Which is best for planetary and nebula imaging? Barlow or eyepiece projection? I have Celestron Travel Scope 70mm telescope and use a Nikon D3300 camera. I need to take photos of planets and nebula with this, and I can't afford an equatorial mount.
Your scope has an objective lens (up front lens) that is 70mm in diameter ( 2 ¾ inches). This objective lens has a focal length of 400mm (15 ¾”). This means, the objective lens projects an image of the objects being viewed 400mm distant from the lens. Both the diameter and the focal length define the astronomical telescope. As to magnification, we can squeeze out about 50X (50 times magnifications) per inch of diameter. That translates to, the maximum usable magnification obtainable when using a 2 ¾ inch telescope is about 135X. We can squeeze out more but it will be “empty” magnification. In other words, objects will appear bigger but there will be no gain in acuity. Let me add that magnification comes at a price. The higher the magnification, the dimmer the resulting image. If If you were to operate this scope at 125X, the image will be quite dim but still usable.
We fit an eyepiece lens tube downstream from the objective. It’s job is to magnify the projected image produced by the objective lens. The eyepiece tube contains several lenses. The eyepiece also has a focal length. Your scope came with a 10mm and 20mm eyepiece. We discover the magnification realized when we mount an eyepiece by division. Mount a 10mm and the magnification will be 400 ÷ 10 = 40, written as 40X meaning objects will appear 40 times closer than actuality. Mount the 20mm and the magnification will be 20X plus this image will be quite bright.
Astronomers usually desire more magnification from their telescopes. The Barlow lens mounts between the telescope body and the eyepiece. This lash-up doubles the magnification of the eyepiece. At the same time, it dims the image plus it adds some distortion. Don’t get me wrong, a good Barlow creates a decent image.
Your scope is an entry level instrument designed for ease of operation delivering a direct view. While it can function as a platform to image with a camera mounted, such operation is daunting but not impossible. Even if you use this scope only for visual observing, you might be disappointed. This is because we are surrounded by images produced by space-based telescopes and by giant mountain-top instruments. Make no mistake, you can achieve wonderful results from small earth-bound scopes, just don’t expect to succeeded on your first few attempts. By the way, your first target should be the moon. Plan an evening when the moon is at ¼ phase (first quarter).
We have, up our sleeves, several methods of mounting a camera to a telescope. Procure a telescope to camera adapter. Most commonly, we dismount the camera lens, mount the adapter with camera, this array replaces the eyepiece. This position is called “the prime focus”. In this configuration the telescope’s objective lens becomes the camera lens. This method should be the one you try first.
Another way to work is called “afocal”. We set the scope up for visual observing. Point the scope at the moon or a planet and adjust everything so you are looking through the eyepiece and seeing crisp image. Now hold the camera with its lens mounted (normal configuration) so it is looking through the eyepiece. You can snap a picture of what you were looking at this way.
Eyepiece projection requires a special camera-telescope adapter. This device holds the camera and the eyepiece. The camera is fitted to this adapter with the camera lens dismounted. This method yields the highest magnifications however the image is dim and you will have difficulties focusing and difficulties exposing.
If you go for the planets and comets and nebula, you will need a motor driven telescope mount. This is because the images you will find interesting are dim so you will need to elongate the exposure so the camera will accumulate light sufficient for the exposure. Time exposure is super daunting and the telescope must track, with precision, the objects you are imaging. This will be an acquired skill.
The Barlow only figures into these methods if you want more magnification.
Astro photography is rewarding but you will not become accomplished at this art/science overnight. Best of luck!
As to the actual question, with a camera attached to the telescope, a barlow that does not use telescope eyepiece projection might be only a typical 2x magnification (roughly, depending on the selected barlow). The eyepiece projection can be many times more magnification (and much more difficult). A small 70 mm telescope cannot support much magnification (see https://www.google.com/search?q=telescope+maximum+magnification )
For exposures longer than a very few seconds, the equatorial mount is a requirement, and magnification increases that need (otherwise the magnification also magnifies the earths rotational movement). See http://www.scantips.com/lights/stars.html
See https://www.google.com/search?q=barn+door+hinge+tracker+astrophotography for an inexpensive DIY mount very suitable for a few minutes of exposure. The distance from hinge to screw is calculated to match the screw thread pitch to one screw revolution per minute (which you provide, perhaps with a quarter turn every 15 seconds).
The Celestron 70mm is f/5.7. If you have a 200 mm or 300 mm f/2.8 or f/4 camera lens, it will be good for the stellar exposures (better optical correction too). Your D3300 24 megapixels should allow extreme crop to magnify greatly. Begin that way to get your feet wet.