DSLR imaging help please!

[Mattjohnson1978]

Can anybody help explain what's going on with my images after processing? If I try with my limited PS techniques to remove the odd glow, I lose detail in the spiral. They are 20x 75 second exposures at 800 ISO.

[Mattjohnson1978]

[carastro]

It looks like vignetting, but it's a bit lop sided, so not quite sure.

Did you do any flats? If it's vignetting the flats should remove that. It's a pig of a job to process images without doing flats.

AV 100 ISO point at a diffuse not to bright white light - same focus - do not move optics. Take about 15 and stack. Subtract them from your light frames using your stacking software.

If it's not vignetting I'll shut up and leave it to some-one else.

Carole

[Greg]

Dew on the front lens?.

[Shibby]

I agree with Carole this will be down to a gradient within the image, most likely due to vignetting. Flats will cure the problem but it's probably too late for that - which is where gradient removal tools come in. I use GradientXTerminator in PS; I think there is a free trial version available...

[Mattjohnson1978]

Thanks for the replies! I must admit, I don't think I've been giving Flats the credit they deserve. I am still very new at this and as I don't possess a light box, the manual (or bible) that I have been reading from suggested that the next best thing is to use a single bias shot and change the normalisation to 5000 and use that as the Flat. Do simulated flats work and if so, I'm now guessing 1 simply isn't enough?

Thanks again

[Black Knight]

I don't have a lightbox either. I take flats using my laptop screen (running Notepad maximized) with a T-shirt placed over the front of the scope and held in place with a rubber band .... no high-tech stuff for me! . I usually go for around 40. I think the normal rule is to aim for around 30-60% luminence on the histogram.

[Mattjohnson1978]

Sounds like an inspired solution, Andy! Same ISO and duration as the lights? With the short nights and the amount of time I'm still spending polar aligning, 40 flats will outnumber the amount of light frames I actually get in, but is more better?

[Black Knight]

Well although I take 5 minute subs, I'm lucky if I can grab 2 hours worth at a time. Which means my flats still outnumber my light frames! Either way I don't think I'd take less than 40. If you use software like BYEOS or APT it's quite straightforward to set up a sequence and then just let it run. Takes 10 minutes at the most .

[Black Knight]

Sorry I missed your other question. I take flats at the same ISO as the lights (normally 800 ISO), but many opt for a lower ISO. For exposure length I set the camera to AV and go from there. As I said, you should aim for around 30-60% luminance on the histogram.

[carastro]

I don't have a light box either, but I do have the luxury of an observatory so I leave everything set up and take them the following day using dull daylight and a piece of typing paper (made into a lens cap), to diffuse the light. The light mustn't be too bright or you'll saturate the flat and then it won't do any good at all. Andy's T Shirt and elastic band sounds a good idea too with notepad maximised. I did hear of some-one once who took down their rig but left their camera in place on the de-rigged telescope so they could take flats the following day by dull daylight.

I use 100 ISO and switch to AV and this seems to give the right exposure. Don't move your optics or focus or it will all be formothing.

Meanwhile artifical flats will do a rough job, but I never had any success in making one. Here is a link on how to make one.

http://www.corius.ne...c/artiflat.html

Carole

[andyboy1970]

I had a quick play and I agree with the earlier post, I think its dirty optics or dew rather than vignetting.

There is lots of detail in there but it gets overpowered by the blotch.

Andy

[Mattjohnson1978]

Thanks Carole. I've not played with the AV functions yet so hopefully it will all make sense when I give it a try.

Andy, do you think it might be dirty optics etc because the blotch appears off centre? I have serious doubts over the collimation and I'm wondering if vignetting would appear like that on a poorly collimated scope? It has been very humid recently so I can't rule out dew I guess. In my set up, it is just primary and secondary mirror and then my camera attached direct. Any recommendations on cleaning those parts?

Thanks again

[andyboy1970]

Do you have a light pollution filter in the optical train? Could be that.

The mirrors have to be really filthy to effect the image that much, however flats will help.

[Mattjohnson1978]

Ok, so I got my scope back out and took some light frames and have had some success!

After creating the master flat, I immediately loaded it into PS and whilst playing with the levels, I saw a very similar pattern forming that I have been getting on my subs, and originating from the same place. The focus had been reset since my last session so I put it roughly in the same place, just to use it as an exercise, but feeling confident that on my next session, the correctly focussed flat frames will help a lot.

Doing a direct comparison between my original stacked file and my new stack which includes the flat frames, I still get a bit of the blotch but I can now get the image a lot darker before losing detail on processing. Even if it doesn't solve my problem completely, I have been taught the value of flat frames!

Thank you all so much for your input. This forum and its members remains one of my most valuable resources :0)

Matt

[RikM]

Flats are a great help to processing. I use the same ISO as my lights but a much shorter exposure so the peak on the histogram is about 1/3 the way across.

The secondary on my 150P dews up something chronic, so I have a foam mat dew shield on it that is a huge help. Normally when it dews the image slowly dims until it's just black though. It doesn't give a bright patch like you have here so I suspect vignetting as well.

Collimation is a must with imaging. Visually, your eyes can compensate for a quite a bit of mushiness and there is a tendency to assume it's poor seeing. The 150P is quite easy to get nailed though using a Cheshire.

[Mattjohnson1978]

I was apprehensive about the collimation but bought myself a cheshire and gave it ago. I just lined the cross hairs up with the centre dot on the primary and left it at that. I could see the whole of the primary off the secondary so i assumed the secondary must be aligned already and just used the adjusters on the bottom till it all lined up. Everything I read on the internet contradicted with other stuff I read...very confusing! Does what I did sound right? And is this something I should be checking regularly? My scope does get moved about a bit, but not knocked or anything.

Matt

[RikM]

I use the sight tube part of the Cheshire to look at the edge of the secondary and adjust until this is central in the tube. Then use the three secondary adjusters to line the Cheshire cross hairs up with the doughnut / ring on the primary. Finally use the primary adjustment knobs to line the black dot up with the doughnut. This last bit it the most critical. You might find that adjusting the primary, throws the secondary out again, so just work through it a few times and it will get closer each time. You should end up with the secondary lined up with the sight tube circle and the cross hairs lined up with the doghnut and the dot. (make sure you line up with the Cheshire cross hairs, not the reflection of the spider vanes!)

[Mattjohnson1978]

I remember using bits of coloured card behind the secondary and in front of the primary to help visually see if the secondary was central in the ep holder, and to me it looked fine. I then used the 3 adjusters for the primary to line the dot up with the ring. Would I be right in assuming that if the secondary was out of alignment, I wouldn't be able to see all 3 retaining clips of the primary after getting the black dot in the ring?

[RikM]

Not necessarily. This 'seeing all the clips' is a bit of red herring really. I can never see all the mirror clips because I have a long Cheshire sight tube. If you take the eyepiece out and just look down the focuser tube then you can see all the clips but you have no way of knowing if your eye is exactly in the centre so you could be miles off. If you have a little colli cap or film canister with a 2mm hole in the exact centre then it might be a way of getting a rough secondary alignment, but the best bet is just to use the sight tube part of the Cheshire and then the crosshairs to line it all up properly.

[Mattjohnson1978]

When I bought the collimator, premium cheshire one from FLO, I must admit I was expecting some kind of directions of use but it kinda just turned up in a box lol. I have only been using the pinhole for lining up the primary. I take it the polished angled plate you see through the side that has the etched cross hair is for the secondary mirror?

Apologies for the simple questions lol

[RikM]

Have a read through Astro Baby's tutorial. http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm She explains thing much better than I can. The 'mirror clips' bit here is using a colli cap though.

[Mattjohnson1978]

Thanks Rik. I am off work next week and plan to replace the "glue-like" lubricant in my EQ3-2 mount and generally try and refine all the motor drives so I will add that on my to-do list. I was also planning on building a DIY guiding system but the webcam I was going to use isn't good enough to register any stars. I have since found out it is a CMOS not a CCD sensor...and its a few years old now too. Just out of curiosity, with good polar alignment, what sort of exposure times can you get on your EQ3-2?

[RikM]

With my 150P and 1000D on board I could get 60 sec easily, 90 sec most of the time and 2 min sometimes if the target object was high enough. Wind is the killer though. I lost about 1/3 to 1/2 my subs so windshear

I never stripped or regreased mine. I had heard that some people who have regreased their mounts with Li-grease have had problems in very cold weather? TBH my EQ3-2 has always worked a treat. If you click on the deep sky images link in my sig, the picture comments should say which mount I used in each case. I still use it for white-light imaging with an ST80 or widefield stuff with just a DSLR. Very underrated mount and I like mine very much.

[Mattjohnson1978]

I will check them out. Thanks again bud and enjoy your weekend