beka

11 hours ago, Stellaris said:

Hi, I use a SW 150P with a Fujifilm x-t2 camera for astrophotography. I recently learned that I could place a Barlow lens in front of my camera for increased magnification, and have tried every combination of lens with the Barlow, no lens just Barlow, with focal increase, without. (Just in case the Barlow is the spacer with the lens right? The lens bit doesn’t go at the end of the eyepiece right?) I can’t seem to be able to produce a single image, not even in the live view. Some people told me it might have to do with the focus, but considering the number of parts involved in the process, it seems fairly doubtful. If anyone’s has any experience with this, or some insight to pass along, it would really help me.
Thanks

Hi Stellaris,

If you use a barlow you should not be using the camera lens.  Your camera can probably come to focus without either camera lens or barlow - attached directly to the telescope focuser (if it has an M42 thread). 

Cheers

13 hours ago, ollypenrice said:

Galileo famously said that nature was written in the language of mathematics and it's hard to disagree. I used to beat myself up over why this should be but, more recently, I've come to wonder if there's really anything very surprising about it. My question to those on here who are mathematically competent (as I am not) is, Can maths not describe anything which is consistent?  If it can, is there really anything more to the ability of maths to describe nature than that nature is consistent?

And then, if nature were not consistent, would anything be possible - including thinking about it? Without consistency, would nature not just be noise?

Olly

Hi Olly,

What about chaos theory? Does this fall into "being consitent"?

Cheers

On 07/05/2023 at 14:50, michael8554 said:

"Hi Michael8554, you mean it should be on the camera side (of the focuser) - for the reason you stated?"

No.

OTA + Crayford + FR + Camera.

Because with OTA + FR + Crayford + Camera, the FR backfocus will alter when you focus the Crayford.

Michael 

... so FR on camera side  

On 04/05/2023 at 10:03, michael8554 said:

Hi Stuart

The Crayford won't alter your Backfocus, but it will eat into it.

The FR must be on the OTA side.

Otherwise when you alter focus with the Crayford, you will alter the FR Backfocus, which must remain at the correct setting for best star shape.

Michael

Hi Michael8554, you mean it should be on the camera side (of the focuser) - for the reason you stated?

Hi SteveBz,

Maybe you can try on the NASA/IPAC Extragalactic Database. There are fit images you can download.

Cheers.

On 18/01/2023 at 18:14, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

So the problem seems to be fixed. It may have been that the camera was not on manual mode. I remember seeing a warning in Ekos but had disregarded it since everything seemed to be working. Anyway I did not think it would lead to errratic exposure time  - maybe the warning should be more strongly worded

Best 

On 18/01/2023 at 20:10, beka said:

I will check if I have not disabled the noise reduction in the camera, I don't have the raw files - just the FITS.

On 18/01/2023 at 18:14, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

Hi almcl,

So both High ISO and Long Exposure noise reduction features were off. I found the "CCD Temp" value in the FITS header, as can be seen by the KStars image viewer and all subs were at 16 degrees.

Best

1 hour ago, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

I will check if I have not disabled the noise reduction in the camera, I don't have the raw files - just the FITS.

Hi All,

I noticed a few times while imaging with my Canon 700D (KStars/Ekos) that one of a series of subs will have much less background noise than those immediately before and after. At first I just though it was the way the FITS viewer displayed the images but when I check later there was a huge difference. I attached a screenshot (to reduce size) with successive images side by side opened in AIJ. I captured the sequence automatically and did nothing between subs. The subs are 60s and maybe max 30sec between the subs while the image downloads over WiFi. Any ideas?

Best

16 hours ago, powerlord said:

I'll be honest chaps, I thought I'd pulled a blinder on this one, but going by the lack of likes I guess not so opening it to the floor with a view to learning from my mistakes - what have i done wrong? What could I have done better?

Maybe you have to post a series of bloopers to be appreciated again - before posting the big one 🙂.  I would like to know which of your scopes/lenses did you use?

8 hours ago, wimvb said:

After that very weak Andromeda dwarf galaxy And XXX, it is now time to tackle its larger and vastly brighter sibling, ngc 147. This is also a spheroidal dwarf galaxy, but with an absolute magniture of -15 (rather than -8 for And XXX). Its apparent magnitude is 9.5

Ngc 147, or Caldwell 17, is situated in the constellation Cassiopeia, at an approximate distance of 2.5 Mly. It was discovered by John Herschel in 1829.

Ngc 147 is host to several globular clusters, some of which are indicated in the annotated Luminance image.

Technical data:

Gear: SkyWatcher 190MN on SW AZ-EQ6, ZWO ASI294MM with Optolong LRGB filters

Total integration time approximately 6 hrs, of which L 2.5 hrs

Processed in PixInsight

Very nice and interesting. I checked the specs of your scope and it seems to be corrected to APS-C size with no additional accessories - and at f5.3. Just wondering why this kind of scope is not more popular for astrophotography. I searched SGL for your image of And XXX but could not find it. I would appreciate it if you could link it. 

Best

2 hours ago, Mr Spock said:

The airy disc size is determined by the formula ϑ = 1.22 λ/d where  ϑ is in radians, λ is the wavelength of the light in meters, and d is the diameter of the aperture in meters. 

From that you will see the airy disc in a 200mm scope is half the size of that in a 100mm scope (for the same magnification).

Scopes with central obstructions will put more light into the first (and subsequent) diffraction ring(s) than a scope with no obstructions. The same applies to the scope's wavefront with a ⅛λ scope putting more light into the airy disc than a ¼λ scope.

Well, the FWHM will be smaller for a larger aperture regardless of magnification . But for the theoretical PSF intensity profile,  there seem to be intensities (like at the level of the red line) where the airy disk is narrower for the smaller aperture. What i would like to know is how that translates to what we see at the eyepiece.

Cheers.

1 hour ago, vlaiv said:

There is no hard limit to what we can detect - no red line which represents "above" and "below".

https://en.wikipedia.org/wiki/Just-noticeable_difference

With visual astronomy, you will often read reports of target appearing and fading out of view. It will become stably visible after certain amount of time - as we build up mental image of it, sort of train our brain to see it. This is combination of two effect - JND and the fact that we can "burn" certain image in our brain.

Further more, besides JND - there is issue of sampling, particle nature of light and the way our brain interprets things.

We don't see continuous signal although it might seem so to us. We see in pixels - or rather special neurons that are evolved to sense light. These are finite in size and are arranged in irregular grid.

Particle nature of light means that light is not continuous signal, but rather arrives in photons and thus have associated Poisson noise.

We can detect very faint signals - just 7-9 photons strong. At that photon rate - noise will be very big, however we never seem to see such noise. This is because of how our brain works in order for us to see something faint. Several criteria must be fulfilled for sensation to happen.

Few adjacent photo receptors must be triggered, and then brain decides on some threshold to produce actual signal - but denoised.

Then there is matter of magnification. You need to increase magnification in order to start resolving Airy disk of source star. To a certain point - star is just point source, and two point sources look the same to our eye - or rather have same shape on our retina that depends on "optics" of our eye. Eye lens distorts point of light just enough so it covers few receptor cells in order to be detectable - and we see it as point because our brain filters things.

When we start increasing magnification - we reach place where airy disk is no longer point source and is resolved. This spreads the light over more surface and reduces photon count. If star is already at threshold visibility - further increase of magnification will push it below that threshold (here threshold again is not clean line but about +/- 7% of intensity and depends on how long you stare at the thing).

All of this shows that you can't explain things with such a rather simple model as constant cutoff point.

 

Hi Viaiv, I understand that our vision is extremely complex. But surely we can speak of broad thresholds, like the 6th magnitude naked eye limit or that specified for their instruments by the likes of Celestron. In my plot the red line could be like the limiting magnitude for the Green scope.

But let me ask a related question. If we had two APO scopes (perfect optics) differing only in aperture and ignoring other factors like seeing, sky brightness etc. Would we see the airy disk of the same star to be the same size in each scope - or will it be "bloated" in the scope of larger aperture? I think the latter 🙂.

Best

18 hours ago, vlaiv said:

Except for the fact that our vision does not work like that...

Our vision will have an intensity threshold for some particular situation, though of course will vary from time to time and from person to person - for example if dark adapted etc. What I was attempting to illustrate was that if  the threshhold for a particular observer at the time of observation (no suggestion that it is linear) is at the level of the red line then it is possible that the size of the disk seen for a certain star might be bigger for a scope of larger aperture vs one with a smaller aperture - which we might interpret as bloat.

11 hours ago, CraigT82 said:

Isn’t the Airy disk size related to the focal ratio only, rather than aperture size? Eg. 200mm f/10 scope will produce an airy disk the same size as a 100mm f/10 scope, and so the seeing blur relative to Airy disk diameter should be identical for both? 
 

Edit: Nope I’m talking rubbish. The diameter at the focal plane would be the same but the angular size subtended in the sky would be half for the larger scope. 

Hi CraigT82, actually the size of the airy disk depends only on the aperture and not the focal length for a point source like a star.

Hi All,

I have heard of that SCT views are "soft"  or "mushy" several times on the forums. I was curious about this because I could not really imagine the views through my SCT being any crisper. On globulars and open clusters, if the faint stars were any more pin point then I wouldn't see them (and I don't use glasses). So I tried to simulate the Point Spread Functions (PSF)  for different apertures of scopes shown in the image below, and this is my take on it...

The curve for the lowest aperture is the flatest Purple curve, the next flattest Green curve has double the aperture, and the tallest Blue curve has three times the aperture of that of the Purple. If the threshold intensity that our eye can see is at the Red horizontal line, we can see that the width of the PSF is larger for the Blue curve than that of the smaller aperture Green - meaning it will appear more "bloated". We can also have a similar situation between the Purple and Green a little lower down.

Thus for certain stars the image would appear more pinpoint in a scope of smaller diameter than that for a larger one. Together with the larger image scale of SCTs, and the fact that larger apertures are more susceptible to seeing conditions, this could explain why we might perceive SCT images as soft, bloated or mushy.

Best

On 21/08/2022 at 16:24, Louis D said:

This just reinforces why I love this T-shirt of mine:

So you can love astronomy and not love CATs???

11 hours ago, Jamesnewbe said:

Hi everyone, new to stargazers and have just ordered my first telescope, the Celestron PS1000 Equatorial Reflector Telescope. 
since ordering it I am have second thoughts about if it is the right one for me.

I paid £130 for it which is roughly my budget. Is it easy to use and will it show me some clear images as I really can’t find any reviews. should I keep it or send it back and get an alternative?

thanks everyone 

Hi Jamesnewbe,

If it is indeed the Powerseeker 127, I have used this scope and it is seriously problematic. The mount is shaky and the eyepieces supplied with the scope are pretty bad. However if you learn to collimate and are very determined, you can still see a lot with this scope. You can use it to familiarize yourself with the night sky and get some idea of the kinds of objects you like to observe (for example planets vs deep sky). You can eventually discover how passionate you are about the hobby, and determine if you need to buy further accessories or a different scope in the future.

Best

I was just wondering about the "mushiness" of SCT views as compared to Newtonians. Is there any comparision of the views between that of an SCT and a non-premium Newtonian of similar aperture, but say with the Newtonian "barlowed" up to the same f-ratio as the SCT? While I have not had the opportunity to look through a large dob, faint stars throught my CPC 1100 seem to be really pinpoint (for example in open clusters like M37).

Best

16 hours ago, timwetherell said:

Got quite interested in Chi Cygni recently. Magnitude varies from about 3 to about 14 which is a crazy range. Apparently it's due to the surface temperature dropping which has the double effect of shifting the emission peak into the infrared and molecules forming in the cooler photosphere that absorb visible light. It's near minima so thought I'd try photographing it in visible and infrared light and sure enough, it's really bright at 1µm and really dim visually. 

Hi timwetherell,

Very interesting. Could I ask the equipment you used? I am especially interested in how you did the 1𝜇m IR.

Best

18 hours ago, hjw said:

...build a permanent setup in my back yard.

Greetings  hjw, could I just ask what material did you use for  your dome? 

Best

25 minutes ago, IB20 said:

Saw that Saturn had just peeped over the hedges around 11:30am ...

Guess you meant 11:30 PM?

I think I can get the value of the eVscope for people that already do astronomy and know some of the science, like those mentioned above who use them to acquire images then post process. But for outreach I feel it is too detached from the science. I think some will remember these which kids were always eager to look through. I feel that these digital scopes are just a step removed.

Best

Hi Parziva1,

I have some experience with the 130 SLT, and have mixed feelings about this scope. The mount is not very robust so it is troublesome to use above a magnification of about 100X.  To get that you would need a 6.5mm eyepiece which should be a decent quality to be able to use comfortably. Secondly, collimation is critical. I don't have any collimation tools so I have just tried on a defocused star, and I feel that I never got it to be optimal. As a wide field scope and with lower magnifications, it is good on things like open clusters but it will not be ideal for planets. For astrophotography, you will not be able to use it with a DSLR as it will not reach focus without a barlow (a 2X will make it f10 and probably unusable on its mount) . If you have a mirrorless though it might be a decent option to get started. Hope this helps you decide.

Best

I think this is a great image though of course each of us may have our own criteria - and I am no astrophotography expert. I imagine you might not get as much scale and resolution on your WOStar71. The corrected image size for the 6.3 reducer is specified as 24mm so in theory it should be okay for the 4/3" sensor of your camera to the edge of the field. The soft stars are also expected because of the 0.75 vs 2.7 arcsec/pixel image scale. Do you know the reason for the coarse noise patten of the background ?

Best!

On 30/04/2022 at 21:52, Grogfish said:

What would a refraction correction even look like? That sounds like a vanishingly small impact to any measured position, so why was it necessary? 
 

Atmospheric refraction can actually significant and observable. For example it can be several minutes between the times the Sun actually rises or sets and the time we see it doing so. The effect of atmospheric refraction is greatest near the horizon because we see objects through a greater length of atmospheric thickness as compared to strait up to the Zenith.

Best