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Starting Out in EAA - Reflector or SCT?

EAA equipment
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#26 OhioKraken

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Posted 19 April 2019 - 07:34 AM

Bought a used Celestron 6 Evolution last evening from a fellow Cloudy Nights member.  Next step is to purchase the camera and focal reducer.

 

It was mentioned in this forum that you can stack two f/6.3 to equal a f/3.3 reducer.  Are there any drawbacks to stacking focal reducers?  Are the images cleaner or sharper without stacking?  Just curious.

 

Thanks again to all for the great advice!

Chris



#27 Stargazer3236

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Posted 19 April 2019 - 08:37 AM

When you stack focal reducers, you will get a lot of vignetting. Just use a small sensor camera, like the 385MC or 224MC camera to avoid vignetting issues. The camera sensor will only use the center portion of the incoming light and you will get marginal, if none at all, vignetting. With a sensor so small, the ASI224MC camera would be a perfect choice when stacking focal reducers.



#28 OhioKraken

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Posted 19 April 2019 - 08:54 AM

Got it...I'm waffling between buying the 385MC or the 224MC.  The 385MC offers a wider FOV but costs $120 more.  

 

I also see 0.5 focal reducers available and they seem much cheaper than the F/6.3 reducers.  Is there a quality difference between the two?



#29 BJS

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Posted 19 April 2019 - 09:36 AM

I would choose the 385..the extra fov will make centering objects a little easier.  

 

The .67x reducers are designed for SCT's; they are also correctors.  The .5x's are just reducers....they won't help correct the image any.  In practice, I don't see much difference.  ymmv. 


Edited by BJS, 19 April 2019 - 09:40 AM.


#30 Rickster

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Posted 19 April 2019 - 12:17 PM

There are a number of trade offs to be considered when choosing focal reducers.  For example:

 

Focal reducers make the image size smaller.  This means that the image of a given object will fit on a smaller sensor.  It also means that each pixel will receive more concentrated light.  The net result is that field of view is increased and exposure time is decreased (photographic speed is increased).  These are both generally considered to be good things in the EAA world.  The trade off is that reducers steepen the light cone, increasing the angle of the light rays.  This magnifies image train misalignments in collimation, focus, sensor tilt, mirror tilt (flop), etc.  It also increases coma.  The bottom line is that it makes the image train "fussy" and less forgiving.   

 

The better SCT focal reducers are also field correctors; for example, the common Meade/Celestron 0.63x (aka 6.3) SCT reducer/corrector.  These will not only "reduce," they will also flatten the image for use with photographic sensors, therein, improving image quality.  The tradeoff is that they typically include 4 active elements and sometimes a 5th passive element.  That means the light must pass through 8 to10 glass surfaces.  Each glass surface reduces the light transmission, and degrades the image, by causing internal reflections.  Stack two of them and you have a lot of glass surfaces in your image train.  Additionally, stacking two of them means increasing the correction as well.  This can lead to an over corrected (distorted) image.  The strength of this effect varies with the spacing between the correctors.  I haven't been happy with the results I have gotten when stacking reducer/correctors, but other CN members have experimented with this and found combinations that they like. 

 

Reducers allow the use of a smaller imaging sensor, which reduces the camera cost.  The tradeoff is that reducers aren't free.  Plus, you need to consider the cost of spacers and the time spent experimenting and the potential reduction in image quality.  Instead of using a reducer, you can achieve the same (actually better) net result by using a larger camera sensor with larger pixels (or binned pixels). Where do you want to spend your money? 

 

As a general rule, my experience has led me to prefer larger camera sensors over reducers. I prefer to minimize the amount of glass in the light path. And good used astro modified DSLRs (large sensors) are becoming less and less expensive as people flock to the latest and greatest astrocams.  But DSLRs have their tradoffs as well.

 

And to be clear, there are good exceptions to every rule.  For example, the common 0.63x SCT reducer/corrector was designed specifically for photographic work with f10 SCTs and has stood the test of time.  It is a no brainer.  In another example, I use an ASI290 mini mono with a cheap 0.5x reducer spaced for 0.8x focal reduction in a 50mm e-finder.  In that case, FOV and speed are more important than image quality.  Both of these examples are near perfect uses of focal reducers.


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#31 Stargazer3236

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Posted 19 April 2019 - 04:41 PM

Got it...I'm waffling between buying the 385MC or the 224MC.  The 385MC offers a wider FOV but costs $120 more.  

 

I also see 0.5 focal reducers available and they seem much cheaper than the F/6.3 reducers.  Is there a quality difference between the two?

The 0.5 reducers do not include the corrector effect. With the 0.5 reducer, you will get some coma at the edge of the field. It will not affect the sensor on the 224 or 385, but any sensor larger than that, you will notice coma on the stars from about 75% out to the edge of the FOV. There is no real quality difference other than coma.


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#32 OhioKraken

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Posted 19 April 2019 - 07:26 PM

So what exactly do I need to connect the ASI385MC to the scope (Nexstar 6 Evo) as well as a f/6.3 or f/5 focal reducer?

Probably will end up ordering both...may start with the f/5.

 

Thanks!

Chris



#33 OhioKraken

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Posted 20 April 2019 - 09:39 AM

Here's what I'm thinking would work.

Attach a 1.25" focal reducer (.5 or f/6.3) to visual backof Nexstar 6.

https://www.highpoin...al-reducer-1-25

Then attach a 1.25" t mount adapter to the reducer.

https://www.highpoin...5inch-tmount125

 

Then attach the ASI385MC to the t mount adapter.

 

Is this correct?



#34 Stargazer3236

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Posted 20 April 2019 - 10:14 AM

If you use just the F/5 reducer, just screw the reducer on the front of the camera, then insert the camera into the visual back. If you are using the F/6.3 reducer, you first screw the reducer onto the back of the telescope, then thread on the visual back and then put the 1.25 nosepiece of the camera into the 1.25" visual back.



#35 Rickster

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Posted 20 April 2019 - 10:42 AM

For a f6.3 reducer, you need about 105mm of spacing between the sensor and the midpoint of the FR lenses  (depending on what brand you buy).

 

http://www.wilmslowa...rmulae.htm#FR_b



#36 Rickster

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Posted 20 April 2019 - 12:20 PM

Here is a good example of how to do it.

 

https://www.cloudyni...-atik-infinity/



#37 OhioKraken

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Posted 22 April 2019 - 08:09 AM

I note (after reading the links provided) that you can use a f/6.3 reducer with less space than recommended between the focal reducer and the camera.  If this is done, the reducer still works but loses some of its reducing power, i.e. a f/6.3 reducer with 67.5 mm of spacing (instead of 105mm) becomes a f/7 reducer.

 

Is it possible to make the spacing greater than 105 mm to approach f/6 or f/5?



#38 alphatripleplus

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Posted 22 April 2019 - 09:48 AM

I have both a C8 and C6 and use f/6.3 reducers. The often quoted 105mm spacing will get you to about f/6.3 on a C8, but not on a C6. By increasing the spacing from FR to sensor you do indeed increase the focal reduction. However, with the C6, the lowest I have been able to get to with a single f/6.3 reducer was f/6.1, and to achieve that the FR to sensor distance was well over 150mm (don't have the exact distance).



#39 Rickster

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Posted 22 April 2019 - 11:47 PM

In basic theory, yes, you can achieve f5 with a f6.3 reducer by increasing the distance between the focal reducer and the sensor.  However, as Errol has pointed out, other factors may limit the focal reduction in practice.  For example, increasing the reduction may result in not being able to reach focus.  And, in the case of the SCT, the focal length of the SCT varies with the position of the mirror. I forget offhand which way it goes, but inferring from Errol's comment, it appears that the focal reducer results in the mirror moving in the direction that increases focal length.  The other factor is that a SCT FR is tuned to f6.3 (typically) and attempting to vary from f6.3 will result in distortion.

 

Regarding the 0.5x reducer.  I assume that you are considering an inexpensive 1.25" reducer (right?).   All I can say is that I haven't been able to get a satisfactory 0.5x reduction from an inexpensive 1.25" 0.5x reducer.  But I haven't tried that reducer with my SCTs.  Maybe someone else can comment there.

 

In general, my experience is that FR specifications are optimistic if you are at all particular about image quality (even when used with small sensors).

 

Once you start taking images, upload one to http://nova.astrometry.net/upload  It will calculate the focal length you actually achieved.  It will likely be different than what you expected.



#40 OhioKraken

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Posted 23 April 2019 - 07:07 AM

Rick,

 

I was originally considering the .5 reducer to reduce the focal length so that my working magnification with the ASI385MC was closer to 90-100X.  However, last evening I broke out the new scope to test out the evolution mount and goto capabilities.  After the normal three star alignment, I was able to see every object I pointed the scope at with a 13mm eyepiece (115X).  Tracking was great as well and I was very impressed.

 

So at this point, I think I will spring for the F/6.3 reducer (Celestron P/N 94175).  If I get f/6.5, I will be right around 115X with the ASI385MC (150mm x 6.5 / 8.4 mm).  Biggest question I still have is the optical train...I will thread the f/6.3 reducer onto the back of the OTA.  Then I can either use a visual back or a t-adapter (Celestron 93633-A) to create some distance before inserting the camera nosepiece into the adapter or visual back.  I don't know how much distance the t-adapter or visual back adds - this doesn't seem to be listed anywhere.



#41 makeitso

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Posted 23 April 2019 - 08:29 AM

This is what you nee: https://www.highpoin...AiABEgL2mfD_BwE

 

You put this on after the 6.3 reducer, then the t adapter for your camera. You may need some spacers to get the right back focus. You could attach the camera directly to this if you’re using a asi camera. 

 

Jack



#42 Rickster

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Posted 23 April 2019 - 10:44 AM

Fortunately, add on T thread spacers aren't expensive (try Ebay).  Unfortunately, they don't all have exactly the same threads.  Getting matching threads seems to be a crap shoot.


Edited by Rickster, 23 April 2019 - 10:44 AM.


#43 OhioKraken

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Posted 24 April 2019 - 06:57 PM

So I ordered a ASI385MC and got a call today...I was offered an ASI385MC-COOL for the same price and accepted.  Guess they're trying to get rid of extra inventory.

 

In any case, it's a great value...the camera is almost half price.  I then realized that the camera is 1.5" taller, a little wider in diameter and 9-10 oz heavier.  I'm not concerned about the heaviness or extra diameter, but am wondering if I made the right decision based on the length.

 

With the cooled camera on the Nexstar forkmount, I have about 3" of space to fit the focal reducer and the optical spacing.  Plus the wires come our the back of the cooled version whereas they come out the side of the non-cooled version.

I think I have two options:

1. I could keep the cooled camera and use my star diagonal to avoid contact with the mount.  It seems like most people don't do this because they are trying to keep glass and mirrors out of the light path.  

OR

2. I could return the cooled version and get the uncooled version and avoid using the star diagonal.

 

Thoughts and opinions?  Thanks!



#44 descott12

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Posted 24 April 2019 - 08:51 PM

Yes I think you will probably want to get the non-cooled version. I have a 294 Pro/cooled and if the 385 is in the same can/format then clearance will definitely be an issue. And I certainly wouldn't want to hang an expensive camera off a diagonal not just for optical reasons but I am not sure how secure it would be.

I wonder if focusing would be an issue too...



#45 DSO_Viewer

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Posted 27 April 2019 - 12:21 PM

So I ordered a ASI385MC and got a call today...I was offered an ASI385MC-COOL for the same price and accepted.  Guess they're trying to get rid of extra inventory.

 

In any case, it's a great value...the camera is almost half price.  I then realized that the camera is 1.5" taller, a little wider in diameter and 9-10 oz heavier.  I'm not concerned about the heaviness or extra diameter, but am wondering if I made the right decision based on the length.

 

With the cooled camera on the Nexstar forkmount, I have about 3" of space to fit the focal reducer and the optical spacing.  Plus the wires come our the back of the cooled version whereas they come out the side of the non-cooled version.

I think I have two options:

1. I could keep the cooled camera and use my star diagonal to avoid contact with the mount.  It seems like most people don't do this because they are trying to keep glass and mirrors out of the light path.  

OR

2. I could return the cooled version and get the uncooled version and avoid using the star diagonal.

 

Thoughts and opinions?  Thanks!

Here is a fantastic article to read about cooled vs non-cooled cameras.

 

https://www.qgdigita...3767,"page":26}

 

Steve



#46 Rickster

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Posted 27 April 2019 - 06:41 PM

I have a question for you guys that live with light pollution.  It seems to me that camera noise would be much lower (by orders of magnitude) than your LP noise (even if you use LP filters).  And therefore, the camera noise would be buried in the LP noise and would be cut off when you adjust the black level on your histogram.   So, is there an advantage to using a cooled camera for doing EAA in a LP environment? 

 

Please forgive my ignorance if the answer is obvious.  I have never done EAA from a LP area.  From my location, even an inexpensive DSLR, like a T3, doesn't introduce enough noise to be a significant factor for EAA.  (Although I can see where there might be some cases where low camera noise would be important to the AP guys, for example narrow band imaging.)



#47 DSO_Viewer

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Posted 27 April 2019 - 08:35 PM

I have a question for you guys that live with light pollution.  It seems to me that camera noise would be much lower (by orders of magnitude) than your LP noise (even if you use LP filters).  And therefore, the camera noise would be buried in the LP noise and would be cut off when you adjust the black level on your histogram.   So, is there an advantage to using a cooled camera for doing EAA in a LP environment? 

 

Please forgive my ignorance if the answer is obvious.  I have never done EAA from a LP area.  From my location, even an inexpensive DSLR, like a T3, doesn't introduce enough noise to be a significant factor for EAA.  (Although I can see where there might be some cases where low camera noise would be important to the AP guys, for example narrow band imaging.)

A cooled camera of the same make always trumps a none cooled camera under all types of conditions esp. when the temperatures start to creep upwards.

 

Steve



#48 Stargazer3236

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Posted 28 April 2019 - 03:07 AM

A cooled camera of the same make always trumps a none cooled camera under all types of conditions esp. when the temperatures start to creep upwards.

 

Steve

However, a non-cooled camera trumps a cooled camera when it comes to short sub exposures, especially when doing EAA (electronic assisted astronomy). Using short exposures, like 2, 4, 8 and 15 seconds, you do not accumulate as much noise as you would when doing long exposure astrophotography in which you need to turn the cooling on to eliminate the noise build up on exposures over 60 seconds. If you are doing autoguiding and setting your camera to take 2, 3, 5, or 10 minutes exposures, you will see accumulated noise from the more timely exposures. plus you will not have to add another cable to the setup to turn on the power to the cooled camera, plus, not to mention adding more weight to your mount to hold the battery in place for the cooled camera.



#49 Stargazer3236

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Posted 28 April 2019 - 03:17 AM

I have a question for you guys that live with light pollution.  It seems to me that camera noise would be much lower (by orders of magnitude) than your LP noise (even if you use LP filters).  And therefore, the camera noise would be buried in the LP noise and would be cut off when you adjust the black level on your histogram.   So, is there an advantage to using a cooled camera for doing EAA in a LP environment? 

 

Please forgive my ignorance if the answer is obvious.  I have never done EAA from a LP area.  From my location, even an inexpensive DSLR, like a T3, doesn't introduce enough noise to be a significant factor for EAA.  (Although I can see where there might be some cases where low camera noise would be important to the AP guys, for example narrow band imaging.)

As DSO_Viewer calmly stated, a cooled camera can be beneficial, but up to a certain point. When you are making EAA sub exposures, you do not accumulate that much noise, depending on your gain settings and histogram adjustments and your sub exposure time. You can take 2 second shots of 300 gain and accumulate between 50-100 sub exposures (subs) and get a better image than a single shot at just one 3.5 minutes sub. The longer the sub exposure, the more the noise factor creeps up. You can of course post process an image to remove most of the noise, but you do not need to spend extra money (unless you are rich) on a camera that can do the same things un-cooled vs cooled.

 

I personally bought the ASI294MC camera un-cooled. It was about $300 less than the cooled version and the benefits of having a light camera and no need to power up the camera were bonuses. I mainly do EAA astronomy and could not be bothered to buy a cooled camera, unless I lived in a remarkably warm climate year round. Living in New England, the Summer is just the annoying factor in an otherwise cool and cold climate in the Spring and Autumn and Winter.

 

In my images, I hardly notice that much noise that detracts from the overall image quality, especially since I can play with the histogram sliders and color saturation to suit my needs on a particular DSO. SharpCap3.2 Pro is an awesome app and gives me the options to adjust my ROI (Region of Interest), plus subtract or add the resolution to mimic anything from an ASI224MC all the way up to my ASI294MC in terms of image area and reduce vignetting when imaging.



#50 descott12

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Posted 28 April 2019 - 06:52 AM

I have the cooled version for the 294 and I think I see benefit from it but it is hard to tell when doing EAA. Usually it is front-mounted on my HyperStar and that setup works well.

 

However, if you are planning to rear-mount on a forked SCT, I would seriously consider getting the non-cooled version. The form-factor of the cooled version is a much taller can shape and there is not enough clearance under the scope. Invariably (and this happened again last night) you will goto something at a 50-60 degree elevation and the camera will hit the mount. Not great for the scope, your alignment gets messed up and, if you view remotely like me, there is alot of wasted time trying to figure out why you are not seeing what you are supposed to be seeing.  Definitely get the uncooled version if this is your planned situation.




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