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Hi, can anyone help with star detection in DSS. I have been  getting some good images using my Star Discovery 150p and nikon d5100. 

I'm using a 2xbarlow as I know I can't gain prime focus on this OTA and keeping my exposures down to 25secs due to field rotation with it being a Altazimuth mount and this can also be corrected in processing. 

The problem I'm having is that when I put everything in to DSS and register the images, it detects hundreds of stars but when I stack them it tells me there's not enough stars and to change the threshold so it can stack more of the images! 

The threshold is already set all the way to the left at 2% 

I'm using 100 x 25sec exposures iso 1600, 25 x dark @ 25secs iso 1600 and 25 x bias @ 1/4000secs iso 1600

No matter what I do it will only stack the dark, the bias and 1x25secs exposure! 

Is there anyone out there that has had similar issues and can shed some light on this please. 

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There is a slight bit of trailing but not much, and as u can see I have tried my best to reduce this as much as possible. 

The only other thing I can think of is lots more shorter exposures to try and reduce it more but loosing a lot of light this way. 

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Hi,

Well, let me say at the outset that you might be expecting a bit much of the Star Discovery mount, because with a 2x Barlow you are effectively at a working focal length of 1500mm, and with a long tube. With Alt-Az imaging you will get two sources of star deformation, field rotation as you point out, and quality of overall tracking. When I image using an effective FL of 500mm with a beefier Alt-Az mount, the Nexstar SE, and with an exposure duration of 30s, I get a significant proportion of my subs showing star streaking due to mount movement. Your 25s exposures should avoid significant star elongation due to field rotation, so long as you are below, say 60°, elevation, but it may not be enough to avoid the elongation due to mount tracking movements.

DSS will give you a quality factor/FWHM for each sub, IIRC; what sort of range do you get?

It does seem a bit odd though that it is 'seeing' hundreds of stars, yet doesn't recognize stars for stacking. I wonder if the star detection threshold is a bit too low, and you are detecting noise rather than stars? You only need a hundred or so stars for proper function.

Ian

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Thanks i know using a Barlow will slow everything down, my scope is F5 so using the Barlow will increase it to F10 but surely i should be able to still get some sort of image that works?

Also the mount is an upgraded more robust version of the Synscan AZ and far as i can tell my alignment is as close as i can get.

Here's a sub of Pliedes x25secs iso1600 the FWHM is 6.61 #stars is 17

Pliedes-21.09.17.jpg

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Cheers guys will try and gain better focus, unfortunately i don't have a mask and the focuser is a standard Crayford rack and pinion type.

but there is one thing i have learnt in the past few days and maybe should have known already duhhhhh!! is to use the live view on my camera to zoom in and gain better focus just not had a chance to try it yet with the typical Manchester sky!!!!!

I will keep trying!!

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1 hour ago, Ulysses31 said:

Thanks i know using a Barlow will slow everything down, my scope is F5 so using the Barlow will increase it to F10 but surely i should be able to still get some sort of image that works?

It's not so much that the Barlow will slow things down, but the long FL will reveal any mount tracking imperfections much more than, for example, at 500mm FL. And those imperfections will show up as poor star shapes. I take it that you are using a remote shutter? Also, try mirror lock-up, it could be shock from that which is wobbling the 'scope.

41 minutes ago, Ulysses31 said:

but there is one thing i have learnt in the past few days and maybe should have known already duhhhhh!! is to use the live view on my camera to zoom in and gain better focus just not had a chance to try it yet with the typical Manchester sky!!!!!

Yup! :icon_biggrin:. In fact, that is all I use to focus my stars.

Ian

PS. Ah, I see you're in Stretford. Our daughter lives there with her husband and our two grandchildren. We'll be up to do our child-minding for the half term!

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Unfortunately the Nikon d5100 only let's you lock the mirror up for cleaning. It won't function with it locked up, only run in quite mode which takes twice as long to take shots! 

Think I am just going to have to keep everything as tight as possible regarding the mount and try to get better focus. 

I also have a extender which will take eyepieces, I can gain focus from 6-25mm eyepieces. Would I get any better results this way?

 

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Any in camera noise reduction should be turned off, sounds like it is still on as images only take twice as long when in camera noise reduction is on.

Also check mount balance as good balance shouldn't need tight fixings. If too tight then the motors are working too hard.

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As said, the star shapes are not great but this still doesn't make sense if what you say "it's detecting hundreds of stars" is true for each sub. Detecting hundreds of stars and then not stacking because there are not enough doesn't make sense.

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Right thought I'd have another go at DSS running a short stack of M27 10 x30 subs @  iso 1600, 2 darks 30 secs @ iso 1600 and 2 bias 1/4000 secs @ iso 1600 with the star detection threshold at 2%

So it's been running for 50 mins and only registered 3 of 10 subs and has registered over 12000 stars for each sub!! 

Will let you know what the finished product is................. 

Think it could be a destroyed computer?????????????

 

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Registered results were :-

Sub 1 10019 stars

Sub 2 13380 stars 

Sub 3 13281 stars

Sub 4 15236 stars

Sub 5 16063 stars 

Sub 6 18911 stars

Sub 7 19437 stars 

Sub 8 20635 stars 

Sub 9 23116 stars

Sub 10 37844 stars

It's seems to be stacking them but taking forever so of to bed.......... 

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So I managed to get it to stack at last although it was only 5 frames out of the 10 and the end result was not really worth writing home about. 

But at least it's worked, I have tried again increasing the threshold but doesn't seem to want to go over 4% any higher than that and won't detect any stars. 

Will try and get out this week fingers crossed for clear sky's and nail the focus. 

Thanks for everyone's imput think I've managed work it out now ?

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8 hours ago, Ulysses31 said:

But at least it's worked, I have tried again increasing the threshold but doesn't seem to want to go over 4% any higher than that and won't detect any stars. 

I'm pleased that you got it to work, but this does suggest that your stars aren't sufficiently star-like to be counted. I hope you have greater success next time, but this astro- photography lark can be a challenge!

Have you looked through the "No EQ Challenge" thread? It's a long thread but there are folk who've had similar problems.

Ian

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1 hour ago, The Admiral said:

I'm pleased that you got it to work, but this does suggest that your stars aren't sufficiently star-like to be counted. I hope you have greater success next time, but this astro- photography lark can be a challenge!

Have you looked through the "No EQ Challenge" thread? It's a long thread but there are folk who've had similar problems.

Ian

Ian, I can't seem to find this thread it's been mentioned before to me. But doesn't come up with anything when I search for it?? 

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