transparency for nebula hunting!

A sky quality meter helps me with obtaining solid data on conditions. I was at a remote, almost pristine 21.92 SQM spot I know well last week for two nights - was really looking forward to it. However, I was getting readings of 21.3 under 'clear' skies, just soup. A few nights ago I went to a new 21.94 spot according to the LP map atop a hill at 500m and got 21.55-21.64, really was looking for 21.75 plus, but that's the best I've had so far this observing season and I've already racked up a ton of miles driving all over the place - very frustrating. The Veil through the 12" dob was excellent at that spot, but I didn't stay long (30-45 minutes) as I had a silly drive back home! Didn't feel worth it in the end and I was beat the next day.

It's been quite warm and humid here lately, I think that's a big part of the issue with transparency, hoping the colder weather ahead will clear things up.

If you can get your hands on a Unihedron Sky Quality Meter with lens (SQM-L) they are very useful for sussing out atmospheric conditions on a given night, and if that dark sky spot is really as dark as the map says. The Light Pollution map might show a certain spot at say 21.5, but in reality it will be lower to some degree depending on atmospheric conditions and local light pollution. Things get darker around midnight as more people and businesses switch off lights. A lot of people use NELM or naked eye limiting magnitude, but for me personally, that's too subjective and dependent upon my eyes and level of dark adaptation. Still, it's a decent benchmark if you don't have the meter. 

As mentioned above, these conditions could still be good for planetary observations, but I like a bitterly cold, clear night with little to no humidity in the air.

As @jetstream says, looking over buildings can cause some wobbles because of heat thermals rising off of them. Open grassy spaces are best in my opinion. Collimation doesn't sound like the issue, and just shine a red torch on the secondary or primary to check for dew. My secondary is usually the first one to get dew on it.

Colder weather on the way no doubt! 👍

 

Edited September 22, 2020 by Ships and Stars

[McEloff]

Thank you very much everyone. 

18 hours ago, jetstream said:

Very light high cloud can cause the haze around bright object and observing over buildings causes wobbly images- if you add both together the views could be challenging IMHO.

It's possible this added to it, it was quite clear (from what I could see) and there is only a handful of houses between me and Mars but it all adds up...

18 hours ago, kev100 said:

Hi there. You’re correct that any or all of the factors you mentioned could contribute to the issue you’re having. Collimating is probably the easiest thing to eliminate. Through the Cheshire, if you can see all the mirror clips equally, the cross hairs align and the centre spot is ... well ... centred, you’re collimated ... You could also check by looking at a bright star .... does it resolve to a pin point , with sharp and clearly defined diffraction spikes?

Dew can definitely be an issue, but if you can’t see it on the secondary (it never occurs on the primary in my experience), then have you checked the eyepieces themselves, being the only other place it could accumulate? As the issue doesn’t occur on dsos, though you don’t mention which, I’d say dew is unlikely to be the cause. Be careful not to breathe on the EPs ...

Stray light pollution/reflections could be the issue. Is your dob is a solid tube version (if not, do you use a light shroud?). Try a different/darker location.... (I know, easier said than done).

I’d say the issue is some combination of atmospheric seeing conditions, plus the fact that Mars is incredibly bright at the moment, and all that light is scattered somehow (you could try fitting the dust cover to the scope and pulling off the small aperture cover to restrict the light coming into the scope...)

I spent much of last night viewing Mars with my (same) scope and from a pretty dark site, and although it was bright and clear a lot of the time, it did shimmer and wobble around horribly at times, with stray light artifacts coming and going. My eyes were watering quite a bit due to the blustery winds  ...

Kev.

I'm fairly certain my collimation is good, I see the clips and spot in the centre, but will check again tonight. Thank you very much for mentioning diffraction spikes! I googled it, and I do see it, it was actually one of things I wasn't sure whether its an issue or not. Glad to know its a good thing!

I've got the flextube, but do have a shroud on it. My backyard is fairly dark... just a few lights I need to look across when viewing Mars, so that may well have affected it. My comparison for that night was M57, M81 & M31 all of which came out quite well. I did have dew on my Telrad, so it was the right conditions for it, but I actually never checked my eyepieces. Will keep an eye out for it next time. 

18 hours ago, Mark at Beaufort said:

I have found that transparency has not been the best over the past week. To assess how good visual observing can be - look at Ursa Minor and count the number of stars.

On a good night I am easily on a 5, but will make a point now of using this as a reference point

[McEloff]

13 hours ago, SiriusB said:

"As an example, when viewing Mars again this weekend I ended up with a 'cloudy' circle all round it."

        That sounds like high haze, not very aesthetic but not necessarily bad for viewing planets. 

You can often get steadier seeing with some haze or even an Autumn mist,which sometimes give excellent seeing.

For faint fuzzies it's a bit of a kiljoy though.

I'll just have to keep on experimenting to see what works for me on Mars (and make sure the other variables are not affecting it). Its a few weeks till it's at opposition, so hopefully I can make the most of that if its a clear night.

 

7 hours ago, Ships and Stars said:

Hello @McEloff, I live in NE Scotland in your neck of the woods and mainly view DSOs such as nebulae and galaxies. I've really struggled the past two weeks with conditions - there's been lots of haze and thin, high cloud that's not easily detectable at night and it's not something a normal weather forecast will clock - they will show clear skies. Well, clear enough you can see quite a few stars and think it's good, but in fact it's been really soupy. That kills transparency for nebula hunting!

A sky quality meter helps me with obtaining solid data on conditions. I was at a remote, almost pristine 21.92 SQM spot I know well last week for two nights - was really looking forward to it. However, I was getting readings of 21.3 under 'clear' skies, just soup. A few nights ago I went to a new 21.94 spot according to the LP map atop a hill at 500m and got 21.55-21.64, really was looking for 21.75 plus, but that's the best I've had so far this observing season and I've already racked up a ton of miles driving all over the place - very frustrating. The Veil through the 12" dob was excellent at that spot, but I didn't stay long (30-45 minutes) as I had a silly drive back home! Didn't feel worth it in the end and I was beat the next day.

It's been quite warm and humid here lately, I think that's a big part of the issue with transparency, hoping the colder weather ahead will clear things up.

If you can get your hands on a Unihedron Sky Quality Meter with lens (SQM-L) they are very useful for sussing out atmospheric conditions on a given night, and if that dark sky spot is really as dark as the map says. The Light Pollution map might show a certain spot at say 21.5, but in reality it will be lower to some degree depending on atmospheric conditions and local light pollution. Things get darker around midnight as more people and businesses switch off lights. A lot of people use NELM or naked eye limiting magnitude, but for me personally, that's too subjective and dependent upon my eyes and level of dark adaptation. Still, it's a decent benchmark if you don't have the meter. 

As mentioned above, these conditions could still be good for planetary observations, but I like a bitterly cold, clear night with little to no humidity in the air.

As @jetstream says, looking over buildings can cause some wobbles because of heat thermals rising off of them. Open grassy spaces are best in my opinion. Collimation doesn't sound like the issue, and just shine a red torch on the secondary or primary to check for dew. My secondary is usually the first one to get dew on it.

Colder weather on the way no doubt! 👍

 

 

Thank you, every day is a school day. It is very possible that there was haze/high cloud around, I just need to make sure I cover all the other variables and then learn what affects the rest. I'm lucky in that I've got farms on 2 sides, but have a handful of houses just below where Mars sat.

According to an online light pollution map I am only at a 20.4 but its good enough to keep my son and I busy for now and it will get better in winter of course. I've not tried to quantify the NELM here yet, but will make a point of it next time.

I'm sorting a battery out at the moment so that we can become mobile and then go look for darker spots. We've got friends who live in a 21.7 area, so planning a trip with the scope there, and I see Slains castle being mentioned as a possible spot, and seeing as that is a bit closer we may take a drive there soon. 

 

Edited September 22, 2020 by McEloff

[kev100]

1 hour ago, McEloff said:

My comparison for that night was M57, M81 & M31 all of which came out quite well.

Hiya,

The fact that dimmer objects appeared okay makes me think that Mars' brightness scattering through high cloud/haze could have been the issue. I was out on Sunday night, on what appeared to be a good and clear night. Whilst the stars and planets looked great, there was definitely some haze in the air as I had absolutely no luck when it came to faint galaxies ... 

Kev

Edited September 22, 2020 by kev100