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sshenke

Refractor suggestions please

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36 minutes ago, carastro said:

Yes, but in the winter I was inside an observatory with dew heaters sometimes wrapped around the tube either side of the focusser as a precaution. 

Are you absolutely sure it is the primary that is dewing up, i.e. you have looked down the tube at the primary? 

Carole  

 

Yes, absolutely, i have attached a picture i took a few weeks ago looking down the ota

20191027_204926.jpg

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4 hours ago, sshenke said:

i understand what you are saying. i wondered about that too, as i cannot see what's going on with the secondary mirror. however, it is absolutely clear that the main mirror dews up within a maximum of 1 to 2 hours. I have been using a fan with heating option to blow air from behind the main mirror, not directly down the tube from the front end

20191120_174930.jpg

Don't be so harsh on yourself.  That's actually pretty nicely done job of wrapping the tube.  Too bad it didn't mitigate the dewing.  You must live in a very damp region.  Do you often get morning fog?

Do you get any dewing on the guide scope's objective lens?

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5 minutes ago, Louis D said:

Don't be so harsh on yourself.  That's actually pretty nicely done job of wrapping the tube.  Too bad it didn't mitigate the dewing.  You must live in a very damp region.  Do you often get morning fog?

Do you get any dewing on the guide scope's objective lens?

Thanks, well for what it's worth, I have wrapped the tube, even if it looks absolutely ridiculous!! I had checked the guidescope objective on last Monday and actually it did not have any dew on it.

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This topic has just reminded me of my endless fights with dew on refractors, reflectors and SCT's.

Nothing really worked better for me than a long, cheap, black art drawing paper dew shield held on with elastic bands. Tried it out of desperation once when at a school outreach session and it stopped the sct from dewing up. The soft black drawing paper came from their  art dept! Worked better than any purchased dew shield!

Worst scope for dewing up I have at the moment is a truss RC. the main mirror goes almost as soon as the secondary. If I heat the secondary the main mirror still curtails observing. Open tubes in this country are a liability.

Don't imagine a refractor will be immune, I have had issues with these as well, though to be fair they take longer to dew up if they have decent dew shields. Once again the long art paper dewshield worked with them too even if they had long metal dewshields.

Ian B

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Interesting that the guidescope does not dew up - is it heated ? They are usually the first to dew up !
You did a great job lagging the scope but both ends are open - I would suggest you make a dewshield for the front of the scope and seal the rear of the scope once the mirror has cooled.

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I think dewing depends to some extent on location. At home I hardly ever have dewing problems with my refractors but at my society observatory (about 6 miles from my home) I need to use dew shields.

SCT's and maksutovs need dew prevention even at home though.

My Kruppax tubed 130mm frac seems to be totally immune from dewing - apparently something to do with the properties of Kruppax.

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10 hours ago, astronomer2002 said:

This topic has just reminded me of my endless fights with dew on refractors, reflectors and SCT's.

Nothing really worked better for me than a long, cheap, black art drawing paper dew shield held on with elastic bands. Tried it out of desperation once when at a school outreach session and it stopped the sct from dewing up. The soft black drawing paper came from their  art dept! Worked better than any purchased dew shield!

Worst scope for dewing up I have at the moment is a truss RC. the main mirror goes almost as soon as the secondary. If I heat the secondary the main mirror still curtails observing. Open tubes in this country are a liability.

Don't imagine a refractor will be immune, I have had issues with these as well, though to be fair they take longer to dew up if they have decent dew shields. Once again the long art paper dewshield worked with them too even if they had long metal dewshields.

Ian B

Thanks Ian, i am going to try the diy dew shield as well

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1 hour ago, dweller25 said:

Interesting that the guidescope does not dew up - is it heated ? They are usually the first to dew up !
You did a great job lagging the scope but both ends are open - I would suggest you make a dewshield for the front of the scope and seal the rear of the scope once the mirror has cooled.

Thanks. I left the rear end open, so as to use the fan with or without heating mode, in case the wrapping didn't work. Will close it off as well and see if it works. Thanks. B

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9 minutes ago, John said:

I think dewing depends to some extent on location. At home I hardly ever have dewing problems with my refractors but at my society observatory (about 6 miles from my home) I need to use dew shields.

SCT's and maksutovs need dew prevention even at home though.

My Kruppax tubed 130mm frac seems to be totally immune from dewing - apparently something to do with the properties of Kruppax.

I am not sure if it is anything to do with location. I live in  the Midlands ( milton keynes). 

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18 minutes ago, sshenke said:

I am not sure if it is anything to do with location. I live in  the Midlands ( milton keynes). 

If you live/image/observe  near water you will get bigger issues with dewing.

Edited by dweller25

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28 minutes ago, sshenke said:

I am not sure if it is anything to do with location. I live in  the Midlands ( milton keynes). 

I was referring to more localised factors such as proximity to water, height above sea level, etc.

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Thank you both. there are no water bodies nearby and i can't think of any local factors that might impact on dew formation.

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58 minutes ago, sshenke said:

Thank you both. there are no water bodies nearby and i can't think of any local factors that might impact on dew formation.

Are you in a local low spot?  I've noticed on my drive in to work that ground fog and frost form much more commonly in a low spot along a small creek, even when it is dry.

I'm so glad my Dob has a Sonotube tube after following this thread.  It also never gets cold to the touch in the depths of winter, either.  It's just heavier than thin walled metal tubes.

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On 21/11/2019 at 10:44, John said:

 

My Kruppax tubed 130mm frac seems to be totally immune from dewing - apparently something to do with the properties of Kruppax.

Yep-something about Kruppax isn’t there?! Magic stuff, doesn’t seem to matter what the temperature is or whether you move the scope from somewhere cold to somewhere warm or vice versa, there’s not a drop of dew. 
 

Not entirely sure how the tube material has such an effect on the glass of the objective but it works. 
 

To OP - the corrector lens on my SCT dews up before any other glass on any of my scopes and I simply can’t shift it. Very frustrating when you’ve got everything set up nicely and then your evening is ruined. 
 

One suggestion-have you cleaned the primary? Dust etc will exacerbate dew issues as the water has something to saturate and cling to. 

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10 hours ago, dannybgoode said:

Yep-something about Kruppax isn’t there?! Magic stuff, doesn’t seem to matter what the temperature is or whether you move the scope from somewhere cold to somewhere warm or vice versa, there’s not a drop of dew. 
 

Not entirely sure how the tube material has such an effect on the glass of the objective but it works. 
 

To OP - the corrector lens on my SCT dews up before any other glass on any of my scopes and I simply can’t shift it. Very frustrating when you’ve got everything set up nicely and then your evening is ruined. 
 

One suggestion-have you cleaned the primary? Dust etc will exacerbate dew issues as the water has something to saturate and cling to. 

i was thinking about cleaning the primary mirror as well, although it doesnt look too dirty.

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Hello all, I have been struggling to obtain good images for some time now, which I have asked help with for , as in the following post:

https://stargazerslounge.com/topic/342321-poor-image-quality-cause/

I thought that dew was the problem, hence I posted in this thread discussing about the merits of buying a refractor instead. Previously I was absolutely certain that there was dew on the primary mirror, but I am not sure now. In fact the dew that I noticed on the primary mirror was seen as soon as I had brought the telescope indoors, so it is very likely that the dew had condensed on the surface when it was brought indoors rather than while it was outdoors.

Last night has been another disastrous (sorry for the pun!) session where I could hardly see the M31, this is really perplexing to me. Interestingly, the guidescope seems to produce better image than teh main camera as you can seen from the attached images: The ASI 120 mini is the camera attached to the guidescope and the ASI 1600MM pro is the main camera attached to the telescope- skywatcher 130PDS. For the same exposure time, or even less, the guidescope camera collects more light than the main camera. I am guessing that this shoudl not be the case? I wonder if anyone can shed light on this problem please. Thanks in advance

 

120mini_3secs.jpg

120mm_as_main_24 secs.jpg

1600mm_13sec.jpg

1600mm_35 secs.jpg

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55 minutes ago, sshenke said:

Hello all, I have been struggling to obtain good images for some time now, which I have asked help with for , as in the following post:

https://stargazerslounge.com/topic/342321-poor-image-quality-cause/

I thought that dew was the problem, hence I posted in this thread discussing about the merits of buying a refractor instead. Previously I was absolutely certain that there was dew on the primary mirror, but I am not sure now. In fact the dew that I noticed on the primary mirror was seen as soon as I had brought the telescope indoors, so it is very likely that the dew had condensed on the surface when it was brought indoors rather than while it was outdoors.

Last night has been another disastrous (sorry for the pun!) session where I could hardly see the M31, this is really perplexing to me. Interestingly, the guidescope seems to produce better image than teh main camera as you can seen from the attached images: The ASI 120 mini is the camera attached to the guidescope and the ASI 1600MM pro is the main camera attached to the telescope- skywatcher 130PDS. For the same exposure time, or even less, the guidescope camera collects more light than the main camera. I am guessing that this shoudl not be the case? I wonder if anyone can shed light on this problem please. Thanks in advance

Screen capture suggests that you are using ASI1600 in 8bit mode and that is not going to produce good results.

image.png.0d1154c57d8fbd1ec850074a36c2e2ef.png

Switch to 16 bit mode and examine image stretched to see captured detail.

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Have you considered making your own dew heater? I have major issues with dew on the secondary, but  loop of resistors drawing around 2 watts, wrapped in shrink tube and attached to the back of the mirror with electrical tape, solved the problem. Going the DIY route also let me use finer wire, so there is no real impact on the image. I imagine something similar could work for the primary?

Billy.

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Thanks Vlaiv and Billy, I will switch to the 16 bit mode tonight and see if that makes a difference. What do you think about the histograms of the asi1600 and asi120, in the images posted above?. I never get a good histogram with the 1600.  as for the dew heating, yes i have used a circular heating pad ( they use it for brewing!!!,) to the back of the primary and it stopped dew from forming, although now i am wondering if dew was present in the primary at all.

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For ASI1600 and histogram, you really need to examine resulting 16bit fits to see if there is something wrong with it.

It will also depend on gain and offset settings that you used. It would be best not to mess too much with those and leave gain at unity (139) and offset around 50-60 (I personally use 64).

12bit range that ASI1600 operates on is not quite as large as 14 or 16 bit of some DSLR and most CCD cameras, but it is still quite large range. You can't expect histogram to be the way you are used to in daytime photography or when you open regular image in photoshop. That does not mean it is bad.

You also need to understand that sub from ASI1600 is going to look rather dark if not scaled in intensity or stretched. That is quite normal and does not mean that camera is bad or malfunctioning. For reference, here is single sub from my ASI1600, histogram and what is really captured for comparison:

image.png.ef7caffec0007be5550a56dd1a038864.png

There seems to be almost nothing in that sub (it's single calibrated Ha sub 4 minutes long).

Histogram also looks very poor - almost "flattened" to the left:

image.png.6b83c8998727b964864d5ad32183ef80.png

But in reality, that histogram is fine, if we "zoom in " on important section of it, you will see that it looks rather good:

image.png.d61ac9466f2b63bcb7746cddfaca6981.png

It has nice bell shape to it with right side being a bit more extended and "thicker" - meaning there is some signal there in the image. Don't be confused by negative sign in this histogram on the left - it is in fact calibrated sub, so dark frame has been subtracted.

Resulting signal in the image when properly stretched is this:

image.png.af5af06943119f2771e79401606322e0.png

As you see, there is plenty of detail in single sub there, although it will not show without stretching.

Here is one sub from the same set, but this one is still in 16bit mode and not calibrated:

image.png.f850bada9f792fc1f06a125063204679.png

You can see that image looks noisier and there is amp glow to the side (all of which calibrate out), and histogram is "bar like" - that is because image is still in 16bit mode uncalibrated  (unlike above one which is 32bit mode calibrated).

Moral of this is - don't judge camera output and quality by what your capture application is showing unless you know what to look for - you need to examine what subs from your camera look like when you stretch them and when you calibrate them to see if there is something wrong with them or if they show enough detail as display in capture application can be misleading.

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