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DSLR imaging issues - help please


LunarLight
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Hey guys

I am having some problems taking images with my DSLR camera. Its probably the case that my polar alignment is not fully aligned but just in case its not I have a question.

On a unguided but motorised set up what is the best ISO setting and exposure time to use for DSO to prevent star trails? I have been using ISO400 or 800 and 60 seconds but the stars have slight trails and are a little bloated so DSS does not recognise them as stars which is incredibly annoying after a night of imaging to find out you cant use them :eek:

Any advice would be hugely appreciated :evil6:

Mark

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Mark, that's a really difficult question to answer. The maximum exposure time is going to be a factor of mount weight handling, and setup. You need to work out the longest you can get for your setup. Check the balance, you want it slightly weighted in the direction that the mount is lifting, so if counterweights are down, you want that slightly (not much) heavier. Aim to get as accurate a polar align as you can, do you have a polar scope that's setup ? If the stars look a little bloated, that might be the focus isn't quite right. Do you have an example frame ?

With my HEQ5, I was on average able to get 2 minute subs, but on one very good night I was able to get 4 minutes, and one night (I didn't tighten the mount head fully) I was only able to get 45 seconds.

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Are you using the camera with it's lens piggyback on the telescope ( ie wide angle images) or using the camera body on the focuser to give prime focus imaging?

I'm assuming you're using the EQ5 mount with an RA drive?

Ken

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On a unguided but motorised set up what is the best ISO setting and exposure time to use for DSO to prevent star trails? I

The ISO setting won't help with trails.

Assuming your camera parameters (23.6mm x 15.8mm sensor with 4288 pixels per row) and your telescope parameters (1000mm focal length) we get 1.1 arcseconds per pixel. ( Calculator for DSLR Astrophotography )

In 60 seconds, stars on the equator would move 60*15 arcseconds, or 820 pixels. Your star "size" would be about 4 arcseconds so say about 3-4 pixels. Your tracking then needs to be good enough to keep star sizes at the same level so that would mean accuracy of 4 parts in 820 or 1:200. That is quite difficult and needs very good polar alignment and no bad kinks in the periodic error (during those 60 seconds) and no wobbles from wind, etc.

Basically, you are imaging at too long a focal length or too small a pixel. Try reducing your images by a factor of 4 in both directions and feeding the smaller versions to DSS.

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jgs001 - ah so its kinda different each time you set up the scope depending on how well you have balanced the telescope?

I do have a polar scope, as for if its set up right im not to sure. When I polar aligned I had polaris in the circle on the reticule so perhaps its not set up right? Im only just starting to understand the stuff I am reading about polar alignment...seems a little complicated lol! Though I eventually have some time off this week so I will look at it properly.

yeah here is an example of one of the frames, lol ignore the terrible light pollution, unless you think that might be why DSS cant stack it

astronomy2002100781b.jpg

Merlin66 - im using a Nikon D300 attached to the focuser for prime focus. Yes thats right I am using a EQ5 mount with RA and DEC motor

Edited by LunarLight
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This might help... Astro Babys HEQ5 Polar Alignment... It's designed for the HEQ5, but it's gotta be similar. That'll help setup the polar scope and thus provide more accurate polar alignment. Looking at the image, the stars do look a little blobby, so may be a minor focus issue, I'm not sure... hopefully someone can confirm shortly. What's your focusing process ? Your camera has liveview, are you using it ?

One thing I do notice, but I've no clue what it means, having never used a newt... looking at the diffraction spikes, you have a double set at top and bottom of the larger stars. Whilst I've no clue what it is, I can't think how it can be good for imaging though.

You've also got some sensor dust, but that's easy to deal with, either by taking flats (although that's a whole new ballgame) or sensor cleaning kit :eek:

Are you shooting in raw or jpg ? you need to be shooting in raw or you lose a lot of data in the compression to jpg and that's not gonna help.

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Eagleseye - hmm I am sure that im just making a fundamental mistake somewhere. Being new to both photography and astronomy its proving quite the challenge ha ha

jgs001 - Thanks for the link I will check it out :-). I have been using live view on my camera though I read yesterday (after taking the images) I need to tighten the focus lock, perhaps that could be it.

ha ha I noticed that to! I have no idea what those double spikes are, never seen them before. This is the first time its ever been like that, could it be the effect of ice or frost on the mirrors? it was freezing when I took the photos

sensor dust... I didnt know that, could I ask how you found that just for future reference

I shoot in raw format :eek:

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For the sensor dust, you've got a couple of darker blobs in the centre, top half of the frame. Scope really show them up. If you really want to check, stick a lens on the camera, use a flash so you don't get bored waiting, set to manual, on both camera and lens, zoom the lens to maximum, set the aperture to maximum (mine go up to f32 at full zoom) and take a shot of a white object (make sure you're within the min focus distance on the lens). Sensor dust will show as small dark dots.

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should I reduce the image size and quality settings on the DSLR before taking the pictures or is it just a case of resizing the images after capture before stacking?

Keep shooting at the highest resolution but resize them before feeding to DSS if you have problems with fat/oblong stars. You can resize them with one of the programs that came with your Nikon, I should think.

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Oh yes. Make sure the camera is set to uncompressed Raw (NEF) files on the camera menu. I also get it to save a basic JPG as well, so I can review the images before I put them into DSS. Don't resize the images before you stack them in DSS, or you will lose hard won data. There is a slider bar in DSS which controls how many stars the software recognises while registering and stacking. Don't worry too much about a few specks of dust on the sensor at this stage. Get used to taking the images and stacking them in DSS first. You can progress onto flat fields, dark frames and all that malarky once your comfortable with the basics.

It is a challenge, but well worth the effort once it starts working.

Dave

Edited by Dave
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jgs001 - ah nice thanks for the tips I will give that a try :-)

themos - see I am aware that it has that function but I dont really know how to use it. I have tried using the edit star function to add stars but when I click on the stars nothing happens it doesnt add them or anything, I have also tried adjusting the bar along the top, the star detection threshold and the various options under register and stack and they still come out with a rating of 0.0% and allows only 1 frame to be stacked for some reason. Im sure its just me doing something wrong to be honest lol!

Its my mums DSLR ha ha I will have to get the instructions and software off her :-D

Eagleseye - indeed I am sure I just need to practise it, just wish there was a few nights where I could get out in a row. Think next time im gonna try a few different exposure times and stuff see what I can do.

I see you have a D300 to, how do your pictures turn out mate?:eek:

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I have also tried adjusting the bar along the top, the star detection threshold and the various options under register and stack
I have found that the worse the light pollution the lower you have to set the threshold in the DSS register stars box before you actually pick up any stars. Have you tried going to just 10% or so (lower percentage means more stars)?

NigelM

Edited by dph1nm
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Looking at the photo you put up I didn't think it was so bad DSS couldn't use it. Shows what I know.

What does worry me is the double diffraction spike John mentioned. If it's just down to focus then no problem I suppose. If it's not then the effect is like the Barhtinov mask. Two of the spider arms aren't 180* to each other.

Have you by any chance tried to offset the secondary by pulling the secondary away from the focuser therefor bending the spider out of line ?

Dave

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themos - I might look into this and see what I can do.

Eagleseye - hey those pictures are awesome! I hope one day to be able to take pictures like that

dph1nm - wait so the star detect threshold means that if you set it lower then it detects more stars? oh maybe that the problem there. I will try fiddling with that when I get back home

davew - oh now that sounds worrying!!! :) no I have never touched the secondary mirror at all. Its a pretty new telescope so if there is something wrong with the secondary mirror then im worried.

I did notice that when collimation my scope the other day as I moved the telescope slightly the collimation point (the red laser dot) randomly moved back and forth as if the secondary mirror was loose... could that be the problem at all? if so I can tighten it up yeah?

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Hi Mark,

Relax for now. When I said worried I meant from an aesthetic point of view. Like, who wants double diffraction spikes ?

All spider arms should be exactly straight, under reasonable tension and at 90* to each other. If you get a straight edge and hold it directly above each opposing pair of arms they should be straight and at 180* to each other. No bow or slight V. Does that make sense ?

The red dot moving around is probably another issue. Is the secondary loose ? If it is, you were lucky to get any photo at all. Do you mean if you rotated the laser in the eyepiece holder it moved ? In that case it would probably be a loose fit. Nip up the eyepiece clamping screws.

Dave

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Eagleseye - hey those pictures are awesome! I hope one day to be able to take pictures like that

Well, you have the same camera as me. So no reason why you shouldn't be able to take images like mine, if not better...

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davew - hey Dave, thanks for the advice on pointing me to the mirrors and spiders. I honestly believe that's what the issue was. I checked and the collimation was WAY out so I realigned the mirrors etc and checked the spiders. Indeed some were not exactly straight. I'm not sure why that happened... Probably from when I dismantled my scope and ran with it upstairs once when it suddenly started pouring rain. :-)

hopefully that will sort out the star bloating (making it easier to focus ) and the weird double diffraction spikes ... But need to wait for a clear night to check... So sometime next year at this rate, what with the weather and all!!

Eagleseye - fingers crossed mate :-) by the way loved the videos you made, especially the star party one lol! Good to see a fair few cans were drunk :-D no party is complete with one ha

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davew - ha ha as frustrating as it can be its is so addictive its unbelievable ha ha :)

Seriously though thanks for the help, this has solved weeks of frustrations. I should be able to get some good pictures now :-)

dph1nm - ha ha thanks for that advice on DSS to man, I realise the mistake I was making and can now stack my images thanks

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