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85 vs 70-200 vs 200-500

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#1 TakingTheMike

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Posted 14 October 2021 - 02:33 AM

Hi all,

 

Apologies if this is a very noob question.  I am an experienced event, travel and sports photographer but quite new to astro.

 

I am unsure which lens I should reach for first when there is a clear night sky.  I have an 85mm f1.8s, 70-200 f2.8s and a 200-500 f5.6.  I don't currently have a tracker.

 

Am I better off with a lighter faster lens that requires more cropping (85), a larger slower lens with less cropping (200-500), or the lens in between (70-200)?

 

Orion Nebula, Rho Ophiuchi Cloud Complex, Pleiades and Helix Nebula are currently on my to-do list.  I live in Auckland, New Zealand so I have picked southern hemisphere targets.  Although I am open to suggestions as well.



#2 piaras

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Posted 14 October 2021 - 04:40 AM

I would start with the 85. It is more forgiving and a better than the two zooms. With no tracking the other two lenses at the longer settings realistically will not work and they are  slower versus the 85. Close down the 85 by one or two stops to reduce lens aberrations and take a have at it. 
Pierre


Edited by piaras, 14 October 2021 - 04:42 AM.

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#3 MikiSJ

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Posted 14 October 2021 - 05:22 AM

I too, would start with the 85, and maybe stop down gradually if you have any issues with color or flatness. 

 

Also, with the multi-leaf diaphram, you might see strange artiifacts on the brighter stars, like Mars.


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#4 BQ Octantis

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Posted 14 October 2021 - 05:36 AM

G'day mate!

 

Welcome to AP! smile.gif

 

Are you shooting full frame or APS-C?

 

The LMC and SMC are probably easier first targets, along with the Great Sagittarius Star Cloud. At this point, Rho Ophiuchi is getting low in the sky—and the light from Venus may very well blow it out once the moon is back to last quarter. The Lagoon Nebula is much brighter and probably more accessible; the Carina Nebula will make an appearance in short order and is way brighter and larger than other nebula (and moves much more slowly being so near to the pole).

 

Get a tracker! Note that 500mm is a tall order without autoguiding.

 

Cheers,

 

BQ


Edited by BQ Octantis, 14 October 2021 - 05:39 AM.

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#5 TakingTheMike

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Posted 14 October 2021 - 05:59 AM

G'day mate!

 

Welcome to AP! smile.gif

 

Are you shooting full frame or APS-C?

 

The LMC and SMC are probably easier first targets, along with the Great Sagittarius Star Cloud. At this point, Rho Ophiuchi is getting low in the sky—and the light from Venus may very well blow it out once the moon is back to last quarter. The Lagoon Nebula is much brighter and probably more accessible; the Carina Nebula will make an appearance in short order and is way brighter and larger than other nebula (and moves much more slowly being so near to the pole).

 

Get a tracker! Note that 500mm is a tall order without autoguiding.

 

Cheers,

 

BQ

I am shooting full frame with a Nikon Z6/Z6ii.  I actually backed the Benro Polaris electric tripod head with an astro expansion kit.  I was sold on the promise of accurate guiding and go-to features, but I am worried the projects won't be as good as they claimed so I might be in the market for a tracker shorter after.



#6 BQ Octantis

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Posted 14 October 2021 - 08:21 AM

I often use my 200mm f/2.8 on my APS-C Canon 600D with 4.3µm pixels as a point of reference. Having shot most easy targets in the southern sky with it, I have a rough idea of per-frame and total integration needed with a stock (non Ha-modded) DSLR. Here's how your setups compare:

 

comparisons.jpg

 

Aside from f/5.6, the your flux is much better than mine (Line 18) because of your larger pixels. I can get away with 30 sec subs unguided; your tracking error tolerance is much higher because your larger pixels subtend larger angles in all your setups except 500mm (Line 13). But the tradeoff is less magnification—so larger sampling and a much wider field (Lines 24 & 25).

 

As to what that means, the SMC is 3˚×5.5˚ (Lines 26 & 27); it takes up most of my APS-C FOV at 200mm:

 

(Click for 100% sensor scale @ 16 bits per channel.)

gallery_273658_12412_117205.jpg

 

The SMC takes up much less real estate in your setups; all bright nebulas will take up even less. So the LMC is probably a more worthy first target IMO.

 

BQ


Edited by BQ Octantis, 14 October 2021 - 04:10 PM.

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#7 Kevin_A

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Posted 14 October 2021 - 08:00 PM

Since you dont have a tracker use the 85mm and you can probably get away with 4 second subs. If you have a 14mm you can do 20-30 second subs.



#8 TakingTheMike

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Posted 15 October 2021 - 02:13 AM

I often use my 200mm f/2.8 on my APS-C Canon 600D with 4.3µm pixels as a point of reference. Having shot most easy targets in the southern sky with it, I have a rough idea of per-frame and total integration needed with a stock (non Ha-modded) DSLR. Here's how your setups compare:

 

attachicon.gifcomparisons.jpg

 

Aside from f/5.6, the your flux is much better than mine (Line 18) because of your larger pixels. I can get away with 30 sec subs unguided; your tracking error tolerance is much higher because your larger pixels subtend larger angles in all your setups except 500mm (Line 13). But the tradeoff is less magnification—so larger sampling and a much wider field (Lines 24 & 25).

 

As to what that means, the SMC is 3˚×5.5˚ (Lines 26 & 27); it takes up most of my APS-C FOV at 200mm:

 

(Click for 100% sensor scale @ 16 bits per channel.)

gallery_273658_12412_117205.jpg

 

The SMC takes up much less real estate in your setups; all bright nebulas will take up even less. So the LMC is probably a more worthy first target IMO.

 

BQ

Thanks BQ, that table is slightly beyond me, you might have to pretend that you are talking to a stupid person.  But I gather that my full-frame lower megapixel sensor is better for low light gathering?  I use photopills to work out the NPF rule, Stellarium and blackwaterskies.co.uk/imaging-toolbox to plan my shoots.

The result image from last night looks okay.  But no doubt I have a lot of work to do in terms of editing, all this stretching of the histogram and Star++ program thingy, and removing light pollution (I live near a city).
 



#9 BQ Octantis

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Posted 15 October 2021 - 04:19 AM

It's the pixel size that pretty much dictates capture durations.

 

My pixels are 4.3 microns. Yours are 5.9 microns. Since they're squares, yours collect (5.9/4.3)2 = 1.9 times more light from the same optic. That means subs only have to be half as long in duration to capture the same amount of light. The tradeoff is less magnification.

 

Magnification cuts two ways. The reduced magnification is more tolerant to sky movement without star trails. But less magnification is…well, less magnification.

 

The full frame sensor just means a wider field. But in the corners and edges, lenses have more vignetting and star distortions.

 

BQ


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#10 Kendahl

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Posted 15 October 2021 - 08:31 AM

Without tracking, you want short focal lengths. The more sky each pixel captures, the longer you can expose before star trails become objectionable. The 70-200 zoom at its shortest and the 85 mm are equivalent in focal length but one may produce better, rounder stars in the corners than the other. You'll just have to experiment, stopping down until the image is acceptable.

 

Your next step will be some sort of tracking device. There are many trackers designed to work with cameras and lenses out to fairly long focal lengths. Some get quite sophisticated, with polar alignment scopes to minimize field rotation during longer exposures. They also get expensive and approach the price of a low end, motorized German equatorial mount. My personal opinion is that, except for portability and low price for the smallest examples, there is nothing a camera tracker can do that cannot be done as well or better by a GEM that can respond to tracking correction signals from an autoguiding system. Orion's Sirius EQ-G is a good starting point. Its biggest limitation is payload capacity which is 15 to 20 pounds of camera, lens and autoguider. Orion and other manufacturers sell mounts with higher capacities and higher prices. If you intend to pursue astrophotography beyond a few casual photos, I suggest you save your money until you can afford this kind of equipment. The most expensive way to progress in this hobby is to repeatedly upgrade rather than swallow hard and buy what you really need the first time out.


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#11 nhmorgan79

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Posted 16 October 2021 - 09:02 AM

I own and have used all three of those lenses for Astro. However, I don't think any of them are really viable without a tracker. The 85mm is the best candidate, but I found it needed to be stopped down to at least 2.5 and since you can only get 2-3 second exposures untracked, you're not going to get much to work with. The 200-500mm especially is going to need a large mount and guiding as well. I also guide for the 70-200. The 70-200 is a heavy lens (heavier than the F mount version), so I think it's too much even for a Star Adventurer tracker. I was surprised when I used my 70-200 2.8s how well it does for astro. No coma, no LOCA, and sharp across the frame even at 200mm.




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