#8" RC hybrid visual/astrophotography setup

139 messages · Page 1 of 1 (latest)

wary shore
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I'm in the position to get a new setup, and I'm pretty (but not totally) set on an 8" RC (probably the Apertura CarbonStar variety). I have suitable eyepieces and a DSLR for imaging (Canon EOS Rebel T5, may also upgrade that too).

I know that for visual use, RCs aren't considered the best option, but I'd like to have a great astrophotography setup that's also usable visually. I use a telescope with a larger central obstruction, closer to 60%, visually and it's better than I expected!

The main thing I'm interested in is figuring out what other accessories I should be getting with it.

  • I'm considering a CGEM II mount – from what I understand this should be suitable in terms of capacity, but I'm not as certain about better options or pitfalls I should be aware of with mount selection.
  • I know RCs are not simple to collimate so I certainly will get one of the recommended collimation devices for them.
  • The main thing I'm not familiar with is guiding. I'm not sure what would be the best solution for this, as I've just been doing unguided astrophotography with an ancient fork mount.
  • I know that the GSO RC scopes are corrected enough that the field is effectively flat for an APS-C sensor, but I'm debating whether to also invest in a focal reducer.
manic dome
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I'd get an eq6r at least or an harmonic mount, the cgem ii is similar to the older eq6 and I'm really not sure if it can track at those focal lengths

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Guiding at over 1m of fl is usually recommended to use an off axis guider, it's a bit more expensive and I'm not entirely sure how you can use one with the dslr, unless the rc has more backfocus than usual

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Most 8" rcs have the primary mirror coupled to the secondary so you have to get a tilt adapter, I still haven't found out if that's been fixed or not

past parrot
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And with harmonic mounts, I’m pretty sure guiding is a must

wary shore
manic dome
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it would have to be a 70mm or more scope, the classic 50-60mm don't have enough fl

shut heron
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I was able to get away with an OAG and 5 minute subs on my EQM-35 Pro and my RC6

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I dont recommend, definitely aim for a better mount kekw

shut heron
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from my understanding, the size of the guidescope will not help in this instance, differential flexture loves to rear its ugly head once you surpass 1m of focal length, and nothing but an OAG will save you.

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I am running the ASI 120MM Mini and the ZWO OAG

past parrot
shut heron
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since it was basically made for the 120MM's tiny chip, the aperture to the prism is just slightly larger than the sensor which is ideal.

past parrot
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5 min subs with a RC6 on an EQM-35 pro?!?!?

shut heron
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Yes

past parrot
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Dude what

shut heron
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this was a version of the Horsehead I did using those 5 minute subs in broadband, on my EQM-35.

Horsehead_10-27-24.png
wary shore
shut heron
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the fact that the image isnt super blurry is beyond me

shut heron
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again I don't necessarily recommend using the EQM-35 with an RC, though. especially not the 8" if you arent going to run a reducer

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I think the only reason I got away with it was because I was using the reducer for mine because I wanted to make it into a light bucket

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the RMS was low enough to not have trailed stars, but the mount has a ton of wobble which presented as bloated stars in the final stack

wary shore
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The little chungus in question

P_20241110_161244.jpg P_20241110_161138.jpg
shut heron
wary shore
shut heron
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if you did you would probably have to weld it to the top of the RC to avoid flexture and even then, I think the primary's in both scopes can move slightly depending on the angle your mount is at

shut heron
wary shore
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Yeah I definitely don't want to cause too much mechanical stress

shut heron
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people told me to piggy back my RedCat on my RC, though

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that might be fun to do at some point but not right now

shut heron
wary shore
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More so on the RC OTA than the mount

shut heron
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with the RC8, you definitely may want something like an AM5N or an EQ6R baseline

shut heron
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I think carbon fiber can support it, I am just not sure what long term stress and wear on it like that can do

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I think the rails are bolted straight to the metal front and back ends of the RC though

wary shore
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I also did consider picking up a Sarblue 60mm Maksutov at one point, and while it's not going yo be the move here I do wonder how suitable it would be for guiding since it has a 750mm focal length

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There's apparently a group of imagers who have a lot of fun with the OTA...which is arguably a toy telescope (though I've used one visually and the optics are actually pretty good)

wary shore
cobalt pumice
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permanent solution is buying a machined decoupler, there's a guy on cloudynights who makes them but its like $500

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this issue still persists, including in the carbonstar RCs

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another solution is to just buy someone's decoupled RC instead of buying new, or if you're getting a large RC go for the truss since it doesn't have the problem

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and yes, OAG is virtually required for this type of scope, especially if you're getting one that flops around (GSO closed tube RC or SCT)

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i would argue its just required at high FL period

shut heron
cobalt pumice
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they range from 6-10" i believe

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trusses are 10"+ and do not have this issue

shut heron
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Really...

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It took a month for my primary to move at all and I think that was triggered by the change in weather.

cobalt pumice
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what sensor and f-ratio

shut heron
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Maybe I'll look into a solution in the future, I want to upgrade to the 8" later.

shut heron
cobalt pumice
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if you're on like a 533 at f/9 you won't really notice

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f/6 probably not that noticeable

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its quite noticeable on MFT (294mm mono + train) at f/5 on the 8" RC

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the other factor is how heavy your train is

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you have the lightest possible train, a 533 color camera

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if you put a mono train on it + a chonk focuser then its a problem

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another factor of course is your own personal tolerance for stars

shut heron
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I have the 533 + a filter drawer + an OAG, the guide camera, the reducer, and an autofocuser

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It's a substantial amount of weight.

cobalt pumice
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i have recommended the 6" closed tube rc before to someone because they were using a 533 lol

cobalt pumice
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mono + FW is heavier

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especially with larger sensors

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and the larger sensors also mean you notice it a whole lot more

cobalt pumice
cobalt pumice
shut heron
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Oh interesting

cobalt pumice
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i personally run a 10" decoupled RC

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with a 268 mono reduced to f/5.3

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these have been my stars for the past 6 months

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so worth it

shut heron
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Those are excellent

cobalt pumice
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apex-l 0.65 is an excellent aps-c reducer for rcs

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there's a tad bit of vignetting but i'll take it

manic dome
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Same for the larger closer 12" iirc

coarse oxide
cobalt pumice
cobalt pumice
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Guiding a C8 with a 30mm guidescope is not a very good idea

coarse oxide
cobalt pumice
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?

coarse oxide
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Well at 3.78um pixels you will get a resolution of about 0.51" per pixel

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If you have anything other than perfect seeing, you will be limited due to that

cobalt pumice
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Are you familiar with how phd2 calculates guide errors

coarse oxide
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Yes, off axis guiding can be good and the closer the resolution of the guider to the main scope the better but what I'm trying to say is that in my experience, a 30mm guide scope can be adequate

cobalt pumice
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It is not adequate at 1500mm because it's incapable of resolving beyond at best 0.8-1", depending on the asymmetric optical aberrations the guidescope might have

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Seeing is a factor but with 1s> guide exposures and multiple stars it can be sufficiently averaged out

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This is also ignoring the potential for mirror flop in a cassegrain

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The guidescope might be adequate if you accept higher FWHMs from stars and potential loss of resolution

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But the whole point of high FL kinda is resolution

coarse oxide
cobalt pumice
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1.6" is not bad

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With nyquist 2-3x that is 0.6-0.8"

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Regardless seeing varies night to night

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Seeing and guiding effects add in quadrature so it's best to minimize what you can

wary shore
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Alright, I guess I'm deciding between an EQ6-R, an AM5N, or perhaps a different harmonic drive mount. Apparently the EQ6-R is discounted at the moment, so that's pretty appealing, but so is the extremely low weight of a harmonic drive setup...

manic dome
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Harmonic>>

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Btw are you in the us or eu?

wary shore
wary shore
manic dome
# wary shore What's your rationale for this?

I have one myself, mount weighs less than the two eq6 counterweights, costed me as much as an heq5 and can carry the same payload as the eq6 with a way better tracking accuracy out of the box

wary shore
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And what's your payload?

manic dome
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the "old" umi 17, 13-20kg payload without and with cw respectively

wary shore
manic dome
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my bad, mostly an evostar 72 with a 533c, I've used it with my old 8" f/5 newt

wary shore
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Pulled the trigger on the Apertura Carbonstar 8" and a Sky-Watcher Wave 150i!

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I'm wondering if I'll regret not getting the pier extension for visual use...

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I still need to figure out a guiding solution though. I'm not in a huge rush for that since I have everything I need to use it visually at least

shut heron
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that's what Ill do

wary shore
manic dome
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Harmonic drive mounts really need guiding

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How much did you pay for the 150i?

wary shore
manic dome
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Kind of expensive, but I got spoiled by proxisky prices

wary shore
manic dome
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The slew speed is really nice, I don't get the manual clutch tho

cobalt pumice
manic dome
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On the carbon star? It's a newt

wary shore
manic dome
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Mh I didn't know there was a carbonstar rc, I thought it was the branding for the newer newts

cobalt pumice
wary shore
shut heron
wary shore