DSLR & lens advice

[MishMich]

I set up my guide cam with my Onyx the other day, as I don't have a guidescope yet, and needed to get it working. To make life simple I decided to mount the DSLR with the Onyx, and use the 50mm prime. This has opened up some questions for me.

Using ISO200, 60 x 30 sec lights, plus darks, flats, bias, flat-darks. Camera set to f/1.8

1.) The light pollution graduates from the bottom right (brightest) to top left (darkest). I wondered what the best way of dealing with this is. Would a clip filter help?

2.) I get some coma around the edges. How do you deal with coma from a DSLR lens?

3.) I have tried processing using DSS, and if I try to get rid of the gradient and light pollution, I lose the small amount of detail in M42, Running Man & the Flame - and I can find no way of enhancing this.

4.) The image has a hint of colour on Betelgeuse and some of the background stars, but the whiter stars have a ble ring, and M42 lacks any colour at all.

5.) The scale of the image means it is not possible to capture the whole constellation (head and bow do not fit. If I got a wider-angle lens, would this then increase the coma? Is there any way of flattening the field using a DSLR & lens?

The camera is still unmodded, which will have to wait until after April now.

M.

[Merlin66]

Mish,

To reduce the edge of field coma you'll need to stop the lens down to say f2.8.

The blue halo's are caused by the chromatic differences in focus from the lens. We used to use a Minus Violet (UV) filter to reduce this "star bloat"

An un-modded Canon is not very efficient in the Ha wavelengths... so you need more ( and more) exposure to pull up the red parts of the image.

Removing LP is a black art!!!

[MishMich]

This is the best I can get to, including processing in PSE7, 85 degree left rotation, downscaling and minimal cropping in Gimp. Original first, processed second.

[skye at night]

1 - Yes

2 - Stop the lens down. ie, increase f number, this narrows the aperture and increases required exposure time, but is the compromise you need to make.

3 - filter should deal with most of this

4 - longer subs

5 - 50mm lens at f2.8

Steve

[MishMich]

Thanks, Merlin, I have a lens filter that is supposed to reduce glare by filtering UV. Do I need something that actually cuts UV completely?

I am now realising that the constellations and objects I want to capture before we move are all in the most polluted & obstructed direction for me. That means I need to get my imaging done at a darker site. That limits the amount of time I'll have for doing this. I guess I need to figure out which constellations will be fully visible, and which won't, and start from there.

M.

[MishMich]

Thanks Steve, that clarifies the situation.

M.

[themos]

Q1 is about gradients and how to get rid of them. Free software IRIS can do this, look in

IRIS TUTORIAL

Q2 is about lens aberrations ( whether it should be called coma I don't know). An expensive APO will create a focus-flat field over the whole DSLR sensor, that's why a small Borg costs so much money. A Canon L series will also be pretty good. As other people said, one has to stop down worse lenses. Most are sharp at f/8. See a comparison of different 50mm lenses here:

Canon 50 mm review

Q3 : don't use DSS to process the result! DSS just registers and stacks.

Q4: the blue halo is your standard chromatic aberration. Stopping down will help. I've also found that the Canon DPP program can correct some lens aberrations provided you are working with a RAW image and have used certain Canon lenses. Check the DPP manual

Canon DPP v3 Manual

Q5 the aberrations will not get worse with a shorter focal length lens but they will not get better either (to a first approximation, anyway). On the one hand each pixel captures a greater angle interval but on the other hand the light has to get deflected more to reach the edge of the sensor and at those larger angles I'd expect more aberration. But I'm not sure of these arguments.

[MishMich]

Thanks, that all gives me something to think about for the next time!

I had a look at the Iris tutorial, and found a way of getting the gradients sorted - although I had to use a mask, for some reason the right click action to select the dark areas doesn't work for me. It just brings up a disabled menu. The remove gradient got rid of most of the problems, but left a kind of wavy effect. Finished off in Gimp.

Thanks,

M.

[Coco]

The problem Ive had with reaaly fast lenses (135/2 85/1.8 & 50/1.2 ) to mention a few is that using them wide open on stars always gave coma and CA.

Stopping them down produced ugly spikes from the iris blades..

Some of my best DSLR images are from my 100-300 F/4 Sigma Zoom... this lens is a gem and sharp from F/4 so wide open is no problem.

Funny that one of the worst lenses for long exposures was my 300/2.8 L ... it had coma impossible to remove.. the Sigma 100-300 trounces it

Recently Ive been using a Vivitar 135/2.8 M42 and Asahi Takumar 200 f/4 with great results.. There very small light weight (beautifully made) lenses.

Cheers

Guy

[MishMich]

Is that M42/0.75mm or M42/1mm?

I have picked up two Soligors, one 135mm f/3.5 & the other 200mm f/4.5. Might give the 135 a try on the EOS. Hard to believe these lenses could be nearly as old as I am, they look almost new. I find the idea of using old SLR lenses on a DSLR quite appealing in a perverse way.

M.

[themos]

M42/0.75mm or M42/1mm?

I forget now but one of them is the standard "T-thread" and the other is the classic M42. When you see M42 lenses advertised, they are the classic M42, not the T-thread. I have a Zeiss 135mm/f3.5 and a Tair 300mm/f4.5, both M42 fitting, and they are good fun with the EOS.

[MishMich]

Been looking at the Canon DPP. One of the options lets you make an adjustment that lightens the vignetting. Now, I guess if you use that, then you need to run the same procedure against the flats as well? Oo-er, missus...

M.

[Merlin66]

The M42/0.75 is the standard T thread, the M42/1.0 is the old Pentax camera thread ( used on early Starlight cameras)

[Psychobilly]

Been looking at the Canon DPP. One of the options lets you make an adjustment that lightens the vignetting. Now, I guess if you use that, then you need to run the same procedure against the flats as well? Oo-er, missus...

M.

The flats should take care of the vignetting and any dust bunnies etc...

You's be better of leaving the light alone and let the flast take care of it...

You can tweak vignetting out in CS as well Filter->Distortion->Lens correction

Peter...

[Coco]

You can also use old Nikkor & Contax lenses with a the correct adapter, I have a few Contax gems..

Lens registration here..

Guy..

[MishMich]

I was looking at the tolerance for registration - has to be within 0.02mm? How do you get to calibrate a mount that closely?

M.