problems at med/high magnification

[DAT]

The last few days many people (in the UK) have finally been blessed with some clear skies.

I have noticed that several people have gotten some great views of various targets.

I have been struggling to get a sharp image at anything above x 100 (more or less). In my 8" dob (1200m fl), I go from using an 18mm ep to an 11 and unfortunately sharpness decreases significantly.

I leave the scope outside for at least 1 hour although I do not use the fan. It is likely to be collimated well too (some kind souls on this forum and another have confirmed). Can tightening the primary screws too much cause this kind of problem ? ( mine are only finger tight). I have finally managed to locate a couple of globulars so it is very exciting but just can't use anything above x100 to see any details.

Could this be the seeing conditions? I observe from a bortle 9 but have in the past been able to use much higher magnification (x170).

Any help is much appreciated

Edited April 30 by DAT

[wookie1965]

There are many things that can do this one being tight screws I read you should be able to put a thin card in between the mirror clips and the mirror, making sure the clips hug the outside edge of the mirror. Seeing and transparency can do the same thing.

I have often looked at Jupiter with a 20mm eyepiece and could see more detail than a 12mm because of the seeing Jupiter went soft and blurred.

Before thinking there is something wrong give it a couple of sessions first you will know then.

Paul

 

[DAT]

35 minutes ago, wookie1965 said:

There are many things that can do this one being tight screws I read you should be able to put a thin card in between the mirror clips and the mirror, making sure the clips hug the outside edge of the mirror. Seeing and transparency can do the same thing.

I have often looked at Jupiter with a 20mm eyepiece and could see more detail than a 12mm because of the seeing Jupiter went soft and blurred.

Before thinking there is something wrong give it a couple of sessions first you will know then.

Paul

 

Thanks Paul, I personally think/hope nothing is wrong, I suppose I am looking for anyone else observing under similar conditions that may be able to confirm or otherwise. So far this has been the case 2 nights in a row, let's see

[dweller25]

Hello @DAT,

I suggest you slightly defocus on a star at high altitude and then compare the view to this chart which should help determine what your issue is 👍

Edited April 30 by dweller25

[Saganite]

Hi,

If you believe the scope to be well collimated and you have enjoyed powers up to 170x then I should imagine your problem is simply one of poor/variable seeing.  Late last year I recall over several nights being unable to use more than 100x on Jupiter, the detail going soft, and the desirable power of 150/170x being quite impossible, and then it improved.  I would though recommend doing as David suggested, and compare what you see against the diagrams.  If you see something like the top left hand corner, then be sure that all is well and and enjoy whatever power gives you the sharpest image.

[DAT]

1 hour ago, dweller25 said:

Hello @DAT,

I suggest you slightly defocus on a star at high altitude and then compare the view to this chart which should help determine what your issue is 👍

Very useful pictures there, I will hopefully confirm tonight but tests I have do look like tube currents

[dweller25]

2 hours ago, DAT said:

Tests I have do look like tube currents

Use the “Fan” Luke 🤣

Edited April 30 by dweller25

[Carbon Brush]

This is a good excuse to buy a second scope😁. Something completely different like a long FL refractor.
If both scopes give poor views, blame the sky. Or maybe get a 3rd scope just to get another opinion?

Seriously though. The sky quality does vary by location and sometimes by the hour.
I have had 'bad' scopes that have fixed themselves a couple of hours later.
I have also had scopes that worked great early on and apparently degraded later.

Don't go jumping in and looking for problems until you have consistently seen the same problems (like collimation) a few times.
Tube currents vary over seconds. Atmospheric turbulence varies by the second. Pinched optics change slowly with temperature. Collimation is permanently present.

HTH, David.

[DAT]

Thanks for all the responses. It’s obviously difficult for me to differentiate between bad collimation (although I am ruling this out), bad seeing and tube currents. However, I have had the same problem several times in a row and at the next opportunity I am going to try putting my scope further away from the house as I am wandering whether heat coming off the building could be affecting the  view. 

[dweller25]

39 minutes ago, DAT said:

Thanks for all the responses. It’s obviously difficult for me to differentiate between bad collimation (although I am ruling this out), bad seeing and tube currents. However, I have had the same problem several times in a row and at the next opportunity I am going to try putting my scope further away from the house as I am wandering whether heat coming off the building could be affecting the  view. 

And put the scope on grass not concrete as that holds heat

[Carbon Brush]

Yes away from the house. A west facing brick wall can be a good early evening radiator.
Tumble drier, bathroom, kitchen or boiler vents are awful air disturbance devices. Yours or others nearby.
Thermals fom a roof some distance away can be problem.

Then of course if you or a neighbour are growing 'certain substances' in your loft that require lots of warmth and didn't invest in insulation🤣
I believe some such installations have been located by police helicopters using infra red cameras!

[DAT]

Well the problem is not observing too close to the house, or on concrete or anyone growing certain substances. Took the scope out 1+ before I started observing, on a wooden base on the grass 5+ metres way from the house and was tested the view on targets straight.

From looking at the diagram above I am not really sure what is going on. I would like to rule out collimation as defocused stars appear as nice concentric rings. This issue has been going on a while and I know in the past I had not collimated the optics properly. Whilst I thought it could be tube currents, the scope was well acclimatised. 

I have loosened the locking screws on the primary and brought it down ever so slightly to see if that makes a difference.

The sky seems clear from where I observe but even on moonless nights,  it is very bright (Wembley stadium is not far off)

However, I know from before Christmas and from viewing with a 130p last summer, that it is possible to see a lot more detail.

I can only make out a few stars in m67 and I think I could just about see one or two in m3 with averted vision although that could be wishful thinking.

That has to be one of my favourite targets so far, spectacular even considering all of the above

 

[dweller25]

You have moved from a 130mm scope to a 200mm scope, in my experience the larger scope will be more affected by seeing conditions.

The Jetstream is often over the UK and it will negatively affect the views in a telescope.

Yesterday was quite windy and that definitely affects seeing.

If you cannot get a high star to focus to a stable view then atmospheric stability is probably the issue and you just have to wait for better conditions.

I have been in this position many times and just packed up !

[DAT]

2 hours ago, dweller25 said:

You have moved from a 130mm scope to a 200mm scope, in my experience the larger scope will be more affected by seeing conditions.

The Jetstream is often over the UK and it will negatively affect the views in a telescope.

Yesterday was quite windy and that definitely affects seeing.

If you cannot get a high star to focus to a stable view then atmospheric stability is probably the issue and you just have to wait for better conditions.

I have been in this position many times and just packed up !

Thanks, with the 18mm still getting good views, just a bit frustrating when there has been so few clear nights not to be able to go higher. With the same scope and the same location I have managed to get x170 so I will persevere