A paltry 30 minutes (6x 5 minute subs) of Eta Carina. I used a f5 8" Newtonian on an HEQ5 Pro, with a Prostar Guide LP-Mono camera as a finderguider, and imaged using a modded Canon 40D DSLR with a Baader CLS-CDD clip filter and a f5 Coma Corrector. Was a first with this combo so quite pleased in the end. Got my mirror far enough down the tube to bring it all to focus and can still (just!) collimate the primary. Had no real issues guiding (no wind) although balancing the setup required three 5kg counterweights spread across the bar. Astrotortilla refused to work but it was still easy enough to track down the Nebula. Processing was quick and nasty using Nebulosity 4 (subtracted darks and bias removed flats, normalized, debayered, stacked, auto colour corrected, stretched etc.) and then a bit of fun with the iPhone and iPhoto. Processing is probably my next area to really explore with a degree of seriousness.
I certainly got more detail this time than I've got previously with the ED80, and wonder what the key factors in getting more resolution and detail might be. A 5 minute exposure on the 8" Newt with the CLS filter certainly filled the histogram to a tad beyond the halfway point so wonder firstly if a longer exposure with the ED80 (at f6.4), that yielded the same 'fullness' of the histogram, would get me a similar result? Or would just more stacked subs add up to the same thing?
The image scale with the 8" Newt is also narrower. I'm assuming this helps get more detail too or is this not true with photography? I have an ED100 coming my way with a similar field of view. If the scale was the same and I exposed for longer, or stacked more subs, (it'd be an f7.5 or so) would this yield the same resolution?
How much longer would I need to expose on the ED100 at f7.5 to match the f5 8" Newt? Or how many more subs would I need?
Given there are going to be windy nights the 8" Newt is not always going to be viable on the HEQ5 Pro. Is my best option in this case slightly longer, or more, exposures on the ed80 or even longer, or more, exposures on the ED100?
Ben, a nice shot for your first attempt with this setup. I have the same scope and I find the Carina nebula a little too large for the 1000mm focal length There's lots of nice whispy bits of nebulosity and colourful stars you're missing out on. The ED80 will give you a nicer crop but of course, less detail. I guess it's all about what you're trying to achieve. It's a bit like painting a scene regarding colour balance and field of view, we all have our opinions.
At f7.5 you're going to have to increase your time more than double to achieve the same exposure at f5. Not sure what iso you were running at but the alternative is to increase the time a bit and double your iso setting. Noise will increase but you can compensate this by doing more subs.
The ED100 will give you more detail than the f5 reflector so if you're happy with the 1000mm cropping you will get a better result. By the way, good work on moving your mirror up, it can be a bit daunting for some to do this, looks like you've done a good job.
Thanks for the advice Michael. I'm prepared to lose a bit of the Nebula just to get some more detail in those small nebulous patches. Through the ED80 it was amazing to the see the full extent of it and those bright yellow stars in particular. Anyway, I've been shooting at ISO800 which, given the noise on the camera and the generally warm conditions up here, seems like a good mark to hit. Surprised I'd get more detail with the ed100 so that's good! So is it longer exposures or more subs that would do the trick? My feeling is that longer subs are the way to go (to really fill out the first half of the histogram) although light pollution might well have a say on how long I can go.
I'd argue that you'll loose detail with the ED100. Resolution can go very much with contrast, your 8" F/5 captures ~4x the amount of light which gives better contrast. The 8" also has a slightly longer focal length and a longer focal length will give better resolution (better image scale).
The ED100 is also a doublet, very little to no colour fringing visually but chromatic aberration will soften images as well and imaging is considerably more demanding.
I do plan to use the dedicated .85x FF/FR with the ED100 so would be effectively losing some focal length there as well. I was planning on the ED100 being more of a visual instrument but if it can catch simialr sorts of detail (or as the 8" Newt (or more than the ed80) then maybe it's a more stable option on the HEQ5? Guess I'll just need to get out there and do it!! I'd love to get closer to some of the smaller galaxies without just getting bigger and heavier but maybe there is no real alternative here.
Colin is spot on, and Benjamin is right to be surprised, resolution is a direct function of aperture; the 8" Newt wins hands down over the 100ED. No
amount of exposure with the 80 or 100 will reveal the detail that the
200mm will.
raymo
Thanks for clarifying Ray. If it's detail I'm after the 8" Newt seems to be the one to use. I can get it really well balanced on the HEQ5 and the finderguider seems to be up to the job for the 5 minute subs I'm taking (unless there is something in the image that seems off - apart from the colour and noise). If the mount was an NEQ6 would I notice a difference with the image? I didn't think I had to throw out any subs from poor guiding, only one which was cloud affected. But maybe I'm not being fussy enough?
I'd argue that you'll loose detail with the ED100. Resolution can go very much with contrast, your 8" F/5 captures ~4x the amount of light which gives better contrast. The 8" also has a slightly longer focal length and a longer focal length will give better resolution (better image scale).
The ED100 is also a doublet, very little to no colour fringing visually but chromatic aberration will soften images as well and imaging is considerably more demanding.
Speaking from a photographic and not a visual comparison, I'm basing my advice on personal experience here, mind you, I am comparing a 100mm f5.5 triplet to my 200mm f5 reflector. The contrast of the refractor rivals the reflector and it matches the reflector for detail. The true test would be to have them set up side by side at the same location on the same night, on the same object using the same sensors for capturing an image.
Ben doesn't mention what type of ED100 he's getting, so it's a bit hard to know what to expect, there are so many variables to consider like quality of the optics, colmination, steadiness of the mount, etc. It will be a matter of making your own mind up with what you are able to achieve with the different scopes.
Sorry for any confusion here, it's the Skywatcher black diamond ED100 with a soon to be acquired .85x FF/FR. It'd be lovely to have a f5.5 Triplet! My reason for picking up an Skywatcher f9 ED 100 was mainly for visual use on the moon and planets, to replace my f5 Skywatcher ST120, which shows quite a bit of CA although fun to use for grab and go galaxy hunting. Just wondered where it would stack up photographically. Have a good picture of that now :-) Am planning to try the Newt and ED100 out sequentially as soon as time and clouds allow.
So, to be clear, a shorter focal length but high quality 100mm refractor on a good mount CAN get more photographic detail and resolution than a lesser quality but longer focal length 100m refractor and be the equal of an 8" Newt? Aperture and good glass can increase contrast and the focal length isn't such a factor in detail, or at least not with these scopes? I had figured the focal length of the ED80 would limit its ability to capture fine detail no matter how long the exposure or number of subs?
Michael, resolution, as I said, is a direct function of aperture. An 8" scope
will resolve objects that are half the angular diameter of those resolvable
with a 4". Light gathering power is a square function, so that an 8" has 4 times the LGP of a 4", but resolution is simply directly related to aperture,
so an 8" has twice the resolving power of a 4". It doesn't matter how
good the 4" scope is, it cannot overcome the laws of physics. A good way
to demonstrate this is to look at the moon's craters. The 8" will resolve craters that are at best just tiny smudges, or even invisible in the 4".
It's all academic most of the time anyway, as atmospheric turbulence
usually limits resolution, rather than the scope's own limitations.
raymo
Okay. I tried the same object with the same exposure time (6x 5 minute subs), and the same camera on a much windier night with my ed80 (with .85x FF/FR) and got the result below. It would seem the noise is much more prominent for the wider field (as I expected given I had to stretch it more) although I find the noisier detail about the same (if that makes sense). The resolution seems more to do with scale given the noise from the Canon 40d. A less noisy camera (or better processing) would be more revealing I guess. Or am I seeing this all wrong!?
A close up of the ed80 pic that roughly matches the scale of the picture from the 8" Newt and certainly seems the detail is less contrasting but also noisier.
I own a Skywatcher ED100 F9 scope. Also a number of reflectors from 6" to 16".
Resolution wise, the ED100 will give your reflector a run for it's money! The ED100, though only a doublet, made at F9 it's still very good. There are a number of losses and aberrations in reflectors that are not present in refractors. Increased aperture, while theoretically increasing resolution, needs better seeing to do so.
That flat mirror in the middle of the light path doesn't help either and knocks the T-stop down. So while a reflector has an F-ratio of F5, it's not as bright as an F5 refractor. The reflector would likely have a T-stop of 6, or more! Refractors transmit more light than reflectors which require two bounces. I would say my BD SW ED100 with focal reducer is roughly neck to neck with my 6" F5 Newtonian.
However, 4" verses 8" is still a considerable gap causing the refractor to need much longer time on the target. That's why my ED100 doesn't get much camera time lately. It can't match the 10" F4.
Give it a go though. It may surprise you. Remember to give it longer time on target to compensate.
Oh, and nice Eta Carina!
Last edited by cometcatcher; 27-04-2017 at 12:00 AM.
