![]() It is more commonly found on smaller / lighter telescopes. The Vixen-style rail is a narrower dovetail bar. These are both industry standard mounting rails. which mounts to the mounting rail on the bottom of the telescope. ![]() Mounts will have either a "Vixen" style or a "Losmandy" style dovetail saddle. This is THE most critical piece of gear for astrophotography. You would still need an equatorial Go-To mount. That $400 price is *just* the optical tube assembly (not a mount). You remove the extension barrel when using the DSLR camera. So they throw in a 2" extension barrel to hold the eyepiece an extra 2" farther away if you want to use it for visual observing. But that's farther than where an eyepiece would be located. This means the image actually comes to focus about 2" beyond the focuser tube (which is perfect for a DSLR camera). the Newtonian "astrograph" design moves the primrary mirror 2" closer to compensate for that extra 2" the camera needs. the focuser tube hits the limit of travel. So you start focusing inward and you can see the image just starting to come to focus when. the whole camera has to be brought about 2" closer to the scope. but the image sensor is about 50mm farther back from that t-ring (about 2".) This means in order to focus. The problem with photography is that a DSLR camera has a t-ring to mount to the scope. typically about mid-travel on the focuser. In the case of a newtonian-type reflector, the issue with a normal Newtonian scope is that the position of the mirrors is designed to bring the image to focus at the eyepiece. which is pretty good as you're starting out because that will be a bit more forgiving when it comes to tracking accuracy.Īn "astrograph" is any class of telescope which has been optimized for astrophotography. That means the light that an f/11 scope would require 8 minutes to capture. an f/4 scope collects light 8x faster than an f/11 scope. Orion makes a 6" f/4 Newtonian "Astrograph" for about $400. (depending on the object it could be 4-8 minutes long at ISO 800). You'll also be contending with high focal ratios (f/10, f/11) which mean you'll need much longer exposure times. 2000mm) the tracking accuracy becomes critical and the mount is very unforgiving of tracking errrors caused by alignment errors, vibrations, flexure, periodic errors (basically mechanical imperfection in the worm gear) etc. When you get into longer focal lengths (e.g. Also, best to keep the focal length short when learning astrophotography (e.g. and only for very light loads (the CGEM or better yet the CGX are the mounts more often used for imaging). This causes a twisted smeared image in a long exposure.Ī minimum mount from Celestron would be their Advanced VX mount. ![]() but the field rotates (it appears to "twist" over time becuase the axes aren't parallel). On an alt/az type mount the computer can move both axes to compensate. that means they neatly cancel each other out and the star is held in place even for a very long exposure. The reason it needs to be an "equatorial" mount instead of the simpler alt/az mount is becuase on an equatorial mount, the axis of the mount is parallel to Earth's axis of rotation. So you can imagine how you'd end up with a smeared image if it doesn't track (tracking is requried - not optional). If not tracking, a star would travel 15 pixels every second. In other words 1 pixel = about 1 arc-second of sky. If we pick the 900mm focal length, you get a field of view 1.4° wide in the horizontal direction and the camera has 5184 pixels per how. The Earth is spinning from west to east at 15 arc-seconds of angular rotation per second of time. but doesn't track (and it's not a solid mount so vibration is likely to be an issue). the mount needs to be tracking and you need an equatorial mount. ISO 800 is typically the highest ISO I would use on that camera.įor any deep-sky object. That's fairly easy and it will nail the moon. At f/11, the exposure duration should be set to the inverse of the ISO. The moon is an easy exposure using something called the "Looney 11" rule. but cost a lot more (typicaly these things start at around $1000 for *just* the optical tube - no mount.) An "achromatic" refractor will still look good in the center of the field. The more expensive "apochromatic" refractors use a combination of better "glass" (typically an ultra-low dispersion glass) and usually at least 3 elements (and sometimes 4) and produce noticeably sharper results. It will have color fringing or color bleeding near the edges of the frame. Also, both scopes you listed are achromatic refractors.Īn "achromatic" refractor is one that uses a very simple achromatic "doublet" configuration (two lens elements). Do you have specific targets in mind? The problem is the lack of tracking.
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