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SCTs, T Adapters, Prime focus and Focal reducers.

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#1 NightOwlPhysicsNerd

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Posted 09 February 2018 - 09:49 PM

Hey folks, I could really use your help here. I have an Celestron SCT 11 that I am finally getting around to hooking my a6500 up too. While I am slowly getting all the other gear together I ran across a bit of a head scratcher.... 

Where is my Prime focal point and does a Focal reducer change that? (well I should say, of course it does but how much?) 

This dilemma came up when I was buying the T adapter for my Sony a6500, there are 2 version available for the wide 2 inch adapters 

Short .... https://www.telescop...us-adapter.html

and 

Standard ... https://www.telescop...us-adapter.html

I will also need a .... https://www.telescop...telescopes.html

that would screw onto the celestron focal reducer.

So which one should I get ?

Or am I losing my mind. 



#2 hdt

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Posted 13 February 2018 - 02:48 PM

Just to confuse you a bit further--Sorry, I can't draw, so you'll have to go along with a verbal description. I'm also going to make a couple of assumptions, and state the assumption.

 

(1) You have a Celestron 94175 f/6.3 focal reducer

(2) You want to do "prime" imaging

(3) My personal experience with a Canon DSLR translates to your Sony A6500, in that both of these have a somewhat proprietary bayonet mount for their lenses; you press a button on the front of the camera and twist the lens about 1/4 turn to remove (or insert).

 

The most direct way to image using your SCT-11 is to:

(a) Remove the SCT visual back by unscrewing it

(b) Screw in the f/6.3 reducer to your SCT threads

© Screw a SCT-to-T adapter (Celestron 93633-A) into the reducer (if you have a Celestron Edge OTA, you'll want the Edge-specific adapter and focal reducer)

(d) Screw a T-ring adapter (telescopeadapters.com TSON) into the SCT-to-T adapter

(e) Mount your A6500 into the T adapter

(technically, it is easier to mount screw the T-ring adapter into the SCT-to-T; mount to camera; then, attach the assembly to the SCT threads, but the (a)-(e) sequence gives you what the optical train should look like)

 

In my experience, the f/6.3 is nearly parfocal--in other words, if I insert the f/6.3 reducer between the SCT  and the visual back, and then put your diagonal and eyepiece back on, you don't have to twiddle much with the focus button to reach focus again. The Celestron 93633-A adapter has nearly the correct length for being parfocal when you have a DSLR (most DSLR's have similar distances from the lens mount to the CMOS sensor).

 

The SCT's have huge focusing distance because they focus by moving the primary mirror, and you should be able to come to focus with the A6500.

 

The two adapters you posted are for putting the A6500 into a 2" eyepiece holder. In an SCT, that's not as desireable because you will add more weight (2" diagonal etc.) into your imaging train.

 

Hope I helped reduce confusion.



#3 bobzeq25

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Posted 14 February 2018 - 04:07 AM

You're not losing your mind yet.

 

Unfortunately you will if you try to learn how to do astrophotography of deep space objects with that scope.  <smile>  It's an extreme example of the two biggest beginner mistakes.  An inadequate mount, and too big a scope.

 

Cameras don't take great images because they're better than your eyes.  They do because they're different,  That difference means you can't just slap a camera on the back of a scope, and take images that are like what your eye sees, only better.

 

Long exposures, necessary for the camera, fundamentally change the nature of the game.  You're trying to take long exposures of a moving target with a long telephoto lens.  A tiny, tiny error (the camera pixels are only .005mm big) will hopelessly blur the image.  Things as small as the changes in atmospheric refraction with changing altitude, count.

 

The mount is now more important than the telescope.  The CPC mount works well enough for looking through, but is fundamentally flawed for photography.  It's the wrong design (an alt-azimuth rather than an equatorial), and too flimsy for photography.  The big telescope magnifies tiny errors.

 

It's all very complicated.  I suggest you get this book, which will introduce you to the subject.  It takes you on one appropriate path, starting with a camera and a lens.  Trying to learn imaging DSOs on a CPC 1100 would be like trying to learn driving in Manhattan with a top fuel dragster.  It's simply the wrong tool for the job. 

 

http://www.astropix....bgda/index.html

 

The 11 is a fine scope for imaging the Moon and planets.  They're sunlit, it's like taking a snapshot in daylight (although you actually take a video).


Edited by bobzeq25, 14 February 2018 - 04:21 AM.


#4 Samir Kharusi

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Posted 14 February 2018 - 09:28 AM

The C11 is also a fantastic scope for long exposure DSOs when used with the Hyperstar! Do not get sidetracked into acquiring a small refractor. Piggybacking camera lenses (primes) is also great. If going the Hyperstar route (560mm focal length, f2) just ask Starizona to supply the precise adapter to mount your Sony camera. Indeed, trying to do DSO imaging as a beginner at f6.3 or f11 is asking for extreme frustration. At f2 all you need is 30 second subs that you can probably execute well with many modest mounts; no autoguiding required. At f6.3 you will need minimal length subs of at least 5 minutes (probably impossible unguided with most mounts under $10k) otherwise the Stacking Efficiency will be very poor.



#5 bobzeq25

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Posted 14 February 2018 - 10:08 AM

The C11 is also a fantastic scope for long exposure DSOs when used with the Hyperstar! Do not get sidetracked into acquiring a small refractor. Piggybacking camera lenses (primes) is also great. If going the Hyperstar route (560mm focal length, f2) just ask Starizona to supply the precise adapter to mount your Sony camera. Indeed, trying to do DSO imaging as a beginner at f6.3 or f11 is asking for extreme frustration. At f2 all you need is 30 second subs that you can probably execute well with many modest mounts; no autoguiding required. At f6.3 you will need minimal length subs of at least 5 minutes (probably impossible unguided with most mounts under $10k) otherwise the Stacking Efficiency will be very poor.

I disagree with Samir on this issue (and not on much else <smile>).  Hyperstar is not trivial for a beginner to get working.  The CPC 1100 alt-az mount is not suitable, even with Hyperstar.

 

When you're starting out, there's a whole lot to learn.  The best setup is the one that gets out of your way, and lets you get on with that.  According to all the experts, and the majority of those who have done it, that's the good mount, small refractor.  Your first year or two is not about making great images, it's learning how to do them.  Careful consideration of concepts like Stacking Efficiency is down the road, there are simple rules of thumb about exposure that work "good enough" for now.

 

If your goal is to image with the C11 and show your son the results, you'll reach it faster if you use the  good mount, small refractor to cut your teeth on.  Or a camera and a lens.  I have numerous quotes from people who've found that to be the case. 

 

A theoretical case can be made for Hyperstar, and a few experienced imagers use it, but the number of beginners who start with Hyperstar is tiny, close to zero.  As a practical matter, it's just not a good beginner setup.

 

A lot of very smart people have been doing a lot of imaging for a very long time.  If ever there was something where following the crowd is a good idea this is it.  Lots of people have idiosyncratic ideas, easy to talk about, hard to make work better than the common practice.

 

Bottom line.  Experienced experts can make things work that raw beginners are ill advised to try.  Not an uncommon situation in anything, perhaps moreso in AP of DSOs.


Edited by bobzeq25, 14 February 2018 - 01:01 PM.


#6 Samir Kharusi

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Posted 15 February 2018 - 04:24 AM

For a beginner, IMHO, there is nothing better to start with than an f2.8 or faster camera lens. F2.8 enables subs around a minute each, and as long as the focal length is not too long, say, 100mm or shorter, most mounts can cope. E.g. one could start with piggybacking using a 50mm lens, then 100mm, then 200mm. But, agreed, an alt-az mount is not suitable for anything more than 30 seconds or so. For a CPC one could try an equatorial wedge. An even easier start would be an f2 lens, 30 second subs. Unfortunately f2 lenses that are fully satisfactory on stars at f2 are rare, but the Canon 100mm f2 is said to be somewhat OK. Never tried it myself. Closing it down to f2.8 lengthens the subs to 1 minute each frown.gif but a CPC on a wedge ought to be able to handle that. Nevertheless, most small scopes are unfortunately far too slow to use without autoguiding. Personally I consider autoguiding as the tertiary stage of astrophotography, even though most of us jump onto that far too early in the game. Myself, I have given up autoguiding 10 years ago. Simply too much bother when you have a Hyperstar. The C11 Hyperstar is indeed very suited for shooting unguided; focal length similar to small refractors at 560mm but f2! 30 second subs. Closest one comes to snapshooting DSOs. My main recommendation to beginners is to go with camera lenses up to, say 200mm f2.8, and then, if you already own an SCT, switch over to a Hyperstar. If you have no SCT then unfortunately you have to conquer autoguiding with a small refractor.


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#7 NightOwlPhysicsNerd

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Posted 17 February 2018 - 05:18 AM

Thank you all for the replies, Eventually I did figure out that the optimal distance from the focal reducer is 100mm... to just about 105mm to the sensor plane. 

I did use a wedge, and while setting it up was.... well not fun... i played with some concepts and came up with a solution.

The solution:

Folk mounted dovetail (VDUP4- V Series), some 3D printing to build an adapter of sorts and an ADM adapter for the pole-master(VPA-POLE), and now It plate-solves pretty quickly.

HOWEVER, it still needed an auto-guider, but guys I have to be honest with you, Auto-guiding using PHD2 turned out to be pretty easy. I have been using MarksAcquaition software, but I think I will move to a QHY or ZWO or somethign of the like in the future, and then maybe move this down to a refractor on a Gem-EQ

NOW, 

A couple of things that I quickly learned... 

The fork is... HEAVY, and honestly I think that contributes to 80% of peoples issues... seriously that thing is crazy heavy as hell. I am significantly stronger than most, and it was still a chore for me.
The mount isn't the worst part, its not what makes the whole thing actually suck, its the godforsaken focuser... i mean ... I just ... I don't even... already ordered a feather touch and a remote.

Let me just say for a moment, if anything keeps me from buying another Celestron, its going to be the quality of the focuser, and the various attachments. I mean its like buying a nice car and finding cloth seats. 

The Wedge is actually pretty easy to deal with... but again, upper body strength is your friend.

As for using a 2 inch rather than a 1.25.... there is a very good reason.... on the 11 inch scope you get some pretty bad vignetting on a Full frame when you aren't using 1.25 tubes with without a Focal reducer, not so much on the Crop sensor of an APSC and none when you use a Focal reducer (well yeah some still actually) it might not matter all that much, but it makes me freaking nuts.

I also have a couple of A7 series that I would like to use, and besides, I have plenty of counterbalance on the front... But it is something to consider.

Lastly, Yes a sub f2.0 is AWESOME for use as a regular night photography (honestly these days, anything less than 1.8 with ISO capable camera like we have now... is pointless), if anyone is interested, the Sigma 16mm f/1.4 DC drops PERFECT shots, ultra low noise ... surprisingly though it seems like there are no stars in the sky.... weird. (star eater joke folks ... I honesty can't SEE a difference)

Thanks again folks for the input.
 




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