teoria_del_big_bang

1 hour ago, dazzystar said:

Should this back focus measurement be provided by the scope manufacturer as I can't see it on the Skywatcher website? My tube is a Skytravel 102.

I think you are still a little confused.
@vlaiv has given a good explanation in his first reply.

Basically if you are using the scope as bought without any additional optics such as a coma corrector, flattener or reducer then really you do not necessarily need to know the focal point.
So long as the scope is suitable for imaging then you can just use the focusser knob to wind the tube outwards, from being fully in,  to achieve focus. No real need to know exactly what that distance is as by design the scope should be capable of achieving focus within the distance the tube can be moved in or out.

To the best of my knowledge the Skytravel 102 is primarily a visual scope for use with eye pieces and not really recommended for imaging which is why you probably do not find any mention of back-focus with it as it just assumes you are using eyepieces and the focal point will be somewhere in the travel of the focussing tube.

The back-focus that people talk about that has to be correct (normally 55mm) is provided by the manufacturer of the flattener, reducer, coma corrector and that is the 55mm you mentioned in the OP. These additional optics are used in imaging to correct for issues of how the light path is focussed that may not be a big issue for visual but becomes noticeable in an image, mostly at the corners or outer edges of the image where stars become elongated, or tear drop shapes, or maybe issues with colour as not all frequencies of light get focussed at the correct point around the peripheries of the image . So the additional flattener, coma corrector or reducer will correct these aberrations, or at least reduce the effect.
But to work correctly and focus all frequencies of light at the same point the distance between the end of the flattener, coma corrector or flattener has to be a specific distance to the image plane (the CCD chip in the camera) and this is often the 55mm you talk about. You still have to achieve focus by using the focus knob to move the tube in and out but 55 mm of the focal point will now be always taken up by this 55mm back-focus required from the additional corrector optics.

Just be aware that some scopes are not ideal for imaging and more usually designed for visual only. That is not to say you cannot use them for imaging but you may need extra extension pieces to add where the eyepiece fits when using a camera to make the camera sit further back than be achieved with the focusser tube itself and that is just due to the fact that the path of light is altered by the eyepieces and the scope is designed for visual.
A good example are the Skywatcher refractors  such as the Sky-Watcher Explorer 130P, 150P and 200P these are really unsuitable for imaging (well I have used them and needed all sorts of extenders to achieve focus with a ccd)  but they do versions for imaging and are called 130-DS, 150-DS and 200-DS.

I don't want to labour that point too much and confuse you more but it is just worth a note as your tube is really designed for visual, although I have no experience of this scope so cannot say whether you will have any issues when using it for imaging but I am sure somebody on SGL will know for sure 🙂 

I think all I have said above is correct but will say I am pretty new to all this and was also asking the same questions only 3 to 4 years ago but I am sure @vlaiv will correct me (I hope anyway) if I have said something misleading.

Steve

THIS  is quite a good and easy to understand explanation of back focus.

Steve

1 minute ago, Andywilliams said:

Thanks for your reply Steve,

Im currently using a manual focus.

i didn’t try any of the rgb  filters .... that was a mistake.

I was using 20 seconds at gain 139 for focussing images. Too short for Ha ? 

I was always under the impression (yep, I don’t really understand the subject of backfocus 😬) ....that backfocus will not affect focus via the drawtube ? 

If it’s just totally a matter of backfocus then it should be easily resolvable !

i know that I’ve inadvertently changed something while my setup was stripped down for repair....it’s just a matter of figuring out what !

cheers,

Andy 

Andy,

For Ha and OIII I should think 20 seconds is easily enough if there ae bright stars for focussing. I think I only use 4 seconds or so.

As I said if the figure for your back focus is 55mm then because of the filters it will be slightly more due to the thickness of the filters, which means it is not good practice to have no filter at all for the L filter and is the reason filter manufacturers make a plane glass, or maybe an IR filter to use as the L and it is best to use filters of all same thickness so that pretty much when focus is achieved on one filter the others should be as near as damned it in focus.
But your filters should be between 2 and 3 mm thick I would think so the back focus will really be between 55.7 and 56 mm.
Does that extra 0.7 to 1 mm make a difference - well to a noticeable extent in the corners yes, the centre portion will be able to achieve round focussed stars but the edges will have some tear dropped shaped stars. This small error will be noticeable if you look closely, unless you do a heavy crop on the final image, but if more than this the effect could be well noticeable, and on your image although the whole image is not well focussed you can see that in middle the stars are out of focussed but roundish but in the corners they are distorted as well as mis-shapen so there is a problem with the back focus.

Start in middle of image

Stars in lower LH corner

Is this the reason for the inability to obtain focus - I don't think so.

So it could be you have two issues, one that means when focus tube is fully extended it still needs to go more to achieve focus and there is a small error on the back focus distance.

I know you are not at home but I really think when you are a picture of the setup might help us determine if there is an issue there.

Steve  

37 minutes ago, Andywilliams said:

my focus at the moment is a manual focus.

So for the Ha and OIII do you take an image of several seconds and then keep adjusting the focus until you achieve focus?
I can see that with no filter you can just use live view and even a mask but I would be surprised if you get enough light passing an NB filter in live view.

Also did the same optical train work okay before you repaired the CCD ?
You haven't just added the coma corrector have you ?

Steve

12 minutes ago, Grant93 said:

The reason I don't want to plug my Pi into the mount, is because when I set up I like to do the manual polar allign with the polarscope, then polar allign with the function on the hand controller, which I can do using the screen on my DSLR, then I can slew to the object I plan on imaging, check framing all without the use of a laptop, and I can keep that inside. And as I only ever want to stick to one object on a given night, I don't feel I need to control the mount from my laptop. I can then go inside, and I am hooked upto the Pi on my laptop via a long long ethernet 😅 (I find the Wifi slow and unrealiable on the Pi). And just control the imaging and guiding from my laptop.

This is just me explaining the route I take, not that I think this is the best way to do it, so if anyone has better ideas, please do say as I am always open to advice.

 

I think if that's the way you want to do it then I would go that route, at least for now.
I was just not aware the internal guider could use the ST-4 cable.
Once you have it working you can then think about whether you want to try EKOS pulse guiding, I think doing it this way does bring some advantages but get it working as it is and then think if you want, or would have any advantage, changing to using EKOS.

I have no real experience of using the ST-4 method and so cannot comment to whether this creates any difficulties for you or whether EKOS had any advantages, I know it is regarded as a bit of an outdated method by some but it is still used by many, maybe others can chip in with advice, but certainly get it working correctly as you are set up now and see what guiding does for your setup.

Good luck when you next get a clear night to try it 🙂 

Steve

7 minutes ago, alacant said:

Hi

Keep it simple? You may find it -considerably- easier to use EKOS' own first-rate guider rather than adding a third party app to the mix. 

Cheers

I agree to keep it simple and use EKOS, but can you use ST-4 guiding with EKOS ?
I did not suggest this because the OP did not want to connect the mount to the RPi, although I think that would be the best route to take.

Steve

It makes no sense that you can get somewhere near focus with no filter but nothing with the Ha and OIII.
It probably is not good practice to use nothing for L as the back focus will be different when using the filters to using nothing. A filter changes the back focus by 1/3 the thickness of the filter, so usually by about 0.7 to 1 mm.

But that said you should still get some blurry image and not nothing.

What are you using to achieve focus, how long exposure are you using for Ha and OIII when trying to focus ?

Do you have other filters that will allow more light to pass such as RGB filters, what is the focus like with those.

As mentioned above the back focus when using filters will not be exactly 55 mm but 55mm + 1/3 thickness of the filters but even when this is wrong by a few mm you should still get focus on the middle of the image but the stars in the corners and outer edges may be elongated.

Steve

12 minutes ago, Chaz2b said:

The thing to note here, is that she’s suggesting the Best scopes and not the ideal or most suitable. Considering the actual large choice it’s still as daunting which ever angle you slide down into this money pit!

The Amazon comes up often as it’s the best price for that particular item at that time, it changes as the prices fluctuate globally, I don’t think she’s plugging Amazon more than any other provider in the list, and Amazon has a whole host of providers.

The other to note, it’s not in anyway a review on each item but an overview regarding the suggested uses.

I would much like to see references made to places such as forums and possibly YouTube and others for further in-depth reviews and literature.

Gone are the days when the curious are simply led towards clubs and individuals for their first foray in astronomy, now it’s all media lead, that’s my opinion anyway.

chaz
 

I admit I was probably being sceptical about nearly all links being to Amazon and also thought maybe just the cheapest option, nut  they aren't even the cheapest option.
The better scopes in the link @vlaiv the ones that FLO supply are significantly cheaper and when I was checking for a new printer on similar sites the prices at Amazon were often well more expecsive than other on line sites I found.

Maybe I still am being too suspicious and not doing the instigators of these type of articles justice but I still have my suspicions that they still have an alliance to certain websites such as Amazon.

Steve

How many threads have we seen on SGL in 2021 about the same issues.
Its so sad.
I can understand a low power LED on a PIR so you can see when you go out in garden at night, for whatever reason, I have to to take the dog out last thing at night but there is no need for what amounts to be enough flood lighting to play football under or to leave them on all night.

One of the threads this year was appalling with somethings like 3 high power floodlights all mounted under the eaves of a two storey house lit up all night.

I would certainly have a chat and say how it affects you and if there is anyway they could switch them off at a certain time even if its just on clear nights, if they are willing to be courteous then they might even give you a mobile number you can text on the nights you have your rig out.
Even mention one light is the main culprit and suggest a better place for it.

Worth a try, they could just totally ignore your requests but many people are still open to negotiation and compromise so certainly worth a try.

Steve

4 minutes ago, bomberbaz said:

I have a mate who is friends with her who is himself an accomplished photographer.  I am thinking this is simply a case of can you string an article together for us and we'll bung you £xx hundred quid. 

Her resume shows she is actually very accomplished and it is disappointing to see people of her calibre posting tripe like that.

Gemma is content director of science and space magazines How It Works and All About Space, history magazines All About History and History of War as well as Science, Technology, Engineering, Arts and Mathematics (STEAM) kids education brand Future Genius. She is the author of several books including "Quantum Physics in Minutes", "Haynes Owners’ Workshop Manual to the Large Hadron Collider" and "Haynes Owners’ Workshop Manual to the Milky Way". She holds a degree in physical sciences, a Master’s in astrophysics and a PhD in computational astrophysics. She was elected as a fellow of the Royal Astronomical Society in 2011. Previously, she worked for Nature's journal, Scientific Reports, and created scientific industry reports for the Institute of Physics and the British Antarctic Survey. She has covered stories and features for publications such as Physics World, Astronomy Now and Astrobiology Magazine.

That's actually even more disturbing to hear that people who really should have integrity can be bought so easily.

Steve 

8 minutes ago, nephilim said:

I agree with you there.
Unfortunately due to the massive amount of scams constantly doing the rounds i'm very suspicious of most online advertising things these days, its turned me into a complete cynic. I'd rather be a cynic than lose money to one of these cons though.

Steve

I am just the same these days and am always so suspicious of anything I read on line unless I am certain it is from a reliable source.
But like you say because for the countless scams about that is just how you have to be.

Steve

In fairness not all links were Amazon but pretty much 80% of them and enough to make me suspicious. and then when I went on Which and a couple of more believable sites non of the top printers in the suspect sites matched up with the top ones on the other more believable sites.
Now, maybe I am being too suspicious and maybe the sites just choose to send you to Amazon because they are the cheapest option, who knows but I certainly avoided them and like that Astro review would be too suspicious to take it seriously, but as you say unfortunately a total newbie would be totally fooled without further research or joining a forum like this.

Steve 

Yes you have to be very careful what you take as genuine reviews on any website.
I recently wanted to buy a new printer (inkjet type not 3D) and found a few sites very similar to this, and at first view they look very professional and totally believable.
When I started to get 2nd thoughts was when every link to buy the printer took me to Amazon, just like the links in the telescope reviews in your reviews.

In the end with enough searching I did find some what i did think were genuine independent reviews, and also paying for a months subscription to Which I decided on a printer and am very happy with my purchase.

But yes many of these are very authentic looking and many with fall for them.

Steve

1 minute ago, Grant93 said:

Well that may explain it then.. I was clicking start guiding and the photographing sequence at the same time 😂 I never realised there was a calibration between 🤭

If you watch the PHD2 screen it will tell you at the bottom LH of the screen when it is calibrating, it will say something like "West step" then a number and a distance., also to begin with at the bottom RH it will have CAL in red until calibration is finished then it turns green.

Steve

1 minute ago, kens said:

Those start trails look remakably like what you would get if PHD2 was calibrating while you were imaging. With ST4 guiding you calibrate on each target so you need to let the claibration complete before starting imaging.

Another possible reason for poor guiding with the longer guide expsoures is that ht e guide star is saturated. Use the star profile view to check that and if the profile has a flat top, reduce the gain on your camera.

If you can attach your guide log that would be helpful to analyse.

Ah, yes I think you are right, I couldn't think where I had seem similar images before but now that has remined me 🙂 

Steve

I think @kens has hit the nail on the head.
But why it does not show the same effect at faster frame rates I am not sure, maybe it just calibrates much faster and so you do not see the effect so much in the final image as there is more exposure when guiding correctly.

Steve

I had the Raspberry Pi4 plugged into the guide cam and 800d. I had the guide cam plugged into the Autoguiding port, and I had an ethernet from the laptop into the house. I don't plug the raspberry pi4 into the HEQ5 as I like to use my hand-controller as I only stick to one object per night and also I like the polar alignment feature on the controller, seems to do a great job. I had Ekos and PHD4 open on the pi.
I am not quite sure what you mean here, please forgive me but can you explain a but further?
You say RPi4 plugged into the guide cam - I presume this is via the USB cable into the USB on the CCD ?
But then you are also using the guide cam autoguiding port. I am also assuming by this you mean the ST-4 port on the CCD, is that correct and if so where does the other end go ? again my presumption is it goes directly to the mount.

1. Guidecam exposure - It didn't guide very well when the exposure was higher than .2s on the guidecam, it would cause the stars to zigzag on the final picture, I thought I was ok with .2s with that but then I read most people use between 1-4s guidecam exposure. How come this would happen with even a 1 second exposure on the guidecam.
If I read this correctly you are using 0.2 sec exposures on guidecam, this seems very fast as I think the norm is between 1 and 4 Seconds, in my case this somewhere about 1.5 to 2 seconds seems to work best.

2. Focus in the guidecam - How in focus does the guidecam have to be? Am I ok aslong as I see stars on the screen that are near enough (but not perfectly) pinpoint?
I think focus needs to be somewhere close but not necessary to have pin point focus.
Some say slightly out of focus actually works better but personally never really seen a difference in correct focus and slightly out of focus.

3. Dithering only being plugged into the autoguide port - As I don't have my Pi4 plugged into the mount, will PHD4 still work with Ekos and dither through the autoguide port between images, or to dither will I have to have Ekos controlling my mount too? (I hope not, as I said above I like using the hand controller).
I don't think so as PHD2 will control the dithering via the guidecam and ST-4 cable to the mount (I say I think as I have never used ST-4 so am assuming this to some extent) 

4. A question I've always wondered but unrelated to guiding, but don't seem to be able to find a clear answer to - Does anyone ever shoot through wispy high clouds? I often watch some of the more popular youtubers, and have noticed they sometimes mention of high wispy clouds, but they're shooting anyway. But some people also say they throw away data if it has any amount of cloud drifting through it. What are your thoughts?
I am sure in the UK many have to do some amount of imaging through some high thin cloud.
It is all a matter of how thick the high cloud is and how much of it is about.
If in the UK we wait for absolutely not a single cloud then those nights would be very few and far between.
For me at least it seems to be a matter of judging how much cloud is about and whether it is worth an all night imaging session.
Then if all goes well I would look at stretched images of each frame I took and decide whether to keep or discard from a final image.
Trouble I find with clouds is that if I decide to leave the rig out all night imaging, unattended, often the clods can momentarily cover my target enough at some stage  to stop guiding as it looses the guide star for a while and thus stops the sequence and the rest of the night my target may be clear but the imaging sequence has stopped.
 

For me what seems odd in your setup is why you have to have 0.2 Sec exposure time and not the more usual 1 to 4 Sec.
What is the tracking like without any guiding at all, if the PA is good then you should get 30S exposures at least without significant star trails.
Remember that PHD2 and a guide-cam are there just to get that bit better guiding than your tracking on the mount, to allow for 5 to 10 minute exposures or higher, it should not be the only thing that is keeping your camera on target. 
So maybe the first approach is to get decent tracking of the mount unguided, then add the guiding.
Are you sure the tracking rate is 1X sidereal and set to Northern hemisphere ?
I am not familiar with your actual mount but don't see why it should be any different to the HEQ5 or IOptron CEM60 I use. 

Maybe if you can show your PHD2 settings and maybe one of your images with zig-zag stars when using 1S exposures on guide cam it may help/

Steve

3 minutes ago, saac said:

I’m going out on a limb here but I’d hazard a guess that few if any of SGL membership are sufficiently skilled in the ways of Parkour to be of any concern to the law 😊

Certainly not myself, I struggle blowing out the candles on my birthday cake now 🙂 

I am sure nobody on this thread is seriously thinking of doing anything along these lines in any shape or form.

Steve

What a great milestone 🙂
Well done and I raise a glass to you 🙂 

Steve

3 minutes ago, dannybgoode said:

I think home security lights should be banned also.  They often actually aid a thief rather than hinder them.  And as for 'decorative' garden lighting etc - yep that can all go as well...

I would be all for this although I think a ban is very unlikely but there could surely be some legislation to the wattage allowed (or better still a luminance value), height at which they are installed and that they should not be on all hours of darkness.
I know it will not happen as it would be an administrative nightmare but it would be great to have to apply for permission to install outside lighting, whether for business or domestic, and then neighbours informed before permission is granted, similar to adding extensions and so on.
That way at least anyone can object and a compromise made before hand rather than having somebodies hobby ruined overnight by a neighbour putting up enough illumination to play a football match in their garden.

Otherwise it is just up to the individual to complain to the neighbour and try to get them to at least turn them off on certain clear nights, and that just relies on an understanding neighbour which everybody is not these days.
If this was all agreed beforehand with the local authorities enforcing the agreement then they would have to adhere to it or risk being told to remove the lighting altogether.

Steve

13 minutes ago, saac said:

I wonder if they can be hired  

Jim 

Well yes maybe if the lights were higher then they couldn't reach them 🤣

Steve

Like I say I am not that familiar with DSS but I think you can add all files for different sessions using the tabs.

I think THIS explains how. Go to the File Group Section.

Steve

I am not sure how to go about this in DSS as I use PI and in PI all this can be done by running one script.

But the basics should still be the same.
Say you have:

• 10 frames of each RGB from night 1, take flats after that session.
• 8 frames of RGB from night 2, take flats again for that session. If the exposure times of flats is the same as night 1 then no need to take darks again.

What I tend to do is see what the ideal exposure for flats is for each filter. I don't like taking too short exposures for flats with CMOS cameras so try to keep the exposures at least 2 secs long by reducing the light intensity on the flat panel. So if the ideal exposure is 2.225 seconds for one filter then I will then probably actually use 2 seconds for that filter, as the ADU will be close enough to the ideal ADU, as it means I can use the 2 second master dark from my library, another filter may be 2.994 for another filter so would then take the flats at 3 seconds for that filter and use a 3 second master dark.

Then for calibration:

• Make master flats for each filter for night 1 using flat frames from night 1
• Make master dark for each filter for night 2 using flat frames from night 2.
• Calibrate all 10 light frames for night 1 with master flat from night 1
• Calibrate all  8 light frames from night 2 with master flat from night 2
• Stack all 18 calibrated frames for each filter.
• Combine each filter for master colour frame.

I guess it makes more sense for somebody who uses DSS to explain the exact steps as I have not used DSS for a few years I am not sure the easiest way to do this in DSS. 

Steve

17 minutes ago, Imd said:

I don't currently redo flats every session. I have multiple filters and I thought dark flat frames were supposed to be captured at the same exposure as flat frames so when you add it all up it can still take time to do the flats and dark flats for each filter. May I'm in to much of a rush 😀

I guess it depends how you take the flats and what camera was being used.
If we were not talking the mono cooled camera then yes ideally take the dark flats each session just same as the flats/ 
Also if using the T-Shirt method and a morning sky so exposure times differ each time you take them then ideally yes you would retake the dark flats at the same exposure.
 

But if you can use a flat panel and the exposure time of the flats can be the same each time then no reason you cannot use a library of darks that you took at the same exposure times.
If using Pixinsight it can also use dark masters at different exposure lengths (and maybe other software can its just that I use PI) but I am a creature of habit and still try to use same exposure length darks as my flats.

As I say this is what I currently do and think what I am saying is correct, I know many do not do all this including some well known members of SGL that produce far better images than myself so for sure none of this is a hard and fast rule of what you have to do 🙂 

Steve

As far as I understand when removing camera from a setup then you should redo the flats but no need to do the dark frames.

Many even take dark frames with the camera removed and with a cover on the sensor and then wrapped in foil in a dark room to ensure there is no stray light getting in there.
So if you take a library of darks to cover any likely exposures you are going to use, including the short exposures for dark flats, then you do not have to take darks again for sometime. 
It is still a good idea to retake these darks every do often as cameras can change over time but probably now only once a year.

Flats however (ideally) should really be taken after every session and if you have a flat frame panel the normally these do not take long. So for a multi session then you would use the same darks for calibration but each night would have a separate set of flats for each session. 
Then so long as you take the flats at the same exposure length for each filter (if mono) then you ideally would already have the correct exposure darks to use so would not need to retake these.
Obviously then you need to keep all the flats until you calibrate your data and then just keep the master flat for that session, if storage is an issue. Personally I like to keep all the flat frames as well as the master just in case I redo the processing at a later date and find an issue with the master I can then examine all the individual flats to see if maybe just one or two of them had an issue that caused a bad master, but as I said if storage is becoming an issue then just make sure you are happy with the master flat before deleting the individual flats.

If for sure you have not taken the image train apart between sessions, or accidently rotated the camera or filters etc, then strictly speaking you can get away with using the same flats for each session. Personally I would still retake the flats as it takes very little time but I know many on SGL that produce perfectly good images do not so I guess that is your decision.

That is my understanding anyway and what I currently do, others with more knowledge than me may say differently, I know from previous similar threads this is not set in stone what people do and many have different views on what is necessary.

Steve