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Yet another DSS 1 image stacked cry for help


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Hi,

If possible I need some guidance and pointers as I cant get DSS to do anything but tell me only one image to be stacked. I have searched forums and tried a million recommended settings and now im hoping that you can give me alternatives. Im wondering if its a focus issue as the celestron 6" OTA doesn't have a fine focus dial.

AVX mount, polar aligned using Polemaster. All good.

Run starsense, works great. dead centre, all good.

Attach DSLR and focus using backyardEOS FWHM method. Can onlu get a low value of about 9 and seems to fluctuate a lot. (tried Bahtinov mask but cant get spikes, just a blob.)

Take preview images, Look ok.

Take stack of 30s exposures and as far as I can see they aren't blurred. I should be able to get batter than 30s with a polar aligned AVX mount (?)

Tale darks

Process in DSS --> only one can be stacked.

Any thoughts? Ive had 4 sessions now, and no stacking able to be done. Can a more experienced eye tell me if its a focus issue?

 

 

LIGHT_30s_3200iso_+21c_20170313-20h48m59s862ms.CR2

LIGHT_30s_3200iso_+21c_20170313-20h50m13s512ms.jpg

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Hi there Martin,

5 minutes ago, MartinFlower said:

(tried Bahtinov mask but cant get spikes, just a blob.)

To see the spikes you may need to ramp up the ISO/gain. It depends how bright the star your focusing on is.

8 minutes ago, MartinFlower said:

Process in DSS --> only one can be stacked.

Have you tried to alter the star detection threshold? If this is set too high then no stars will be detected and DSS will fail to stack significant fames. I can't remember exactly where the dialogue box is for this...if memory serves me correctly then after you select register and stack it is in advanced options.

HTH Dan :happy7:

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Looking at your other images I think it would not hurt for you to do a star test next clear night. Re fine focus you might like to read up on the peanut butter lid mod for the focuser I'll see if I can find a link later. You've got nice gear to work with.

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It is due to focus and star size. If you look at the 1:1 size of the image stars are huge and a bit oddly shaped - so DSS just thinks its some sort of nebulosity rather than stars.

You try two things: First, of course is to work on your focus to get as tight stars as possible. Second, and it can be done to material that is already acquired - try binning / lowering resolution of your shots to get smaller stars on 1:1 image (order of 10px across or less).

HTH

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So consensus is its a focusing issue.

 

Im in a fairly heavily polluted area (doncaster backyard). My FWHM values seem to vary, so could this be due to pollution? On the OTA, there is only one focusing dial - will look up peanut butter lid method !

cant see how tracking can be so bad unless I have a faulty brand new avx mount?? Polar aligned using polemaster seems bang on.

 

 

 

 

 

 

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5 minutes ago, spaceman_spiff said:

Yes I also see there star trailing there. Maybe shorter exposures would be better. Also, try to find a bright start to focus on...such as Sirius ,the Bhatanov mask spikes should become apparent.

Im concerned that more experienced eyes than mine think im getting trailing on 30s exposures? with a polar aligned avx mount

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Are you trying to focus on a bright star, if not l suggest you do so I.e. a really bright star, you should be able to get spikes.  I find FWHM jumps around too, so l always use a Bahtinov mask.  The stars are slightly oval rather than round so maybe that's why DSS is having problems

 Carole

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Trailing can be caused by couple of reasons. One is polar alignment - in this case stars are elongated in DEC direction. It can be Periodic error of mount - in this case it's not quite trailing, but rather stretching in RA direction (sometimes mount is just a bit ahead of wanted location in other times it lags behind wanted position - so overall effect is star smear in RA direction).

Then there is seeing (severe cases like imaging over houses that burn fuel for heating and have chimneys), and also wind that can cause misshaped stars.

Also note that you are imaging at fairly big focal length (1500mm if I'm not mistaken), coupled with modern DSLR (smallish pixel size) you could be imaging at around 0.5"/pixel - that is really demanding both on mount and on seeing. You should aim for around 2"/pixel at this stage (hence idea to bin x4 your subs for example), and move on to higher resolution with enough experience and confidence in your gear.

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7 minutes ago, vlaiv said:

Trailing can be caused by couple of reasons. One is polar alignment - in this case stars are elongated in DEC direction. It can be Periodic error of mount - in this case it's not quite trailing, but rather stretching in RA direction (sometimes mount is just a bit ahead of wanted location in other times it lags behind wanted position - so overall effect is star smear in RA direction).

Then there is seeing (severe cases like imaging over houses that burn fuel for heating and have chimneys), and also wind that can cause misshaped stars.

Also note that you are imaging at fairly big focal length (1500mm if I'm not mistaken), coupled with modern DSLR (smallish pixel size) you could be imaging at around 0.5"/pixel - that is really demanding both on mount and on seeing. You should aim for around 2"/pixel at this stage (hence idea to bin x4 your subs for example), and move on to higher resolution with enough experience and confidence in your gear.

Thanks for that. I should be bang on polar aligned due to polemaster. PEC is a possibility.

Im using a 0.63 focal reducer to enlarge the FOV, as you say the 1500m is pretty demanding.

 

Can you explain for a newbie what 'bin 4x your subs" entails?

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35 minutes ago, MartinFlower said:

Thanks for that. I should be bang on polar aligned due to polemaster. PEC is a possibility.

Im using a 0.63 focal reducer to enlarge the FOV, as you say the 1500m is pretty demanding.

 

Can you explain for a newbie what 'bin 4x your subs" entails?

Binning of pixels is term that originated from CCD cameras - they have an option to collect signal from adjacent pixels into a single "unit". This for effect has - increase in S/N ratio, but also lowers resolution.

Process can be emulated in software (hence often name software binning) - by using specific form of reducing size of image - summing or averaging adjacent pixel values. When you normally reduce size of image, you have selection of "filters" - like bilinear, bicubic, lancoz, in most common image processing software. Binning is a bit like that, except it is yet another way to reduce image size (in integer steps, so you can only reduce image 2x2, 3x3, 4x4, etc ...). In your case reduction in image size will help DSS figure out what the stars are (it has expectations on stellar image size to be in certain range). You can reduce your images with any conventional filter, but best way to do it for astronomy imaging is by binning because it increases your signal to noise ratio in certain way that will benefit quality of result.

DSS already has a form of binning - in RAW section of preferences -  there is last option "Create superpixel" - it works for color frames that need debayering in such way that it uses a form of binning (2x2) to create color image from raw data. Unfortunately there is no option to set higher bin values (not that I'm aware of). So if you don't have enough processing experience this might be a bit to much, but I'll explain the process in any case.

1. You would need to debayer your raw images manually (well not really by hand, but rather using some software that has this capability) to create 3 sets of images from each sub - R, G and B image

2. You would then need to bin each of R, G and B of each frame to a certain size (I think that good value would be around 4x4 - so 4 times smaller subs).

3. You would then do a stack of each R, G and B channel separately (R images for each sub go into one stack, Blue go into another, and G as well).

4. Align R,G and B stack and compose RGB image in some software and process as you normally would.

It might seem a bit too much to do, but I do something like that on a regular basis for each of my shooting sessions (and a bit more, since I calibrate my images by hand as well).

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You say in the first line of your post that you need guidance and that is probably what you need, a guide camera.

Long focal length imaging needs a solid base, presume you're using a tripod, if so don't extend the legs fully and make sure it's on a solid base.

Never  managed to get FWHM focusing to stabilise using SCT usually experiment to get it right.

As said above you need the distance from the focal reducer to the camera sensor about right, celestron do an adapter for this.

I don't think your mount is faulty, you really need guiding at that focal length despite what the manufacturers may claim :grin:

Try doing a 5 minute exposure on a bright star to get a better idea of what's going on.

Dave

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Some advice: do the registering, computing offsets and stacking in separate stages in DSS. Do not use the batch option.

(1) After registering the list of files will show you the number of stars detected. If any of these are less than 8 then DSS will not stack that frame (it may not stack some which say more than 8, but it will definitely throw all those with 7 or less out). So if there are some you need to lower the % detection threshold and try again. I would also not have more than a few tens of stars myself but this probably isn't the problem in your case.

(2) Computing offsets - after this stage you will get the dX and dY filled in. If any of these say NC then that frame will not stack. You need to go back and try and get more stars detected on that frame, either by re-registering or by using the star editor (red cross icon on lower RHS of the image display), then redo the compute offsets.

(3) Only once you have managed to get a dX and a dY for all the frames you want to stack should you push the "Stack checked pictures button".

Finally, if you really cannot get enough (8) stars in common between the frames then try the superpixel mode in the debayer section.  This is much better at detecting blobby or trailed stars.

NigelM

 

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Could you check what version of DSS you are using. I had this ages ago and it turned out I was using an older version. Updating DSS solved it.

 

Edit: Sorry sure from memory I was using the wrong shortcut icon on my Desktop.

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8 hours ago, Davey-T said:

You say in the first line of your post that you need guidance and that is probably what you need, a guide camera.

Long focal length imaging needs a solid base, presume you're using a tripod, if so don't extend the legs fully and make sure it's on a solid base.

Never  managed to get FWHM focusing to stabilise using SCT usually experiment to get it right.

As said above you need the distance from the focal reducer to the camera sensor about right, celestron do an adapter for this.

I don't think your mount is faulty, you really need guiding at that focal length despite what the manufacturers may claim :grin:

Try doing a 5 minute exposure on a bright star to get a better idea of what's going on.

Dave

Its an avx mount using a C6 SCT as the lens. not a separate telephotyo lens.

 

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ah. THanks all for the advice everyone.

  • set superpixel on in dss (the only thing different !)
  • detection threshold set to 2%
  • #stars detected are n the 20-40 region across the light file list

This time it gave me multiple files to stack , so it looks like the superpixel setting. Does this make sense?

 

The final picture came out very green through pre photoshop. Is this an expected thing?

. I will do the star test tonight, it looks pretty clear at the moment.  Im using the celestron T adaptor and the celestron reducer and give the chtinov mask a 3rd go

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