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Optimal Sub exposure length...

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#1 CloudGazer

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Posted 22 October 2020 - 03:49 AM

Hi everyone,

 

I've read Chuck Anstey's rather technical article on sub exposure length (and number), but I'm unable to calculate my own optimal exposure length, as I don't have the tools to normalize pictures… And I'd likely make too many mistakes along the way. 

 

According to light pollution map, I'm in a Bortle class 5 zone with a sky brightness of 19.9 mag/Arcsec².
I'm using a stock Canon 6D through an old f/7 triplet, shooting at ISO 800.

 

Any good suggestions on sub length?

 

I actually made a test, shooting 2x20min at M45, one consisting of 32x30sec., the other 8x120sec. 

Temperature was about 10°C/50°F.  All stacked in deepskystacker, and the resulting stack was considerably more bright with the long exposures - Makes sense.

I ran the same actions on them in PS, doing 9 consecutive curves raises.  After that, I set blackpoint in levels at approximately the same spot (5x5 sample).

 

The results seemed to indicate that 8x120seconds was a little better than the shorter exposures.  The stars are generally smaller, and the longer subs have surprisingly more blue in the nebulosity/dust… I've tried redoing the blackpoint several times on the shor-sub picture, no change.  But I might be completely off on the wrong track here, or missing something obvious.

 

 

All comments or suggestions appreciated.  Next time the moon&clouds allow, I might try 180sec. Vs. 60sec instead…

 

Attached Thumbnails

  • Exposure comparison.png


#2 sg6

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Posted 22 October 2020 - 06:11 AM

Just looked at the artical and one aspect stood out: It was from 2007 and the term CMOS does not exist in it, whereas CCD does.

 

Most cameras now are CMOS and they are different to CCD. You cannot carry over CCD ideas to CMOS and expect the same result(s). Different technology.



#3 the Elf

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Posted 22 October 2020 - 08:23 AM

Wellcome to CN!

As you for sure have guessed you are not the first asking this question. There is a very recent thread going on where you find a lot of good information:

 

https://www.cloudyni...ith-cooled-osc/

 

In there you will find this video:

https://www.youtube....h?v=3RH93UvP358

 

Dr. Glover presents a table of optimal exposure times in there.

 

In the thread you will also find statement that all this is focused on the impact of noise. But there are other considerations. Here is my list of constraints:

 

- minimum number of subs for good pixel rejection and effective dither, 20+ imho

- maximum number of subs limited by computer and processing time, for my 3yo laptop 200 until I run out of disk space

- avoid saturation of bright objects using lower ISO or shorter subs

- minimum impact of read noise, that is what the video is about

 

For beginners there is a different problem very often: the sub exposure length is limited by the tracking capabilities of the mount, by periodic error or by poor polar alignment. You need to get a sturdy high quality mount first, master auto guiding next and after that it is time to think about the noise. Which mount do you have? Do you auto guide? Do you use PEC? What is the focal length of your scope? You might want to put a list of your equipment in your signature and your location to your profile. This helps us to help you.

Temperature is probably not relevant because even at Bortle 5 the background brings in more noise than the dark current. Glover explains it in the video.

 

If you are not only new to CN but also new to the hobby here is a general overview I wrote for newbees:

http://www.elf-of-lo...ingStarted.html

Mind the book list at the end! Hope this helps.

Clear skies!



#4 ryanha

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Posted 22 October 2020 - 08:33 AM

This article is great:  Picking the correct exposure for Deep Sky by Dr. Robin Glover (creator of SharpCap)

 

And it is recent and talks about the difference between CCD and CMOS.

 

The gist is that with CMOS which has low read noise, once you get to a point where your signal is much greater than the noise you have diminishing returns for longer subs.  And shorter subs have the advantage of introducing less tracking error and less chance of other light influences ruining the shot).

 

Your longer subs having smaller stars is not what I would expect.  The star size should be affected by

- Focus (should be the same each way)

- Tracking (should be the same or worse with longer exposures)

- Registration (aligning/stacking).  I guess this could make stars bigger if the registration process is bad or not rejecting bad frames in your shorter subs?

 

--Ryan


Edited by ryanha, 22 October 2020 - 08:35 AM.


#5 endlessky

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Posted 22 October 2020 - 10:03 AM

The key with subexposure length is trying to achieve a mean background ADU count high enough to swamp the read noise of your particular camera, at the ISO/gain you usually shoot at.

 

There are some ways to determine the read noise of the camera (PixInsight has a script: you provide two bias, two flat frames and two dark frames; it tells you all you need to know about the sensor). An alternative is this website: https://www.photonstophotos.net/ - there you can find a lot of information about many DSLRs.

 

Once you know your read noise, you try to swamp it by some chosen factor (opinions vary, these are some commonly used values: 5*RN, 10*RN, 5*RN^2, 10*RN^2). I tend to stay towards the 10*RN^2 or 10-15% higher than that, just not to have too many subs to process. You pick one of these factors, take a test exposure, measure the mean ADU with some software (PixInsight Statistics process, for example - or many acquisition softwares have their build in histogram reader). If it's in the ballpark, you are good, and that would be your optimal subexposure length. If not, adjust exposure by raising it (if the ADU is too low) or lowering it (if it is too high). Then, the "rule": the more subs (the longer total integration time) the better applies.

 

Possible drawbacks to exposing for longer than necessary (according to the factors above):

 

- saturating too many stars

- guiding/tracking issues are more likely to happen

- satellites/planes having more time to leave their marks on your pictures

- wind gusts

- animals

 

Only drawback I can think for shorter exposures, but long enough to swamp read noise:

 

- many subs = more hard drive space and better CPU/RAM for processing



#6 nimitz69

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Posted 22 October 2020 - 10:14 AM

I’ve settled on 2 min subs for LRGB and 6 min for NB.  My skies are 19.5 - 19.8.  Total integration time trumps sub length all else being equal and that’s what I focus on.  I recently finished the data collect for M13 - 15.4hrs and just started on M31 ... 5 hrs and counting ...



#7 CloudGazer

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Posted 23 October 2020 - 08:44 AM

Wow, so many good replies - Tons to learn here, now I just need some integration timelol.gif

 

With the slide Robin shows 21 minutes into the presentation, I can finally get an approximation of the light pollution I've been looking for...

The rest of the stats of the 6D I've found online, now I can calculate an exposure around 50 seconds will be optimal, if I accept 10% of total noise as read noise.  But once I finish with the articles, I might be even wisercool.gif

 

Thanks a lot for all the help!

 

My setup is newly acquired, although some is bought used.  I had a few things lying around from early attempts on film 20 years back, but not much could be used (except some experiencewink.gif )

 

EQ6-R PRO

Old TS 130mm f/7 Triplet APO with stock Canon 6D

Guiding through an old 300mm Sigma f/4 lens with adapter for ZWO ASI120mm mini camera.

 

I'm using ST4 for guiding mostly, as my EQASCOM skills is not up there yet and it often fails calibration or wanders off... Once I get it up and running, I plan to train the PPEC in the mount to improve guiding.  With ST4, I (mostly) get less than 2" error, which I figured was okay for a newbie.



#8 Madratter

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Posted 23 October 2020 - 08:56 AM

First, for most cameras and conditions there isn't an optimal exposure time, but a range of times.



#9 the Elf

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Posted 23 October 2020 - 12:16 PM

For a simple and quick light pollution estimation go to clearoutside.com and enter your loc. Right in the second line under the coordinates it will show an estimate in all different units, including Bortle.

The main purpose is a forecast for astronomy but for my place it is often too optimistic.

 

http://clearoutside....cast/50.7/-3.52


Edited by the Elf, 23 October 2020 - 12:17 PM.


#10 the Elf

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Posted 23 October 2020 - 12:23 PM

Have you got a field flattener for the scope? A regular scope (as opposed to a flattfield astrograph) comes with a curved image that does not match the flat sensor. A flattener corrects for that. It needs to fit the scope, there are no general flatteners. There are flattening reducers that give you a shorter focal length and so make the scope faster. If you haven't got a flattener yet, consider a flattening reducer. More signal improves the image a lot and a shorter focal length is easier to deal with.



#11 choward94002

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Posted 23 October 2020 - 01:02 PM

There are literally dozens of factors involved in getting subexposure times, ranging from the optical train to the sky conditions to the object detail desired down to the local weather conditions ... it's really a good part science mixed with the art!

 

A good analogy is asking "what's the travel time from New York and San Francisco?" ... you have mode of travel (car, bus, plane, ship, bike, walking) the route you take (through the mountains?  desert?  maybe sea around South America?) the time you go (winter? summer?) ... you wouldn't answer that by trying each one, you'd get a map and some weather reports and do some number crunching!

 

Similarly, I take a *lot* of pictures of a *lot* of objects (I've got a database of 742 objects to date, with tens of thousands of pix of those) and that would be impossible without using a planning program like SkyTools4 (ST4) telling me when to take the pix and for how long to get the best possible result.  You generally have between 12 and 17 nights per YEAR when a given object is at just the right location and the night is just right for that "perfect shot", and recall for stacking you're going to need a *lot* of those "perfect shots" so it only makes sense to make those nights count.

 

M76-Guided.jpg
 
Here's an example:  This is M76 which I'm building up my stacking library for ... ST4 tells me exactly what day(s) to shoot, when to shoot and for how long and which equipment to use to get the very best possible sub for my later stacking.  Note all of the factors that it's tracking ... sky altitude, moon location, seeing conditions, SQM, filter, binning level, level of detail I'm trying to get out of the target, even air temp and humidity.  Note the pix that resulted from that sub (stretched, of course) ... that's not guesswork, that's simple science
 
Yes, it's $200 ... but if your free time is so cheap that you're willing to spend days and days experimenting to find what I can calculate in seconds then I have a house that needs painting that I'm more than happy to pay you $200 once you're finally done ...

Edited by choward94002, 23 October 2020 - 01:08 PM.

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#12 ssa2294

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Posted 23 October 2020 - 01:43 PM

Hoping someone can help me understand this all. Next year I am looking at getting dedicated CMOS, possibly ASI2600MM (when it comes out). Using that as an example and Janco's calculator found in his post Ideal Exposure Calculator

 

 

Sky Brightness:  20.5 mag/arcsec2 (Using the Elf's suggestion for using Clear Outside)
F-ratio:  (Scope is ES ED80 F/6)
Pixel Size:  3.76 µm (ZWO's specs for ASI2600MC)
QE:  80 % (ZWO's specs for ASI2600MC)
Select Filter: L 300 nm (no idea whether to use different, but 300nm is fine for now)
P:  2.00 e/pixel/sec
Increase in noise allowed (%):  10 % (Uncertain for now, this seemed subjective for the user to determine acceptability?)
4.8 -

 

What I get:

Calculated Ideal exposure for specific gain  
Gain:  (Right now 0 experience with dedicated CMOS, use to using ISO800 with Nikon, just selecting something here).
Ideal Exposure:  30.4 sec

 

I do not have a 2600MM, heck it isn't even released, no experience here to speak of. Obviously I could not use Sharpcap's sensor analysis to fill out the chart, but using the existing chart for now, I am heading in the right direction?

 

Up until now I relied on using Backyard Nikon's histogram to get luminance up to 1/3rd based on whatever ISO I had selected. As complete nub when it comes to dedicated cameras, I would have thought previously "I will just use the histogram and all will be good". 



#13 CloudGazer

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Posted 23 October 2020 - 05:29 PM

Cool, clearoutside looks better than the app I use today (Located in Denmark).

I do have a field flattener (3.7") that came with the scope - It seems to work okay, although stars around the edge of the FF cam are not perfectly round.

 

I will look into skytools... It may be a bit early for me to start out with a tool like that - I still want the 'hands on' feeling at this stage!  But I can see the idea of a planning automation tool - Especially here at 55°N, where time and targets are a bit more limited.



#14 russellmm

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Posted 23 October 2020 - 10:28 PM

I have spent some time trying to understand optimal exposure but in the end I just kind of gave up and went with what some have said with regards to where you histogram sits. Attached is a single Raw image inside of APT with its histogram. This is typically where I try to be on my images. I have no idea if its right or wrong

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  • 2020-10-23 20_22_59-- APT -.jpg


#15 endlessky

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Posted 24 October 2020 - 02:51 AM

When I was relying on histogram reading to check if I was exposing enough, I would aim to stay between 1/4 to 1/3 from the left.

 

Since there are 5 boxes, 5/4 would be 1.25, and 5/3 1.66. The peak seems to be at 1.5 (one box and a half), so I would say your exposure is pretty spot on.


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#16 AhBok

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Posted 24 October 2020 - 08:13 AM

To the OP:


Using the histogram on a DSLR or planetary cam works great, but optimal imaging deep sky with a CMOS cooled cam takes a bit more work. If the calculators are giving one a problem, you might want to start with unity gain and 60 second test exposures, increasing exposure time until you overexposed a more than a few stars.

I highly recommend Robin Glover’s exposure time video. It is the most thorough, yet easy to understand treatment. Your ideal (minimum) exposure time look about right for a mono camera, but you might want to double or triple that time for a color cam with a Bayer matrix.
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#17 the Elf

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Posted 24 October 2020 - 08:58 AM

For planning free stellarium is a good and easy start. In the upper right of the main window is the camera and eye piece simulator. Enter your scope, reducer and camera sensor data and you can see a preview. Quite a lot of popular targets are in this program as photos. You can even add your own backyard from a panorama image for perfect planning.

Ruben Kier's book "The 100 best astrophotography targets" is a nice list of suggestions. At amazon the full TOC is visible in the preview. It is sorted by month. Just pick a few objects, open stellarium, Hit F3 and enter the name.



#18 sn2006gy

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Posted 24 October 2020 - 12:03 PM

Hoping someone can help me understand this all. Next year I am looking at getting dedicated CMOS, possibly ASI2600MM (when it comes out). Using that as an example and Janco's calculator found in his post Ideal Exposure Calculator

 

 

Sky Brightness:  20.5 mag/arcsec2 (Using the Elf's suggestion for using Clear Outside)
F-ratio:  (Scope is ES ED80 F/6)
Pixel Size:  3.76 µm (ZWO's specs for ASI2600MC)
QE:  80 % (ZWO's specs for ASI2600MC)
Select Filter: L 300 nm (no idea whether to use different, but 300nm is fine for now)
P:  2.00 e/pixel/sec
Increase in noise allowed (%):  10 % (Uncertain for now, this seemed subjective for the user to determine acceptability?)
4.8 -

 

What I get:

Calculated Ideal exposure for specific gain  
Gain:  (Right now 0 experience with dedicated CMOS, use to using ISO800 with Nikon, just selecting something here).
Ideal Exposure:  30.4 sec

 

I do not have a 2600MM, heck it isn't even released, no experience here to speak of. Obviously I could not use Sharpcap's sensor analysis to fill out the chart, but using the existing chart for now, I am heading in the right direction?

 

Up until now I relied on using Backyard Nikon's histogram to get luminance up to 1/3rd based on whatever ISO I had selected. As complete nub when it comes to dedicated cameras, I would have thought previously "I will just use the histogram and all will be good". 

Seems right.

 

BTW, the ED80 will struggle for flatfield on the 2600 and if you shoot LRGB it won't have consistent stars across the RGB since the two element can't correct completely.  The ED80 absolutely shined on the ASI533 as that smaller sensor had no curve, no bad stars, no problem being lit up by the image circle. I tried the orion field flattener, the apex-l and the televue reducer/flattener - they all worked beautifully with 533 but couldn't get aps-c lit up, flat or had more than 6% fade off in vignette (upwards of 10-15%)

 

Modern sensors are fast.. after 30 seconds you've well saturated read noise and probably start hitting a good mean for broadband targets.   Right now i'm in light polluted skies so i tend to shoot 60 to 90 seconds at gain 0 with the Optolong L-PRO for broadband targets.  I can pretty much stick to 5 minutes for NB regardless - maybe slightly shorter if i have some bright stars i don't want to blow out. Straight up L filters will be fast fast fast.

 

I've learned to just invest heavily in modern computer. I'll be jumping on the ryzen 5700 and getting 128gb of ram to stack all the images from aps-c.   It's a good trade off for me, more computing time than clear skies so i'm not afraid of shorter subs to maximize dynamic range, reduce noise and shoot for integration times.


Edited by sn2006gy, 24 October 2020 - 12:05 PM.

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#19 the Elf

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Posted 24 October 2020 - 02:22 PM

Here is some info about gain, again by Glover:

https://www.youtube....h?v=ub1HjvlCJ5Y

 

The key takeaway is at 6:00 there are sensors with two modes, the special one called high conversion gain. You want to pick a gain just above that level if your camera has got this feature. For the 6200 this is at 10db (=100 units) with the DR just slightly lower than gain 0

https://astronomy-im...Performance.png

If you invest in more computing power or in the imaging train is your personal decision. I am happy I can use a lower ISO with my new camera and come away with 1/4 of the subs now. For environmental reasons I do not update my computers often and my sub count limit is 200. The comfort zone ends at 100. For a decent recent one this may well be 500. It depends on the sensors pixel count of course. Using AOI settings is recommended for small objects. DSLRs don't have that.



#20 ssa2294

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Posted 24 October 2020 - 08:00 PM

Seems right.

 

BTW, the ED80 will struggle for flatfield on the 2600 and if you shoot LRGB it won't have consistent stars across the RGB since the two element can't correct completely.  The ED80 absolutely shined on the ASI533 as that smaller sensor had no curve, no bad stars, no problem being lit up by the image circle. I tried the orion field flattener, the apex-l and the televue reducer/flattener - they all worked beautifully with 533 but couldn't get aps-c lit up, flat or had more than 6% fade off in vignette (upwards of 10-15%)

 

Modern sensors are fast.. after 30 seconds you've well saturated read noise and probably start hitting a good mean for broadband targets.   Right now i'm in light polluted skies so i tend to shoot 60 to 90 seconds at gain 0 with the Optolong L-PRO for broadband targets.  I can pretty much stick to 5 minutes for NB regardless - maybe slightly shorter if i have some bright stars i don't want to blow out. Straight up L filters will be fast fast fast.

 

I've learned to just invest heavily in modern computer. I'll be jumping on the ryzen 5700 and getting 128gb of ram to stack all the images from aps-c.   It's a good trade off for me, more computing time than clear skies so i'm not afraid of shorter subs to maximize dynamic range, reduce noise and shoot for integration times.

I am confused as I had thought the point of a triplet was for all 3 colors to 'correct', that this would be a problem when using a doublet.  



#21 the Elf

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Posted 25 October 2020 - 06:58 AM

I am confused as I had thought the point of a triplet was for all 3 colors to 'correct', that this would be a problem when using a doublet.  

Two different things to consider:

a) how dispersive is a given type of glass? Dispersion is the physical effect of refraction index changing with wave length. The result of this is chromatic aberration. There are some glasses that have far less of this effect than others. Here is a list:

http://www.hoya-opti...sreference.html

Mind the "code" columns. The second number is 10x the abbe number. Higher is better. The best glass available today is FPF53 with an abbe number of 95. According to the table Hoya FCD100 is the same.

So one thing you can do is to avoid excessive CA by using good glass. The combination that works well for astrophotography is doublet made from an FPL53 element plus a lanthan element. There are a few scopes out there with this combination.

 

b) Correct the effect by combining two elements that have dispersion but in opposite direction like crown glass and flint glass. A combination of two elements can only correct two colors. With three elements a much better correction is possible. See wikipedia, achromat and apochromat.

 

As you for sure have guessed a triplet that contains an FPF53 is best. In general triplets are often better than doublets. But this is not a physical law. If someone makes a cheap triplet from cheap glass it can be worse than an excellent doublet made of good glass.

As was explained by others the image quality is always good at the center. The larger your sensor the more you have to invest in good glass.
 


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#22 sn2006gy

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Posted 25 October 2020 - 10:03 AM

I am confused as I had thought the point of a triplet was for all 3 colors to 'correct', that this would be a problem when using a doublet.  

The ED80 is a great scope for its price/value.  It has a single element FPL53 glass and not sure what the second one is.  It worked amazingly well with the 533 for many reasons - the 533 is a small sensor so its very forgiving and with the ED80 the 533 is a light sensor.

 

If you stick with the ED80 and want a larger camera, it will struggle for two reasons - the Orion focusers can't really handle the image train weight and the image circle isn't large enough for APS-C to be fully illuminated and flat.

 

The ED80 with a 1600mm/mc or a 533mc is a great imaging scope.  Anything larger and it struggles.  Chromatic aberration seems to be OK with tiny sensors as the correction seems to fail further out from sensor.. so again, small sensors work well to hide all of that.

 

To hold the asi2600 on an ed80 you would definitely need a Moonlight focuser upgrade. The sag on the factory focuser varied as you travelled across meridian and required careful shimming and image clean up - pix insight often didn't like stacking those images because star alignment was difficult. This is mostly because the 2600 compared to the 533 or 1600 is MUCH heavier/beefy camera. A lighter camera is not much of an issue - not because it doesn't sag but because sagging leads to so little image issues it's not a problem..

 

tl;dr - bigger sensors = bigger problems ;)



#23 ssa2294

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Posted 25 October 2020 - 10:30 AM

 

If you stick with the ED80 and want a larger camera, it will struggle for two reasons - the Orion focusers can't really handle the image train weight and the image circle isn't large enough for APS-C to be fully illuminated and flat.

 

 

 

You are referring to the Orion ED80, I have the Explore Scientific 'ED80'. I don't know how much different the two really would be, though I can see why people want to change out the focuser (I dislike how it attaches loosely to the tube). Now I am worrying about what 2600+EAF+EFW+spacers etc are going to tip over my mount and flatten my cat frown.gif  

 

Back to the original topic, what has  me worried most when switching to dedicated/mono is over exposing. So if I use that calculator to help me determine Ideal exposure, is there a means I can calculate to prevent over exposure? I guess what I am looking more for is a range instead of a single set value (i.e. one box = ideal and another box = max/risk if over exposure).



#24 Peregrinatum

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Posted 25 October 2020 - 12:00 PM

diluting RN sufficiently is the floor, then its balancing out the other factors like over-saturation, guiding error, risk of unwanted signal, etc...  I try to maximize the exp per frame



#25 StarAlert

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Posted 21 November 2020 - 02:37 PM

Seems right.

 

BTW, the ED80 will struggle for flatfield on the 2600 and if you shoot LRGB it won't have consistent stars across the RGB since the two element can't correct completely.  The ED80 absolutely shined on the ASI533 as that smaller sensor had no curve, no bad stars, no problem being lit up by the image circle. I tried the orion field flattener, the apex-l and the televue reducer/flattener - they all worked beautifully with 533 but couldn't get aps-c lit up, flat or had more than 6% fade off in vignette (upwards of 10-15%)

 

 

I just bought the Apex L to use with TAK 100DL and a TEC160. Do you know if it can be interchanged with the Televue reducer/flattener? That is, can the Apex be used in an NP101 or NP127 with good results? 




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