vlaiv
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Posts posted by vlaiv
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2 hours ago, bosun21 said:
I am looking at getting a camera for EAA with a larger fov than my 585. Do you think a 183 would be a good choice for this? My scopes would be 10" go to dobsonian f4.8 and my 120ED with a 0.85x reducer for f6.37.
I think that best EAA camera is one that is large and has low read noise.
If you can get even larger sensor than 183 - then get that one, otherwise - 183 will be good enough.
For EAA you want "sensitivity" and this means large pixel size coupled with large aperture. You have scopes with enough aperture, but problem is to find camera with large pixels. Most modern cameras have small pixels - and this is where binning comes to the rescue.
Drawback of the binning is that you reduce number of pixels - and that is why you need large camera.
Say you want to go with 2"/px with your 10" dob for EAA. That will give you really fast setup. FL of dob is 1220mm and in order to get 2"/px you need pixel size that is ~12um.
You simply can't get that easily with cmos - but if you use ASI183 for example - that will be bin 5x5. Now instead of 5496×3672 you get ~1100 x 734px - not bad - but larger sensor would let you bin and still get a lot of pixels - or decently sized image.
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I found actual FPSs for ASI183 in its manual:
It can achieve good speeds with ROI - not the best, but good enough.
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Just now, bosun21 said:
On looking at the specs for the 183 it looks not as slow as i believed.
I don't think that any of of the cmos sensors are particularly slow. They are just limited by USB bandwidth and if one is using and utilizing USB3.0 and using ROI - they can get good speeds.
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3 minutes ago, bosun21 said:
I can't recall the figure off hand but it was nowhere near the 150fps I needed for the planets. My 585 isn't the fastest planetary camera but it will still give me 180+ FPS.
According to ZWO website:
I don't see why wouldn't it give 180+ on smaller ROI if it gets 100+ FPS on 1280x720 which has x3 more pixels to read out and transfer than 640x480.
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19 minutes ago, bosun21 said:
The 183 will be a poor choice for imaging the planets though due to the relative slow frame rate.
What would be frame rate on 183 on 640x480 ROI for example?
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On 25/09/2024 at 10:01, Stuart1971 said:
Before you start spending a ton on money on that mount, just to get you RMS down by .2 or .3 RMS, ask yourself a few questions, forget what other people say they get with this mount, as people will always quote those low figures, as they have seen them for 10 mins on one night, I have had 0.2 RMS from my mount, for an hour or 2 on the odd night when seeing was really good, and in certain parts of the sky, but mostly it’s around 0.6 to 0.9 which for my imaging scale of 1.4”/pixel is perfect.
So ask yourself, what is you imaging scale, and your seeing, in the UK the seeing at best is probably around 1.5”, so really your guiding does not need to be any better than that in theory, as you are seeing limited, and if you imaging scale is lower, you are still seeing limited, so guiding at 0.7 to 1.2 RMS would be fine…
I think some people get too hung up on getting there RMS figures as low as possible, if you ]r images are good and the stars are round then just enjoy, all what is more important is to keep your RA and DEC rms figures as close to each other as you can, as if you have one that is double the other then you can get elongated stars no matter what your guiding is like, so that is something else very often missed..
Mount performance always adds up to seeing to increase FWHM and reduce sharpness of the image - no matter what the seeing is.
If one is disproportionally large compared to the other - sure, other will not have much effect, but do keep in mind that FWHM to RMS of Gaussian curve is x2.355 - so seeing that is 2" is the same as guiding that is 2 / 2.355 = ~0.85" RMS - as you can see, they tend to be rather similar and have similar impact on star size in the end.
On 25/09/2024 at 10:39, Wonderweb said:Hi Stuart. Thanks for the info. I totally agree that the limiting factor is probably the seeing, but I was curious as to what was available and above all, if maybe an engineering solution (lapping the gears) would improve my tracking accuracy rather than relying on a pec training to make the best of a bad job.
Depending on which camera scope combo I use my pixel scale can be anywhere from 1.7"/pixel down to 0.4"/pixel (although i dont often attempt the lower end). I mostly operate at around 0.75"/pixel so would like to try and get things as stable as possible.
I would always take reported RMS figures with a grain of salt. It really depends on guiding parameters and guiding sensitivity is much less with higher DEC. Most accurate figures for mount guiding performance come from:
- guiding near meridian
- guiding with enough resolution (meaning low enough guide resolution to give precise measurements)
- when the seeing is taken out of the equation - by using longer exposures (at least 2 seconds) and multi star guiding (in best case at least dozen stars).
It is also worth remembering that it is not all down to mechanical precision. There is also part of precision that depends on electrical side of things.
Stepper motors are all more or less rated to +/- 5% accuracy in full step position. This figure is worth remembering and comparing to actual step size in arc seconds.
For EQ6 mount that is: 720:1 is mechanical reduction, further 200 full steps per revolution gives us 360 degrees / (720 * 200) = 9 arc seconds per step.
You will hear that precision of EQ6-R is 0.140625" - but that is with 64 micro steps or 9"/64 = 0.140625". Let's calculate actual error that is 5% of one step or 5% of 9 arc seconds = +/- 0.45".
+/- 0.45" is declared error on stepper motors in EQ6 - and that alone prevents the mount from being guided below 0.5" RMS consistently.
We are guiding at ~15"/s - so above deviation happens about every one and a half second. It won't always be exactly + or - 0.45 but it will be in that range.
Then there is matter of stepper motor drivers and precision of micro stepping. This depends on electronics but also on the power supply. "Dirty" power supply will cause larger error by introducing noise into voltage itself if electronics is not properly filtered with capacitors.
Final remark - you are over sampled at 0.75"/px. You want your sampling rate to be around FWHM/1.6 - so measure FWHM that you can achieve and divide with 1.6 and then aim for that resolution / bin accordingly (prefer binning in software as it is more flexible).
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12 minutes ago, heliumstar said:
@vlaiv I mean 60 cm x 40 cm.
Ideally I would like to image without barlow but of course not a deal breaker. Any other sensors or anything else that would be suitable?
Just run the math again and see what would be suitable.
As far as imaging and sensor spec go - you want couple of things. It is basically the same as planetary imaging with few differences (more like lunar than anything else).
1. Mono is better than color for white light as you won't be capturing color information
2. You want good QE and decent frame rate, and low read noise
3. Adjust your F/ratio according to pixel size (or your target image size).
4. Look for the sensor that will give you imaging in one go rather than mosaics as it is easier.
Sun will provide you with a lot of light so you can keep exposure very short. You don't need very high transfer rates as you can afford to image for longer than just few minutes. Low read noise helps but is not essential as there will be plenty of light so signal will be strong.
If you want to go for 60x40cm and want to go with 300dpi print - let's say that sun diameter in the image will be 30cm to have some room around - that will mean that sun needs to be 30/2.54 * 300 = 3540px wide.
Since max solar diameter is around 1950 arc seconds max (32' 32" when largest and 31' 27" smallest) this leads to 1950 / 3540 = 0.55"/px, which for 805mm of focal length gives 2.15px.
Again, 2.4um pixel size seems like a good fit. Whole disk will be somewhat smaller and you won't need to use for 300dpi and 60cm x 40cm. ASI183 still seems like obvious choice - but this time, Sun will fit onto sensor in single go.
For 60x40 print - my choice would be 183 mono sensor - no barlow and I would crop / scale image for 300dpi print in the end to frame the Sun nicely.
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1 hour ago, Phillyo said:
I wish I understood all the different charts and spot diagrams etc.
I can explain what those charts mean, but it is not easy to interpret them in terms of absolute quality.
This is MTF vs distance from center in sagittal and meridional direction (S and M respectively). 10 and 30 stands for 10 line pairs per mm and 30 line pairs per mm.
This is what above diagram "speaks" about. MTF of 1 is perfect image. Anything below 1 is contrast loss to some degree. Meridional direction is "in direction of sensor center" and Sagittal direction is perpendicular to that. 10 line pairs per mm represents level of detail that is order of 50um on sensor. 30 line pairs per mm represents detail that is about 16.66um - or rather should be equated with pixel size.
In another words - above diagram shows that if you have line pairs - meaning black / white lines that go towards the center of the image - like this target:
at the edge of the sensor at 22 mm with lines being 16.66um wide - you'll get only 80% contrast - so you won't image black and white 0 and 1, but rather 0.1 and 0.9 for example (in 0-1 range or perhaps 25 and 225 in 0-256 range).
From what I can see from the graph - lens has rather good correction all the way to the edge of full size sensor as it has the same MTF so star size won't change much.
However, in order to asses absolute performance of the lens - we should compare it to 85mm perfect aperture and that would mean creating ideal MTF of 85mm aperture and seeing what sort of MTF we get for 10 LP per mm and 30 LP per mm.
I could do that, but it would take me some time to perform the calculation and simulation.
This graph is rather simple one:
It represents "profile" of flat frame. It is level of illumination from center all the way to 22mm away from optical axis (I think Y field in degrees is mislabeled and should be distance from optical axis - same as in first image).
This one is result of "ray trace" sort of thing. A lot of rays are cast and traced and their end position is plotted. Now, this does not give us full picture as it does not include interference effects of light - but it does give us some information.
First - we can see the same thing like in MTF diagram. Star image is roughly the same in center of the field as is on the edge of full frame (22mm away from optical axis). It does not blow up significantly nor distort.
Second thing that we can do - is compare RMS radius of ray trace to actual RMS radius of star for perfect lens.
RMS radius of actual star profile is about 2/3 of airy disk radius. For F/4.8 system linear airy disk radius is 3.2um. RMS of that will be about 2um. In above spot diagram we see that RMS radius is below this value (good thing) all the way up to 18mm. Only on far edges stars will be a bit larger than with diffraction limited scope.
I have only one objection and that is the choice of 200um for box size in spot diagram. I think that is due to marketing - but it hides actual spot diagram shape. By choice of box size we "zoom in" or "out" the diagram. It has nothing to do with performance of the optics - but people like to see "tight" points rather than some weird scatter plots - although latter are more informative.
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3 hours ago, heliumstar said:
Something that will look good printed on let's say at least 60x40.
I'm guessing that you mean 60" x 40" there or 60 cm x 40 cm?
In any case, what you can expect from 115mm scope would be max of 0.36"/px as far as sampling goes or about 1800" / 0.36 = ~5020px across the disk (if disk is 30' angular size).
If you print that at say 150 dpi - you get solar disk of 5020/150 = 33.46" which should fit nicely on 60" x 40".
I'd go with ASI183 mono there and wedge and Baader solar continuum filter. Although FPS is not high on ASI183 - it does not matter as you can image for longer periods of time, not just minutes.
Scope is natively F/7 and you want it to be at least F/10 to F/12 for pixel size of 2.4um of ASI183, so x1.5 barlow - barlow element so you can adjust distance between it and sensor.
You'll probably need to take two passes as sensor is not large enough to cover whole solar disk in one go. It has enough width but not enough height so two panel mosaic.
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2 minutes ago, ONIKKINEN said:
Guessing this contrast between the spot and the surrounding bright material was what you observed?
Yes. Visually it appeared as GRS is smaller in size and "detached" from the rest of the features on SEB
Actually, when I reduce the size of your image - I get pretty much the same view I had with ~4mm eyepiece at 660mm of FL with 110mm of aperture:
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I observed Jupiter last night with 110ED scope and GRS was on display. It looked like it shrunk and had some sort of prominent bright edge - which I thought was strange at that moment.
Came here to verify what I saw and I must say that I'm enjoying the quality of the images very much, besides having verified what I saw last night.
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31 minutes ago, Ratlet said:
Just out of curiosity, would this be why the askar 71f would work for visual and imagining? The back focus for imagining requires an extension?
Yep.
It is four lens design with integrated flattener. It also has quite a bit of back focus - 100mm from M54 thread.
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33 minutes ago, Elp said:
I always thought these designs weren't for visual, otherwise you'd have lots of Redcat owners waxing lyrical about visual use.
Well, in general - they are not.
Having flat field usually degrades performance of the scope - they are often no longer diffraction limited systems. It is also quite difficult to get eyepiece in exact position. People that use short focal length refractors with long FL wide field eyepieces need to correct for field curvature as well and most of them use TSFLAT2 because it has enough working distance (I think it is about 90mm or so) - so that they can put it in front of diagonal and get the eyepiece in the ballpark where it should be.
Exception to this rule is for example TeleVue NP101 - which is excellent imaging and visual scope - but it does not have separate field flattener and corrective lens is at exact distance to focal plane so there is no need to calculate backfocus.
These two scopes might be good enough for visual:
https://www.firstlightoptics.com/askar-telescopes/askar-fra300-60mm-f5-petzval-astrograph.html
Both have integrated flatteners and a bit more working distance than 55mm (could possibly work with T2 short optical path diagonal).
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14 minutes ago, Stuart1971 said:
What do you mean “functions” like a refractor
Scopes with built in flatteners are PITA for visual, especially if back focus is 55mm (again - no way to easily adopt diagonal and EP).
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What's your exact goal here?
Small scope that would work as both visual, guide scope and imaging scope?
How about 60mm class scope?
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30 minutes ago, Ratlet said:
I wonder how that would do for imaging?
Maybe with small sensor - anything larger would instantly suffer from field curvature and I'm not sure if any field flattener could be adopted to work with such scope.
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4 minutes ago, Ags said:
I was thinking of (at least) the TV 40 mm plossl.
Why?
That is a bit of a waste of light. Most such small scopes are F/4, perhaps even a bit faster. With such F/ratio - longest focal length that you could use would be around 25-28mm, depending on how dilated your pupil becomes (for 6mm you get 4*6 = 24mm).
In the same price range you could get something like
or (for a bit more)
24mm version
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1 hour ago, Ags said:
Not sure that's always true. I think for a typical 30 mm plossl, the focal plane is at the bottom of the barrel.
Focal plane is usually at the field stop of the eyepiece - at least for "simpler" types.
If you can see the edge of field stop sharply when looking thru the eyepiece - it is at the focal plane where the sharp image forms.
Look at the plossl eyepiece from the bottom and you will see that the field stop is roughly at the shoulder level.
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Alternatively - don't get Evoguide, get this instead:
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8 minutes ago, Ags said:
Given the Evoguide has 55 mm backfocus, adding a Baader T2 amici prism with 47.5 mm optical path would leave 7.5 mm path on the eyepiece side? I have a short T2-1.25"" adapter, so would this be enough optical path for most eyepieces?
I don't have experience with Evoguide per se, but keep in mind that focal position of eyepiece would be around it's shoulder (ideally at the shoulder).
This means that you need to sink in about 20 or so mm of nose piece of EP somewhere - but with only 7.5mm of path you won't be able to do that and protruding nose piece of EP will hit the prism.
Your best bet is to "convert" eyepiece by unscrewing the nose piece and making suitable adapter for it to attach directly to T2 thread.
I've already done something like that for a friend to convert spotter scope for use with astronomical eyepieces.
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13 minutes ago, Lee_P said:
ZWO customer support suggested I open the camera up and inspect the board.
I'm looking at the image and reading the above as : "ZWO customer support suggested I open the camera up and insect the board." and wondering how does one go about "insecting" the board and which insect should they use
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3 minutes ago, Mircea said:
Maybe I'm wrong but I see a short T2 thread on the focuser, right below the shoulder of the 1.25'' / 2'' adapter.
I think you are right. It looks like it is however M54 thread not T2 as whole 2"/1.25" adapter fits into it and 2" is 50.8mm.
In any case - it will be usable for attaching the imaging accessories.
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32 minutes ago, Ags said:
on the other hand it is cute.
Yes it is
I like the image where you can see that wider then the OTA itself
Here is what I really think of this scope. I think that focuser is holding it back. I think that it will be excellent beginner imaging scope. I know that people will say it's slow at F/12 - but as we have established (hopefully) by now - it's not the F/ratio that is the speed of the scope.
- It is well baffled and given its construction - flats will be easy to do (very little chance of stray light issues).
- same thing goes for imaging unlike newtonian which can have issues with stray light if longer dew shield is not installed
- it does not need correctors of any kind. No need for field flattener and no need for coma corrector - it is all the expense that you'll have on the optics side of things.
- it is light weight for sure. It has more aperture than 80ED and is color free. It is cheaper as well.
- I think it will be able to illuminate at least APS-C sized sensor - which is most of DSLRs / Mirrorless that novice imagers might use
I just wished that focuser has some sort of thread for threaded attachment of accessories to avoid any tilt. By the looks of it - it does not seem to be the case. It looks pretty much like regular monorail/crayford focuser.
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More than 50% secondary obstruction?
That is going to severely impact visual performance.
White light imaging camera recommendations
in Discussions - Cameras
Posted
I agree that 294 is not the best choice due to issues, but I think there is alternative - at least for EAA with OSC sensor.
https://www.altairastro.com/altair-hypercam-269c-colour-camera---tec-cooled-1097-p.asp
That camera is rarely mentioned - but I think it has very good specs for EAA with above approach.
There is also fan cooled version - which is somewhat cheaper:
https://www.altairastro.com/altair-hypercam-269c-colour-camera----fan-cooled-7949-p.asp