Do you have specific targets in mind? The problem is the lack of tracking. Also, both scopes you listed are achromatic refractors.
An "achromatic" refractor is one that uses a very simple achromatic "doublet" configuration (two lens elements). It will have color fringing or color bleeding near the edges of the frame. 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 ... 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 moon is an easy exposure using something called the "Looney 11" rule. At f/11, the exposure duration should be set to the inverse of the ISO. So ISO 100 means use 1/100th sec. Or ISO 200 then use 1/200th sec. etc. That's fairly easy and it will nail the moon.
ISO 800 is typically the highest ISO I would use on that camera.
For any deep-sky object ... the mount needs to be tracking and you need an equatorial mount. The 70EQ is equatorial ... but doesn't track (and it's not a solid mount so vibration is likely to be an issue). The 80LCM is motorized ... but not equatorial.
The Earth is spinning from west to east at 15 arc-seconds of angular rotation per second of time.
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. It turns out 1.4° x 60 (minutes) x 60 (seconds) = 5040. In other words 1 pixel = about 1 arc-second of sky. If not tracking, a star would travel 15 pixels every second. So you can imagine how you'd end up with a smeared image if it doesn't track (tracking is requried -- not optional).
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 ... that means they neatly cancel each other out and the star is held in place even for a very long exposure. On an alt/az type mount the computer can move both axes to compensate ... but the field rotates (it appears to "twist" over time becuase the axes aren't parallel). This causes a twisted smeared image in a long exposure.
A minimum mount from Celestron would be their Advanced VX mount ... and only for very light loads (the CGEM or better yet the CGX are the mounts more often used for imaging). Also, best to keep the focal length short when learning astrophotography (e.g. 500-700mm range). When you get into longer focal lengths (e.g. 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.
You'll also be contending with high focal ratios (f/10, f/11) which mean you'll need much longer exposure times. (depending on the object it could be 4-8 minutes long at ISO 800).
Orion makes a 6" f/4 Newtonian "Astrograph" for about $400 ... an f/4 scope collects light 8x faster than an f/11 scope. That means the light that an f/11 scope would require 8 minutes to capture ... it can capture in just 1 minute. It has a 610mm focal length ... which is pretty good as you're starting out because that will be a bit more forgiving when it comes to tracking accuracy.
An "astrograph" is any class of telescope which has been optimized for astrophotography. 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 ... typically about mid-travel on the focuser. The problem with photography is that a DSLR camera has a t-ring to mount 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 whole camera has to be brought about 2" closer to the scope. So you start focusing inward and you can see the image just starting to come to focus when... the focuser tube hits the limit of travel.
To get around this... the Newtonian "astrograph" design moves the primrary mirror 2" closer to compensate for that extra 2" the camera needs. This means the image actually comes to focus about 2" beyond the focuser tube (which is perfect for a DSLR camera). But that's farther than where an eyepiece would be located. 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. You remove the extension barrel when using the DSLR camera.
That $400 price is *just* the optical tube assembly (not a mount). You would still need an equatorial Go-To mount. Tip: Get the BEST mount you can afford. This is THE most critical piece of gear for astrophotography.
Mounts will have either a "Vixen" style or a "Losmandy" style dovetail saddle ... which mounts to the mounting rail on the bottom of the telescope. These are both industry standard mounting rails. The Vixen-style rail is a narrower dovetail bar ... but thick. It is more commonly found on smaller / lighter telescopes. The Losmandy style rail is much widder ... but a little thinner. It is more commonly found on larger / heavier telescopes. As these are industry standards... you can buy a mount from one vendor and a scope from a completely different vendor ... you do not have to buy the scope and mount from the same company.
I will caution you that a $400 telescope isn't a lot of money for an imaging scope. Newtonians often need a coma corrector for imaging or you may see degradation of the quality of stars near the edge of the field (and coma correctors can cost a few hundred dollars. Baader planetarium makes one that costs about $200.)
And keep in mind ... this really is doing things on a budget.
Meanwhile back to your question on exposure.
You cannot adjust the f-stop when shooting through a telescope. What you do instead is pretend that you are taking a manual exposure and use the focal ratio of the scope. E.g. if you used that f/4 Newtonian ... then it's f/4.
The ISO for *that* particular camera should be ISO 800 (or less). For a moon photo you wont need ISO 800 (I do my moon exposures at ISO 100 becuase it's fairly bright and it's a fast exposure). But for deep-sky objects ... things are much fainter and you have to increase the ISO. The optimal ISO for *that* camera is ISO 800. This has to do with how that camera applies gain. Light is technically analog and the sensor receives it as analog. But because it is a digital camera, it has to run the data through an analog-to-digital conversion (ADC). It can either apply some analog gain before it does the ADC (which we call "upstream gain") or it can apply some gain *after* it goes through the ADC (which we call "downstream gain"). It can actually do a little of both. THAT camera sensor applies mostly "upstream" (analog) gain UNTIL it hits ISO 800 ... at which point it switches to "downstream" (digital) gain. The problem with digital gain is that you lose dynamic range faster.
In other words ISO 800 will give the optimum amount of gain possible before you start sacrificing a lot of dynamic range. This is true of any Canon DSLR that has the Canon 18MP sensor. (so this ISO doesn't apply to all Canon cameras but it does apply to the T6).
Exposure duration will vary based on the target.
When I did my Andromeda image ... I was shooting 8-minute exposures at f/5.4 (the f/4 scope could shoot that in 4 minutes ... and maybe 3 minutes would be better). But if you tried to do that with an f/11 scope ... first you'd discover that the galaxy doesn't fit in the frame ... but also you'd have to run very long exposure times because f/11 is "slow").
Just a little over a week ago I shot M42 (Orion Nebula) ... which is "easy" becuase it's bright (for a deep-sky object) but "hard" because it requires a lot of dynamic range. So you end up taking several different exposures and using HDR to combine them. My longest exposure times were 4 minutes (the f/4 scope could shoot those in 2 minutes).
By the way ... you don't just take one shot. You take lots of them. Ideally shot after shot for at least an hour ... and preferably at least 2 hours. (the guys who are really serious will take 6-8 hours worth of data and sometimes across multiple nights.) This data is combined via image "stacking" software which improves the image quality by reducing image noise.
Some faint objects can require very long exposures.
Collecting the data is half the battle... processing the data is the other half of the battle. (Processing usually consumes the most time.)
Tim Campbell
5D III, 5D IV, 60Da
