RE: [Four-Corners-Stargazers:955]

[jeff]

I've got the capability to go up to 280 with lenses but I've never gone above 200mm. But, you can't really see individual dsos in great detail at 200, either. 

I prefer star fields. What's your interest? I wish i had a 400mm for clusters, for example.

Happy Connecting. Sent from my Sprint Samsung Galaxy S® 5

-------- Original message --------
From: R06caddy <r06c...@gmail.com>
Date: 6/10/17 4:10 PM (GMT-07:00)
To: four-corner...@googlegroups.com
Subject: [Four-Corners-Stargazers:955]

How much magnification do I really need as a beginner.  The 600mm for example gives me roughly 12x on a full frame camera.  Most of what I’ve done is the milkyway, the aurora borealis, and night scenics all of which are wide angle lenses.

 

Bob

[R06caddy]

I come at this as a photographer with long experience in terrestrial nature photography.  I spent several months at Boulder on a research project (nothing  to do with astronomy) and during that time I went to programs at the planetarium there.  I always wanted to get more into astronomy but never had the time or dark sky until now.  I guess what I’m saying is that I don’t know enough to have an interest!  I have enjoyed milkyway and aurora borealis photography but that has been with 16-35mm focal lengths using the 500/focal length rule of a fixed mount.  I like the deep space photos that take hours and involve filters but I’m years away from that.  I’m hoping to get my feet wet with the photographic lenses I have and only buying a computerized equatorial mount to avoid star trails. 

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[Charles L Hakes]

The better question is how many arcseconds/pixel do you have? To start with, you need to know the pixel size of your camera (microns is the standard unit here). After answering that, divide your field of view by the number across. My quick calculation without knowing details of your camera is that with a 600mm lens you would be getting just under 3”/pixel, which would be VERY hard for just a tracking mount to keep up with unless it was a very expensive mount. Without any tracking the stars on the celestial equator will move about an arc second in one second. (And yes, those two are related!)

I occasionally use a 480mm scope with a field flattener that makes it more like 380mm, and I can easily see star trails in a minute if I am not using the autoguider.

Here is a shot I did unguided at 110mm several years ago. My notes say I was able to get 4 minute exposures.
https://www.fortlewis.edu/observatory/image_detail.asp?ID=179
There are a variety of other wide-field shots on the “wide field” link at the top of the page, bust most of those are wider.

Bottom line is lower magnification is much easier.

By the way, an excellent rule of thumb that is only sort of related is to match your f-stop number to the pixel size in microns. You cannot get better resolution than that, even in the daytime.

Charlie

[R06caddy]

Canon 5dV 5.36 µm square is the pixel size. Diagonal field of view 4degrees 10 min. or 15,000 sec. frame is 35mm or 1:1.5 so angle the long way is 12,480 sec. Not sure how to calculate from there but 30.4 megapixels equals 6750 pixels x 3500 roughly. If I'm doing this right, it is less than 2"/pixel. It sounds like a much shorter lens is needed unless I want to get into guiding. The more I look at this, I'm thinking this is going to be a learning summer and not a summer to buy equipment.

[Charles L Hakes]

That sounds about right with that many pixels. Many (most?) dedicated astro cameras have bigger pixels for more sensitivity, and direct binning into 2x2 or 3x3 blocks for even more. There is certainly lots you could do with much shorter focal lengths and get comfortable with darks and flats and stacking. I use DeepSkyStacker and am happy with that.

Charlie

[R06caddy]

When you talk about an expensive mount are you talking about a Losmandy or something more or less. Is it better to just figure on guiding and use a less expensive mount?

[Charles L Hakes]

Compared to less expensive mounts, I guess Losmandy would count as expensive? ;) (Have you checked the Astro-Physics website? $) But for long exposure deep sky, I would count on guiding no matter what mount you use. All the ones mentioned are by “mainstream” companies, and have lots of supporters. The more massive the mount, the more stable it will be (against wind, vibrations, bumping, etc.) Yes, for lower focal lengths that is less and less necessary.

I don’t recall seeing it put like this, but since the mount is maybe the most important part of imaging, make sure you spend as much on the mount as on your imaging scope. (?)

Probably just muddied the pool for you.

Charlie

[R06caddy]

Yes, I'm learning that you need to put $$ into the mount. Would I be better off starting with a 200 or 400 or something even shorter.

[Charles L Hakes]

There is exactly the problem. The shorter the focal length, the easier everything is. The longer the focal length, the more really cool targets you have to chose from.

Charlie

[R06caddy]

I have some stuff on order. How would I go about learning how to use it?? Bob Hutson