Along the Border With Camera and Lens

[SteveNickolls]

I’ve had a couple of very rewarding sessions this May imaging along the border of Cepheus and Cassiopeia shooting with my modified Canon 700D DSLR, Astronomik 12nm Ha clip in filter and 135mm Samyang lens. On both occasions I used my old CG-5 mount as the pack horse.

The first image, taken in the early morning of the 9^th May 2018 took in a number of Sharpless regions including The Cave Nebula (Sharpless 2-155), Sh2-154, Sh2-161 and Sh2-157,  the Claw Nebula. This took just an hour and the image was composed from 12x 300 second light frames taken at f/2 and ISO 400. These were stacked in Deep Sky Stacker (DSS) with x14 dark frames and x50 flat and bias frames. The image was processed using StarTools.

The second image, taken over the late evening and early morning of the 13^th/14^th May is of Sharpless 2-171 which straddles the border of the constellations and the faint cluster NGC 7822. This image was made up from 24x 300 second light frames, again at f/2 and ISO 400. These were combined in DSS with 22 dark frames and x50 flat and bias frames and subsequently processed in StarTools.

When darker nights return in the Autumn I’m going to resume my trips along the border and maybe try my hand at a mosaic. I can wholeheartedly encourage others to do the same.

Cheers,
Steve

[happy-kat]

A mosaic sounds a great challenge. If going out and about if using that lens won't the Star Adventure be the mount to choose for lightness to carry etc?

[whatablanker]

Great images. How do you do your flat frames without messing the focus up? I use my table with a white screen but holding it against the lens and trying not to push the lens in is a pain in the proverbial so an easier way would be greatly received?

[SteveNickolls]

5 hours ago, happy-kat said:

A mosaic sounds a great challenge. If going out and about if using that lens won't the Star Adventure be the mount to choose for lightness to carry etc?

Hi happy-kat, thanks for your comment. The Star Adventurer would have a distinct advantage when it comes to framing the image as the ball head is so flexible. I'm looking forward to the autumn when the nights begin to get dark at reasonable hours :-)

Thanks whatablanker for your kind comment. If I don't alter any element I reuse stock flat and bias frames that I took with the camera, Ha filter and lens all assembled and not adjusted after an imaging session. My process for taking flat frames has been to place them a few inches from a large matt white tile illuminated by a powerful light source some distance way taking care to have an even illumination. I set the camera to AV and let the camera take say x50 flat frames. The exposure level of the flats is to have the camera histogram around a third the way across.

My set up with the camera and lens is all held in place using one of these-https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9891_TS-Optics-Telephoto-Lens-and-Camera-holder-with-Vixen-style-dovetail-bar.html It has practically meant that with care I only need to focus on one night as the focus wheel is never touched so with care I can take the camera and lens off the mount and place it in front of the tile to take some stock flats which I can then reuse many times. With flats it is important to use the same ISO setting as for the light and dark frames. Eventually I may have to take new flats if I break up the combination to fit a different lens or filter, or dust settles on the protective uv filter in front of the camera lens.

In the past I have tried pointing the camera and lens towards a blank computer screen but this created moire effects. I guess you could try a couple layers of cloth or even better a layer of opaque perspex to help blur the screen.

Hope this helps :-)

Cheers,
Steve

[happy-kat]

I just turn the camera and lens to point at zenith and balance my tablet straight on top of it running lightbox.

[The Admiral]

A couple of lovely images there Steve. The modded Canon, filter and Samyang seems to be working out well. I too look forward to seeing some other panels in this region.

Ian

[iansmith]

Nice images. 

Cheers, Ian

[The Admiral]

On 16/05/2018 at 20:22, SteveNickolls said:

In the past I have tried pointing the camera and lens towards a blank computer screen but this created moire effects.

Provided that the panel is close and the lens is at infinity, I shouldn't think the moire is a result of interference between pixel pitches. You'd be much more likely to see that if the lens was focused on the screen. As such, adding diffusing media won't help. I think it's more likely due to a "collision" between the screen refresh cycle and the moving shutter slit. The way around that would be to ensure that the exposure is sufficiently long as to encompass many refresh cycles. In other words, you'll need to turn the screen brightness down (or display a grey rather than white screen), or lower the ISO, or both. Adding diffusing media would of course also have the effect of reducing screen brightness. I used a laptop screen close up to the scope to do my flats, and I did need to make sure that the exposure time was reasonably long in order to avoid patterning.

I'm not sure why the ISO needs to be the same as for the lights. After all, all ISO does is change the gain for the whole sensor output, and I can't see that it would affect the uniformity of response across the sensor. I have seen the view extolled though, and I must admit I prefer to keep settings identical so far as possible, so I'd prefer to try dimming the screen in the first instance. Not exactly practising what one preaches though .

Ian

Edit. Just been having a quick read (https://www.cloudynights.com/topic/483388-flats-and-iso/) about ISO for flats. First impression is there's more to this than meets the eye (or should that be camera)! As always .

[SteveNickolls]

Thanks Ian for the above advice on taking flats. As regards the ISO value when for flats I'm only following what the FAQ in DSS says. I'll read the link with interest-cheers.

Steve