Decision Making Help - Flattener f/7 or Flattener/Reducer f/5.6

Brian Boyle:
I really hope I don’t send this thread off on the wrong tangent, but I have a slightly different take on “fast” v “slow”.  I am probably alone (and it may be due to my background) but I don’t think in terms of fast and slow, but rather narrow v wide field.  

You can always make any telescope “faster” by binning up (at a small cost in read noise - unnoticeable if you are sky-noise limited). In terms of speed alone an f/7 can be made into an effective f/3.5 simply by binx2. 

The real difference between “fast” and “slow” is arcsec/px and hence field of view, given a fixed detector size.  My suggestion would be to get a corrector based on the typical resolutions/field size you want to image at.  

When faced with the same quandary in moving from f/5.5 to f//8 I asked  myself “what pixel scale do I want?” and “what size of field do I want?”

For my RC8 f/8 this drove me to a .67 x reducer/flattener as I wanted to maximise
field-of-view, consistent with optimal sampling of the best seeing I was likely to get.  (And I am still waiting for!)

Tangents welcome Brian!  I started the hobby 4 months ago, so LOTS of things to learn.

I have a Nikon D780, which will be my OSC for the foreseeable future, and I just purchased the ASI6200MM.

D780 = 5.92 µm pixels
ASI6200mm = 3.76 µm pixels

So for the different focal lengths with the flatteners and reducers:

• Focal Length  -->  D780 ; ASI6200mm

1. 925mm -->  1.32 "/pixel ; 0.84 "/pixel
2. 740mm -->  1.65 "/pixel ; 1.05 "/pixel
3. 666mm -->  1.83 "/pixel ; 1.16 "/pixel

Astronomy tools with my ok to poor seeing suggests 0.67 "/pixel to 2.5 "/pixel, so for the ASI6200MM I will be potentially oversampling on poor nights, but otherwise all combinations look to be in a good range for both sensors.

I am a real advocate of focal reducers, so I'm a little bias.   I agree with  what Brian is saying about pixel scale and field of view.  However disagree about binning.

Yes, double your pixel size and you will increase chip sensitivity,  but at the cost of resolution.  But that is relative to your focal length and pixel scale.  Binning is mostly use when the focal length is too long for the pixel size and rendering over sampling.  Binning then creates a better sampling.

When using a focal reducer I think there are a few things to consider.

1. Imaging time as you mentioned.  F7 to F5.6 can greatly reduce the time needed for individual subs and can also reduce the overall time.  However, depending the camera chip pixel and scope focal length it may cause under sampling.  I live in the north east jet stream and seeing is rarely better than FWHM 2.  The FWHM of a stacked image can be as bad as FWHM 5.  Therefor I get little resolution improvement from smaller ARC/SEC/PIX a longer focal length will achieve.  Depending where you image, this become relative.

2. Decreasing the focal ratio increases the Signal/Noise ratio.  For those that claim aperture is the only factor in light gathering, they or wrong, and don't understand how the light spread is changed over the number of pixels.  Please refer to Craig Stark's lecture. https://www.youtube.com/watch?v=EO4QFb3ydNM&t=38s

3. Matching the reducer/flattener to the optics.  A number of companies make reducer/flatteners.  The best choice is one designed for your particular scope, but that is not always possible.  All flatteners have a focal length and that length needs to be accomplished from flattener to chip in order to achieve best flattening of the field curvature.  Otherwise you will have coma.  Reducer's on the other hand do not flatten and can work at different distances from the chip.  I use the Astro Physics 27TVPH.  It is a reducer (not flattener) and achieves greater reduction the more distance between it and the chip.  It is optimizes for a reduction of 0.75x.  I use it with a AP130GTX F6.3 at a reduction of F4.7. (4.54 pix) rendering a arc/sec/pix 1.53.  So ever with the reduction, I'm well below what my skies seeing will allow.   Using a shorter focal length like the Tak FSQ106ED with a 0.73x reducer/flattener ( F3.64) and smaller pixel camera I get 1.99  arc/sec/pix.  Even though the reduction is greater, due to the smaller pixels (3.69) I get better sampling for my skies.  

I used a Televue 0.8x TV102 reducer/flattener on a Astro Tech AT111ED and it worked great.  I then bought a TMB130SS and the Televu reducer/flattener over flattened the curvature and created coma.  Not all flatteners work equally on all scopes. I then use the AP 27TVPH and even though it was not a flattener the field curvature was better and the coma was gone.  Note:  I saw no increase in resolution between the 130 aperture and the 111.  The sky has the last word.  However due to better glass the color was better.  You should see a big difference between 81mm and 130mm.

4. Decreased focal length makes guiding easier.

5. And when you need the longer focal length, you have it.  Pixel scale should always be best sampled to your sky conditions.  Field of view will vary with the particular object or the composition of an object.

I know the above is my personal experience, but the concepts apply to all.  For me, using  a focal reducer is a no-brainer.  The difficulty is finding one that works well with your optical design and achieving the correct focal distance from flattener to chip.  Base on your sky conditions you may see no lost of resolution due to the increased arc/sec/pix.  Again the Sky always has the last word.  I feel sometime like I am warring a tuxedo to a Texas Bar-B-Q. 

Lynn K

Lynn K:
I am a real advocate of focal reducers, so I'm a little bias.   I agree with  what Brian is saying about pixel scale and field of view.  However disagree about binning.

Yes, double your pixel size and you will increase chip sensitivity,  but at the cost of resolution.  But that is relative to your focal length and pixel scale.  Binning is mostly use when the focal length is too long for the pixel size and rendering over sampling.  Binning then creates a better sampling.

When using a focal reducer I think there are a few things to consider.

1. Imaging time as you mentioned.  F7 to F5.6 can greatly reduce the time needed for individual subs and can also reduce the overall time.  However, depending the camera chip pixel and scope focal length it may cause under sampling.  I live in the north east jet stream and seeing is rarely better than FWHM 2.  The FWHM of a stacked image can be as bad as FWHM 5.  Therefor I get little resolution improvement from smaller ARC/SEC/PIX a longer focal length will achieve.  Depending where you image, this become relative.

2. Decreasing the focal ratio increases the Signal/Noise ratio.  For those that claim aperture is the only factor in light gathering, they or wrong, and don't understand how the light spread is changed over the number of pixels.  Please refer to Craig Stark's lecture. https://www.youtube.com/watch?v=EO4QFb3ydNM&t=38s

3. Matching the reducer/flattener to the optics.  A number of companies make reducer/flatteners.  The best choice is one designed for your particular scope, but that is not always possible.  All flatteners have a focal length and that length needs to be accomplished from flattener to chip in order to achieve best flattening of the field curvature.  Otherwise you will have coma.  Reducer's on the other hand do not flatten and can work at different distances from the chip.  I use the Astro Physics 27TVPH.  It is a reducer (not flattener) and achieves greater reduction the more distance between it and the chip.  It is optimizes for a reduction of 0.75x.  I use it with a AP130GTX F6.3 at a reduction of F4.7. (4.54 pix) rendering a arc/sec/pix 1.53.  So ever with the reduction, I'm well below what my skies seeing will allow.   Using a shorter focal length like the Tak FSQ106ED with a 0.73x reducer/flattener ( F3.64) and smaller pixel camera I get 1.99  arc/sec/pix.  Even though the reduction is greater, due to the smaller pixels (3.69) I get better sampling for my skies.  

I used a Televue 0.8x TV102 reducer/flattener on a Astro Tech AT111ED and it worked great.  I then bought a TMB130SS and the Televu reducer/flattener over flattened the curvature and created coma.  Not all flatteners work equally on all scopes. I then use the AP 27TVPH and even though it was not a flattener the field curvature was better and the coma was gone.  Note:  I saw no increase in resolution between the 130 aperture and the 111.  The sky has the last word.  However due to better glass the color was better.  You should see a big difference between 81mm and 130mm.

4. Decreased focal length makes guiding easier.

5. And when you need the longer focal length, you have it.  Pixel scale should always be best sampled to your sky conditions.  Field of view will vary with the particular object or the composition of an object.

I know the above is my personal experience, but the concepts apply to all.  For me, using  a focal reducer is a no-brainer.  The difficulty is finding one that works well with your optical design and achieving the correct focal distance from flattener to chip.  Base on your sky conditions you may see no lost of resolution due to the increased arc/sec/pix.  Again the Sky always has the last word.  I feel sometime like I am warring a tuxedo to a Texas Bar-B-Q. 

Lynn K

Hello Lynn,

Thank you for the informative post!

How do you find out what your typical FWHM is for your skies?  There is a clear sky chart a few miles from here, so I have being saying I typically have ok to poor based on that.  I do know I have some negatives against where I live (extreme humidity, 1.5m above sea level, wind, etc.), so maybe it's even worse than ok to poor?

I've also edited my original post to clarify that WO has 3 offerings for the FLT 132.  They have a dedicated flattener or a 0.8x or 0.72x flattener/reducer.  I use a reducer/flattener on my 478mm f/5.9 81mm aperture WO GT 81 even though the focal length is already short, and I have been really happy with the 380mm f/4.7, but I am still very inexperienced as it's my only scope and I've only had it for 3.5 months or so.

Thanks,
Alex

Hi Alex, I must confess that I have no empirical quantified method of measuring atmospheric seeing.  My FWHM is based on 16 yrs of imaging in the Pa Lehigh Valley at 600 ft. elevation.  I use Maxim DL for acquisition, focusing,  calibration and stacking.  When I focus, I use a medium to small star.  The best I can ever get is 1.2 FWHM.  But that is rare.  1.7 to 2.5 FWHM is average and often no better than  3 FWHM.   Maxim's stacking routine allows you to set parameters of FWHM, roundness and contrast  averages of each sub frame.  My experience has been that the sub frame average is much worst than the star focus FWHM.  In other words, even though I got a 1.5 to 2 FWHM when I focused, if I set the Maxim parameter to 2 FWHM most if not all the sub frames will be rejected.  In order for 70% to 80% of the sub frame to pass, I will need to set the FWHM parameter to 3.5 or higher.  And as I mentioned. that can be as high as 5 or worst.  On rare occasions I get a stack averaging in the 2.5 FWHM range.

So, I am sorry, but my evaluating seeing is not an empirical method, but based on years of Sub frames average.  Using high quality optics, I know that is not the cause of the poor FWHM.  When I put my name on the AP130GTX waiting list, My thinking was "Why Not".  8 years latter when I received notice that my number had come up and the scope was ready for purchase, I debated whether to get or not.  Of course it was the cost, but more as whether the scope would give me any advantage based on my poor atmospheric conditions.  It dose achieve a darker background base on visual observations, which in important.

A couple of years ago I visited my son in  Hawaii and traveled to Maui, Hawaii.   I was able to observer the skies at 9000 ft. om Mt Haleakala. Since Mt Haleakala is near to equator and at 9000 ft., my scope Fl/camera pixel 1.55 arc/sec/pix would more likely be achievable. [b]  [/b]But, is a wasted effort in my backyard.

So, in my backyard using a focal reducer, even though it increases the arc/sec/pic,  looses no resolution because the seeing will only render 2.5 FWHM at best and on average 3 to 4.  And a big gain in field of view and S/N.

Lynn K.

My opinion/experience is similar to Lynn K.  During my first year of AP I found of my subs for a given target will vary from 2.8”-4.0” given a SCT, small refractor or Newtonian and from 60s-450s subs.    As a result sampling at 3.35x give 0.8”-1.2” as a target arc-sec per pixel resulting in a target focal length between 650-900 when combined with  most newer 3.75um sensors.    Second, guiding error and star eccentricity wasn’t great at <1.0” arc/pixel lending me to a 1”-1.2” target.    At that point I decided a 5”-6” class refractor @f5-6 would be enough to maximize what I could achieve at my location 95% of the time.    So if your subs are >3” go for the reducer.    If they are less go for the FF and native FL to keep max resolution…   or get both… Reducers for extended objects, FF for galaxies.