| Flattener or Flattener/Reducer | |
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| Flattener, try f/7 | |
| Reducer, you want that f/5.6 | |
| Don't be a cheapskate, get both | |
| Wait, isn't there a 0.72x reducer, why didn't you mention that one? | |
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I have been eyeing getting a larger refractor. I was able to get ahold of a William Optics FLT 132 that should be coming next month. I was hesitant on the WO FLT 132 without the feather touch option, but it sounds like the new ones for this year have a dual speed R&P focuser like the GT 81-IV that I have, and it has been quite good to me. The next hesitation was around the slightly slower scope. Today with the GT 81-IV and reducer I am at f/4.7. The FLT 132 is f/7. From reading, the math sounds like ( 7 / 4.7 )^2, which would mean it is 2.21x slower on diffuse targets if I am understanding things right. I'm assuming that would mean to get equivalent to the 300s subs I take now I would need around 600s subs. I have done some test shots at 600s, and the subs looked fine, albeit at 382mm focal length vs. the 925mm on the FLT 132. It also might be that maybe I don't need to take 300s subs with my new camera, but that is what I had been doing and things still looked good at 300s, so I haven't experimented yet. Seeing is often poor and good seeing is rare, so any lost time seems troublesome. So with the FLT 132 on the way, assuming I only get one flattener or reducer for now, should I consider getting the regular flattener and trying it at f/7, or should I get the 0.8x reducer and try it at f/5.6 vs. the f/4.7 I have today. That would put it at 1.4x slower, which seems a lot faster than a 2.21x reduction. The only drawback would be losing that 185mm from 925 down to 740mm focal length. Editing to help with some of the great responses: William Optics offers:
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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!) |
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Brian Boyle: 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:
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. |
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Hi Alex, It looks like you have a very good setup. Congrats on the ASI6200MM - I think that is a very wise choice. I am saving up my pennies for that little beauty too. If this were me, I would definitely go for the 0.66 reducer given the smaller pixels on the 6200. Once you get the hang of mono - and it is pretty easy - you wont want to go back to OSC! You can then observe with/without reducer on the bad/good nights. Even at 1.16arcsec/pix you are going to get some wonderful detailed shots, and there is always drizzling to use if you are ever under-sampled. If you are over-sampled then just bin-up. CS Brian |
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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 |
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Brian Boyle: You are right about the "fast and slow". I have a F4 10" and a F8 10" and they collect the same data on the same targets when using the same sample rate except one has 4x the FOV. All things equal, they collect the same amount photons since the aperature didn't change. |
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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 |
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Despite the imperfect medium of the internet, I suspect Lynn and I are in 100% agreement. In SNR the sky (nearly) always wins. NB from dark sites are the exception. Completely acknowledge that the downside of binning is resolution, but since the thread was implicitly about losing resolution through focal reduction to improve SNR, I thought I would frame my advice in the same vein. Notwithstanding that, as Lynn says, get the reducer. |
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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. |
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Hello Lynn, thanks for the response. I was hoping there was some tool that I did not know about, but I suspected it was from experience. I've only recently, the past two sessions, started paying attention to FWHM. I had only been taking data for 3-4 hours with my DSLR in an evening and processing it. If the focus was a bit off from perfect, it would still make a nice image with a OSC. Now with the monochrome camera, I've found that having the focus off night to night even a little can result in halos around stars for whichever filter was out of focus. So now I am paying close attention to FHWM to make sure my focus is as sharp as possible to minimize the post processing efforts. Seeing was exceptionally good last night for me, and I ended up between 1.8 and 2.5 FWHM. I also received my ZWO EAF last night and installed it just before I needed to setup. Thanks again for all the responses! So much to learn! Clear skies, Alex |
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| 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. |
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I went ahead and ordered the 0.8x reducer/flattener since it was in stock. I was told the flattener would be back in stock by the time I would get the scope, but I would hate to get the scope and not have anything to use with it. I'll probably grab the flattener at some point in the future. |
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Digging this thread up hoping for an update.... I also have an FLT132 and ASI6200MM. I'm currently using the Flat8III and I'm getting absolutely beautiful, small, round stars all the way to the edge of my full frame. The results with this OTA/Flattener/Camera combination are fantastic. I have a separate OTA that I use for wider FOV. While it produces lovely images, they are just slightly "behind" what I'm getting with the FLT132/Flattener8III. I'd like to use 0.8x William Optics FFFR or the 0.72x William Optics FFFR to increase the FOV with my FLT132. My concern is star shape across the entire sensor. The IMX455 sensor is very, very unforgiving and every little aberration becomes visible. I'd like to know if the star shapes across the full frame of the ASI6200 are still small and round when using either of the William Optics reducers. The OP (Alex) says he purchased the 0.8x FFFR. Can the OP comment on the performance? |
