More advice, please.

[Terrierist]

Hi, All.

I live in a suburban part of Liverpool, as such, light pollution is quite heavy.

Thanks to the advice of members on this forum, I have grabbed a couple of images with the limited equipment (AP wise) I have. My equipment is - Skywatcher Mak-Cass 127 mm, AZ Gti Az/Alt and Nikon D 7500

I've been struggling to get sharp images and I think that is down to trailing, I've been setting exposure times far too long and this has led me to some disappointing results, I understand that it won't be the best system long term, but it is what I have now.

Last night I shot 40 lights, 10 darks and 12 bias trying to catch Soul Nebula and after uploading to Astrometry.net I found I was a little bit off, but still on a part of the nebula. The 40 lights were shot at 800 iso, 10 second exposure and the 'scope has 1500mm at f11.6. I am satisfied with the focus and sharpness of the stars compared to previous images, but disappointed overall with what I set out to attempt to capture.

There was no nebulosity present in the image, am I correct in thinking that with such short exposure times the nebula would not be visible, would light pollution have anything to do with this, also? 

I'd be grateful for any advice, the image is attached with some reference stars marked and the FITS file is here as a Dropbox link if someone could take a look at it for me. The FITS file was generated in astro pixel processor 

 https://www.dropbox.com/s/pvwf8xm9ejoikg6/St-avg-400.0s-NR-x_1.0_1.0_dr-hat-full-eq-add-sc_BWMV_nor-AA-RL-noMBB.fits?dl=0

Next time out I think I'll go 15 seconds ISO 1600 with twice as many exposures and evaluate from there, I really think that an EQ mount and shorter focal length 'scope will be coming in the future.

Thank you in advance for any advice given.

 

Kev

[Tim]

Hiya,

You are really going to struggle to get anything of note on the Soul Nebula, it isn't the brightest even with a hydrogen sensitive camera and faster scope.

Perhaps best to concentrate on targets with brighter elements? 

[happy-kat]

Yes I would try a different target, have you seen the No "eq" DSO Challenge thread? You may be adding noise with those darks, I prefer to use flats and dark flats with my dslr, darks for me add noise.

Given the level of DSLR you own so you have any lens too? You could try to camera with one of your lenses anything up to 200mm could work well.

[Demonperformer]

As said, a brighter target will be easier. 40 lights @ 10 seconds is less than 7 minutes. People will take 7 hours of data using an Ha filter (which increases the contrast with the sky) on an object like this.

You can take images of difficult objects using very basic equipment (I have seen a photo of the horsehead nebula taken using just 2-sec exposures) but that is not the easy way. An EQ mount with a faster scope and a red-sensitive camera makes a world of difference.

[happy-kat]

Telescope is optional the have been cracking images of the H&S using a fast 135mm camera lens.

[almcl]

I have had a go at your .fits file, and it looks as though the sky glow has rather overwhelmed the signal.  There's also quite an amount of vignetting and an odd artefact at the top right corner. 

If you are able to take flats these will help with the vignetting and any dust bunnies, but the sky glow might require a light pollution filter.  I had a similar experience (living in a heavily light polluted suburban area) and the use of a clip filter certainly helped.  Not sure what's available for your Nikon, perhaps someone else can advise?

The other thing is that for large targets like the Heart & Soul, a long focus camera lens (as Happy-Kat mentions) may give you a better chance of getting the target in frame.  Alternatively, go for some of the smaller faint fuzzies, where your long focal length will be more of an advantage?

[Terrierist]

All..

Thanks for the advice and input, it is appreciated.

I guess I'd bitten off more than I can chew, but if you don't try, you never learn!

@almcl Thank you for your help and looking at the FITS file. I've thought about the light pollution filter and other bits and maybe it is time to say enough is enough and start to save for a mount and scope that I can obtain decent (to me) images. Put the money towards that.

@happy-kat You kindly pointed me towards the no EQ topic, I learned a lot there. I have a slowish (F4) 200mm lens that I will give a go when we get a clear night, it will be interesting to see how that performs.

Cheers to all.

 

Kev

 

[happy-kat]

That's great, if you can just mount the camera and that lens directly onto the mount. F4 is way faster than your f11.6 telescope so you can afford to stop it down a bit. Also being a much shorter focal length it will be more forgiving. Good luck with trying it hop[e it works out.

[Demonperformer]

Agree. Your f/4 lens is letting in more than 8.4x the light of your scope. Even if you stop it all the way down to f/8 you are getting twice the light through. If it has a setting of around f/6.3, that would probably be a good starting point (3.4x more light that your scope).

The focal length being only 200mm compared to 1500mm basically means that any errors in tracking will be only 2/15 as significant. In effect, your tracking will support exposures 7.5x as long. This, of course, will only come into play once you have a filter that will block more of your light pollution. If nebulae are your "thing", then it might be worth considering a Ha filter rather than a standard LP filter. This will be more expensive, but will greatly increase the contrast, but that does depend on how well your camera responds in the Ha waveband (the 'normal' canon has a filter that blocks this wavelength [which can be removed], but I don't know about the Nikon).

[Terrierist]

Thanks, John..

All the above makes perfect sense and I think I will give it a try if and when we get a clear night this week