vlaiv

17 minutes ago, Boomerrat said:

Thank you for all your help, I was thinking that I'd have quite a detailed view of the moons surface? How big do I need to go to get that type of detail? Only ask as I might get one for myself... hooked already!!

You should be able to see quite a bit of details on the Moon. Atmosphere will start to be limiting factor for even such a small scope. To get the idea of what you might expect, take a look at these videos:

https://www.youtube.com/watch?v=wbbeU6JJE5A

https://www.youtube.com/watch?v=z74yn4JdSGQ

https://www.youtube.com/watch?v=vTwM1UGkEfA

There are also few others on youtube - just search for Mak 90 or the scope you are interested in. This will give you rough idea what can be seen, but are not 100% correct representations because of different methods used to record and process those videos.

Well, my new Az Gti arrived, and it's rather nice looking and operating mount.

I would like to add tactile hand controller to it, but purchasing SynScan V5 hand controller at more than 150e (delivered) is too expensive for what I had in mind.

There is a SynTrek controller in my box of old bits and pieces (from Heq5 syntrek model that I no longer use because I control that mount via eqmod / laptop), but as far as I can tell, that one is build for 5V TTL while Az Gti works with 3.3V TTL and I'm not sure they will be compatible.

There is another issue of them having different connectors - so it's not simple matter of plug and try - hand controller is using RJ45 connector, and AzGti is using RJ12. Luckily there is pinout for both, so I should be able to make "adapter" with a bit of skill and few bits (male RJ12, female RJ45 and a piece of cable).

and

But can it damage mount if it provides 5V TTL? (as far as I know latest Skywatcher mounts use 5V tolerant 3.3V TTL, at least AZEQ6 and EQ8 do, according to EQMOD page):

This hand controller aside, I have idea of making something very interesting that will serve the same purpose:

There is now arduino model with Wifi (no need to purchase shield) for less than 30e (maybe even less on aliexpress) and there are keypads for it like this:

I'm no stranger to programming, and AzGTI is using UDP protocol, and I'm fairly certain that I could at least sniff out "commands" if I don't find protocol specs online for it. This means that for about 30-40e and a bit of programming I can have wireless tactile hand controller for AzGti.

Only problem with this approach is of course  - ugly looking keypad not suited for astronomy mount. Does anyone know something similar that will have up/down/left/right + few function buttons keypad that can be used with arduino controllers? I was not able to find anything online.

Or maybe programmable wifi remote controls? There are cheap ones for about 10e or less - but those work as HID devices and you can't program them in traditional sense (although you can assign actions for each button via software - but that is not something that I'm interested).

Another option would be to get remote control (some generic) and then just use keypad matrix of it and attach it somehow to arduino - but I don't know enough to do so myself, and most of boards for remote control are "integrated" with other electronics and don't have matrix separate to extract it easily.

In any case, feedback would be welcome on any of above approaches, or alternatives that will get job done (but are not overly expensive).

14 minutes ago, Boomerrat said:

Thank you for the replys, it is a mine field when starting isnt it! It is for my 11yr old daughter so want her to use it without too much fading around. I think she will be more interested In planets really so the 90 is better suited i think?

How far out can you see with the 90 and is it easy enough to set up?

Any telescope is capable of showing things that are as far as edge of universe - provided that those things are large enough and bright enough, so you don't have to worry about that aspect.

If you think that your daughter is more interested in planets, then yes, 90mm mak is going to be better option. Mind you that there is only 4-5 planets worth observing - Venus, Mars, Jupiter, Saturn (Mercury, Neptune and Uranus are usually "spotted" rather than observed as they don't show any features, well Mercury can show phases, but it is really hard target for beginners due to proximity of the Sun) and the Moon of course.

Couple that with the fact that planets are not on display all year round (there are periods when you can't observe planets at all), and you can start to see why primarily planetary observing might not be something that will occupy child completely. My view is that you should not choose scope solely based on the fact that she is interested in planets more.

However 90mm Mak will show you other things as well - almost as good as 114 Newtonian (very small difference in light gathering capacity) and just half a century ago 90mm Mak would be considered pinnacle of starter scopes (most had 3" achromats and were happy with those).

There are better scopes out there for that sort of money - ones that are great starter scopes but also scopes for life, but they are manual scopes and heavy/bulky (not all of them are heavy/bulky but best ones are), but I believe you are right to choose scope that is small and has motor tracking and goto for a child.

Out of two scopes, 90mm Mak would be my choice, and in fact I just recently got myself just a tad larger version of such scope - Mak102mm for casual observing (and other astronomical purposes).

Very nice start and I completely understand your urge to stretch image as much as you can, however, my advice would be not to do it.

There is certain draw to match what can be seen in images found online with your own data and that drives people to push their data too much. It can become habitual thing and it is hard to later "relearn" to stretch data only to the point it will let you.

You should aim to have your image relatively noise free even before you start denoising part of your processing. Be also careful of where you set your black point, in fact, there should not be a single pixel with 0,0,0 in your image - as even natural sky without any LP is not completely black.

1 minute ago, Richard136 said:

Thanks, Paz.

Still juggling between the AA 70 and 72 (and the TS equivalents like you have), and the WO ZS61... 

Saw the ZS 61 and it's very slick and obviously ultra portable. The only question is it is too small for visual use as a grab-and-go?

Weight and length-wise, the AA and TS scopes seem very similar indeed; maybe 10% one way or the other

I used to have 60mm F/4 achromat scope. Well it was not quite scope - more guidescope / finder sort of thing, but it did show quite a few messier objects from my light polluted balcony (hand held even).

Having said that - I would not call it grab&go scope - it was more quick peak scope than anything else. I think you will need a bit more aperture than 60mm to keep you busy/interested longer than 5-10 minutes.

19 minutes ago, newbie alert said:

Quite agree Vlaiv.. and your comment about its easier to join pixels( binning) than to split them I've never thought of..

That is sort of something that I gathered from thinking about drizzling and analyzing that approach. You can easily split pixel - just make it two adjacent pixels with same value, but when you do that and stack such subs even if you dither each sub, do you really recover high frequencies / detail in similar sense as you recover SNR with binning and lower resolution? I'm not sure.

Technical side of splitting pixels is easy, but getting true resolution out of it - that is probably rather hard.

3 minutes ago, wornish said:

On a reasonably good night using the gear described in my signature I am able to get most stars around 2" to 2.5" FWHM .  I am more than happy with the star size it produces.

I was thinking of going for either the iOptron RC8 which has a focal length of 1624mm.  Tempted by the RC10. which has a FL of 2000 so that would need a reducer. It is also quite heavy and would be pushing the weight limit of the mount so probably not a good idea.

Would my guidcam attached to the 50mm/160 FL finderscope be up to the job or would an off axis guider be better?

 

Sorry for the never ending questions 

 

 

 

That means that on reasonably good night your sampling rate with current gear is somewhere between 1.2"/px and 1.5"/px.

Which of the two scopes is really down to size management. While RC10 has more aperture (hence better in that sense, and corrected over larger circle) it's going to be heavier and slower to cool down (larger mirror, but it is open design so it cools reasonably quickly).

With RC8 and ASI1600 (btw, that is combo that I have), you can get following sampling rates:

- native ~0.5 (0.48 but I'm rounding it up)

- bin x2 - 1"/px

- bin x3 - 1.5"/px

- Using FF/FR like Riccardi x0.75 (which works good with this scope and with RC10 version) will give you 0.64"/px without binning

- 1.28"/px with bin x2 (this would probably be best option for high resolution work as nights when you can go below 1.2"/px are rare)

- 1.93"/px with bin x3

Using ff/fr will give you reasonably wide FOV (well, it's anything but wide, but it's not unusable in size):

This was created on astronomy tools Fov with x0.73 reducer (for some reason there is no option for x0.75 in drop down list).

RC10 will be just a bit "shifted", meaning it will give you options with FF/FR that are same as RC8 without one (but again, you can calculate).

In the end, I'm working on software that will be able to do various fractional bin modes to help get closer to ideal sampling rate for given data (one can stack their data - measure FWHM and then decide to bin subs in certain way and restack binned like that to get good sampling rate and best SNR).

I'm late to the party and did not read most of what has been written so far (sorry about that), so I might mention something that has already been addressed, but here are my finding so far:

- Flats are severely under exposed - both ISO800 and ISO100. Not sure if that is the cause of the problem - might not be, but it is something that you want to correct.

Flats have proper looking histogram, but I believe they are well under exposed. They should be at about 3/4 to the right of histogram, but on scale 0-16384 (14bit DSLR) it looks like this:

Otherwise, when zoomed in on relevant part, it looks fine:

Three peaks as it should have, nothing clipping, etc ...

- Second observation for all those who will attempt to process above data - don't even try to use ISO100 flats. Master flat should be calibrated with flat darks, or in this case, master bias will be enough. Both set of files (flats and bias) need to be shot at same ISO settings. Above included files contain ISO800 bias and both ISO800 flats and ISO100 flats. ISO800 flats will be fine to use with bias, but not ISO100 flats because of mismatch in iso setting.

Third observation is that something is seriously wrong with flats and I'm not still sure what it is, here is what flat fielding looks like (single sub after calibration with single dark, flat and bias):

This clearly shows that there was under correction of flat calibration in corners, but there was over correction in the central part - dust doughnut is brighter than it should be.

In most cases one has over or under correction over whole field and that is consequence of either improper calibration or some issues with darks or bias, or maybe some light leak, but this is quite a bit different.

My guess is that it has something to do with the way flats were taken, or the flat panel itself (although I'm not sure what it could be).

Can I ask what scope is this and what the setup looks like and under what conditions where flats taken?

In particular - was there change in anything in setup between lights and flats - for example - focus position?

3 minutes ago, wornish said:

Thanks Vlaiv for the explanation.

You should create a blog or perhaps a separate section on here that explains all this and gives the formulas to use.  I am sure many members have the same struggle to understand all the factors involved and the trade offs.

I think you might have just convinced me to spend money on a longer focal length scope.

Do bare in mind that "/px is not the whole story. There is also issue of FOV and sensor size.

It seems that some people also don't understand image size in relation to how it's displayed on computer screen - fact that you can display it at 1:1 (screen pixel to image pixel) or 100% zoom level or you can display it to fit the screen (like when you open it in picture viewing app or here on SGL forum).

I've often seen that people oversample their images but are quite happy with the way image looks on their screen - because they view it on screen size. For example, they shoot image with high pixel count camera, something like 4000 x 3000 or similar, but they view that image on computer screen that is 1920x1080 or similar. That image gets reduced x2 for display by application used and consequently pixel scale is changed. If original image was good to be sampled at 2"/px but was sampled at 1"/px instead it will look ok at screen size when it is reduced to 50% of its size. It will not look ok on 1:1 pixel or 100% zoom - it will look blurry.

In my view, it's best to aim for proper sampling and make your image look at 1:1 or 100% zoom (or here stands to indicate that it can be named either or, and not to imply these are two different things - it is the same thing but someone might call it 1:1 pixel mapping while others would prefer 100% zoom) - then it will look good on both 1:1 and screen size, and I think there are people out there that don't just look at image as a whole, but also enjoy to zoom in as far as image will let them and observe all the small details in object, or even background galaxies or such small features that are spotted on 100% zoom - so yes, your image will be viewed like that by someone.

To recap (and explain why is that important when choosing focal length):

- make your image look good at 100% by properly sampling (and binning afterwards if you need to)

- make your image at least 1500x1000 in pixel count. Today's display devices will make image look too small if you don't have at least that much pixels in your image and people might be tempted to hit zoom button - and that is just going to make your image larger but blurrier. You want to keep your image with enough pixels so there is no need to hit the zoom button.

These two things are important when choosing focal length to be matched to certain sensor. Take for example ASI1600 that you have - it has 4500px in width, this means that you can use bin up to x4 and should not really use more than that. If your mount and skies can produce images that are 2"/px in quality (stars with FWHM of 3.2") then you should not exceed 1500mm of focal length.

Why is that? 1500mm of focal length gives you sampling rate 0.52"/px, so you can bin up to x4 and with x4 binning you will have 2.08"/px, but it also means that your fov will no longer be 4600 x 3500 but rather something like 1150 x 870 and that is already smallish image.

Now, you can go ahead and use even 3000mm focal length, but in order to get image according to above (I believe sensible, but not set in stone criteria) you will need to do mosaic, as each piece of mosaic will give you something like (when binned x8) 550x430 px and you will need 3x3 mosaic to compose image that is 1500 x 1200.

Hope that makes sense. There is one more consideration with focal length - FOV, it's best to keep fov such that intended targets fit inside - otherwise you will need to do mosaic regardless.

2 minutes ago, newbie alert said:

How can fl have nothing to do with it when it controls your sampling, it's part of the digits that give you the sampling..

Yes you're right to say the mount has no idea what it's controlling on top but all mounts have limits of performance.. otherwise no-one would have issues trying to image long focal length scopes ..the job is far easier with a wider fov

A mesu will handle it better than a heq5 and neither will no what it's carrying on top

Indeed, FL goes into sampling equation, but so does pixel size, and this second part gives you so much flexibility as you can choose how much you want to bin your image. In fact you can set target sampling rate and bin to that exact figure. It involves fractional binning, and there is rather nice way to do it when combined with stacking.

Mount error is in absolute units and not relative units - it is measured in arc seconds. So is seeing. Focal length maps those errors to certain number of pixels. This turns it into relative measure - per pixel. So if your error is some arc seconds in size - you can choose to make it one pixel large, two pixels large or less than one pixel large. Point is that you have fixed focal length (or maybe small flexibility by adding focal reducer or barlow) but you can in principle change your pixel size by manipulating data. It is easier to join pixels than it is to split pixels in math, and that is why it is easier to use longer focal length to get coarser sampling then it is to use short focal length to get finer sampling (involves drizzle algorithm and it won't quite work well in amateur setups).

What I was trying to say with this, and again you are right - mesu will be better than heq5 for example, but that does not mean that you need to use long focal length scope on mesu only - you just need to "accommodate" pixel size to the level of precision achieved depending on focal length you are using. That and the fact that using longer focal length scope will not make mount error larger in arc seconds (it will in pixels if you leave pixel size what it is).

8 hours ago, Craig123 said:

Would you say that sub I uploaded of flaming star looked to be suffering from light leak. To me the gradient on the left  seems far less on this  last try than  was evident on the original jellyfish sub. 

If its still there I'm stumped as to why.. 

Yes, it is there and yes it's going to hurt your data. You need to solve that.

As you can see - it is stronger than target signal and that is really not good.

With one of darks you posted removed it looks like this:

5 hours ago, Craig123 said:

Thanks for the info on this. Here is a 5 minute dark  i've just done with camera off the scope and wrapped in foil aswell ... first time i have tried this and it seems better.

darkoffscope_00002.fits 31.26 MB · 1 download darkoffscope_00005.fits 31.26 MB · 0 downloads

Yes, this now looks much better - look at it being stretched:

Also note something:

That is comparison of mean values between your old flat dark and this new dark of 300s - one is 1022 and other is 1068 ADU. That is what you would expect from normal dark, very little change because dark current is low.

Now that you found that you have a light leak - that is good first step. You can do darks like this and that is guaranteed to make you good darks, but problem is that you still have light leak on your scope and your lights and flats will be polluted with this light leak. That is something you need to sort out if you want decent images.

1 hour ago, wornish said:

I am struggling with this discussion.  Like the OP I would love to go for smaller targets, but have held back from going for a longer focal length scope.

Surely the longer the focal length the smaller the image scale for a given camera. (that is defined by the cameras pixel size)

I agree that a mount has no idea what scope is on it.

Mounts can guide to a given tolerance depending on :

a. The mounts physical design and constrruction

b. The guide camera focal length

but when it all boils down given a reasonable mount and good guiding what matters is

c. The Seeing. 

 

If the seeing in your location is average ( i.e. you are not on Hawaii, in the Atacama desert etc) then seeing is the major factor that defines what you can resolve.

Isn't it simply wasting your money if you think you can some how magically fix the physics, despite what all the brochures promise?

What am I missing?

 

 

 

My point with that post was - don't think of your focal length when you think if mount is capable of delivering. Think your sampling resolution.

Why do I say that? Because it is ultimately the sampling resolution that needs to be matched to level of detail, and while sampling resolution depends on focal length of the scope - it is not only factor. There are other things that go into sampling resolution "equation" - and some of those things you can control.

What does that mean? Someone might give you following recommendation - don't use 2 meter focal length scope because you have average seeing - you won't be able to guide /track it properly with your mount. But that is just simply flawed reasoning.

Let's say that your mount and sky conditions on particular night support 1.5"/px resolution (there is in fact direct link between optimum resolution and star FWHM on a given night and it goes like FWHM / 1.6, so if your FHWM is 2.4" then you are ok with 1.5"/px, but if your FWHM is closer to 3.2" then matching resolution is 2"/px).

Now you have 2000mm focal length and you want to sample at 1.5"/px. You need something like 15um pixel camera and you will get that sampling rate. You can't just take camera for every possible resolution, but you can get rather close - for example if you have cmos sensor with pixel size of 3.8um then you can get very close to 15um by simply binning x4 - that will give you 15.2um pixel size or resolution of 1.57"/px - that is close enough.

Another way to do it is to use smaller scope - let's say scope with 500mm focal length and pair that with 3.8um camera - you will also have sampling rate of 1.57"/px that way.

You can achieve proper sampling rate with both long focal length scope and short focal length scope if you manage your data in certain way (by binning for example) - and both those scopes will record properly what seeing and tracking/guiding has to offer on particular night.

Advantage of long focal length scope will be aperture, but disadvantage will be narrow FOV. If target fits narrow FOV - than that is not disadvantage at all for that target.

You are right - you can't beat the seeing nor can you fix mount issues in software or by choice of scope, but you can "adjust" your data to conditions on a given night and still have your stars look nice and tight, but on a smaller image simply because there is no detail to "zoom in" more.

On a night of a good seeing and when conditions are good - then it pays off to have that much focal length  - it is very easy to go high resolution with long focal length scope, but almost impossible to go with short focal length scope (you need a barlow / focal extender for that and that won't change aperture of small scope so it will still gather same small amount of light).

Btw, just as a comparison - original image in this post had resolution of about 2"/px. This is why stars looked ok when I scaled it to 25% of original size. With same aperture scope under good conditions and when your guiding is better than 0.5" RMS you can get stars like this:

This is sampled at 1"/px by the way, so if we bring it down to 2"/px like I did in my example, these stars look really tiny and pinpoint:

And this was shot with 1600mm of focal length on Heq5 mount. So this is not out of the reach - one just needs to pay a bit attention to guiding and if night is good - stars will be tight - regardless of focal length as long as you know how to match sampling rate to conditions.

3 minutes ago, newbie alert said:

Hard to setup but no harder to use than what you are doing at the moment 

Cem45... not too sure what the payload is and you may well be under but it's not just weight but focal length that you're asking it to achieve.. look at your image scale

Please don't continue to keep that way of thinking alive - focal length has nothing to do with performance of the mount.

Put two identical scopes (in terns of size and weight) on the same mount - one with 200mm FL and other with 4000mm FL (I know, such scopes don't exist, but let's imagine some crazy CN 212 that can be both hyperbolic newtonian and folded maksutov type scope in the same package). How on earth would mount know the difference between focal lengths of the scopes and why would it behave differently because of that?

Again, it's not focal length that is the issue - it is sampling rate (which in part depends on focal length - hence origin of the myth, but you can control it via other means - like using focal reducers, binning, using different camera with different pixel size, etc ...).

I use OAG and never understood all the "it's difficult to use" hype... Never had issues with not being able to find guide star and with guiding itself.

How about TS 80mm F/6 apo triplet? I have one and it is very good imaging scope. Will need field flattener, and I think x0.75 Riccardi FF/FR will do excellent job there making scope F/4.5, very fast little thing.

Altair Astro has the same scope (I think) under their brand name here:

https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-80-480.html

21 minutes ago, Craig123 said:

Hi Vlad. Hope your well...

I had a go at IC405 the other night and tried to eliminate the light leak you have highlighted above.

I think it was the laptop near the manual filter wheel. Anyway I tried a few calibration frames  which I have added here if you have a moment to have a look and see if they are closer to what is required. They are single frames dark, flat dark, flat and light frames before I try and stack them.

Also is an automatic averaged stack of 50 darks using Sharpcap capture dark which I tried for comparison.

Seems like I'm getting closer but I didn't remove the camera from the scope yet incase I need to re do anything. The light panel I'm using isn't too bright (Gerd Nuemann which got slated on SGL the other day ) so the flats are  all the way up to 5 seconds which seems very long but the Histogram analysis in sharp cap says its acceptable and ADU is around 25000...I hope ??

Many thanks

Craig

 

ic405ha.fit 31.38 MB · 0 downloads IC405darkframesingle.fit 31.38 MB · 0 downloads ic405flatsdarks5secsingle.fit 31.38 MB · 0 downloads ic405flats5sec_00006.fit 31.38 MB · 0 downloads autostackdark_50_frames_-20.0C_2019-12-19T21_53_38.fit 31.38 MB · 0 downloads

Not sure what is going on with your subs, but here are my findings (or rather what I feel is not right):

1. There is massive gradient / light leak in dark sub - visible here:

2. I can't see details of file capture because these files are not straight out of the camera - their fits header shows that they were loaded in PixInsight and saved from there (can't tell if they have been manipulated in any way):

I have no idea what was set point temperature and what was actual temperature and don't know what gain and offset were used and if they match across the subs (lights and darks; flats and flat darks).

3. Mean value of dark sub is higher than mean value of light sub - this should not be so.

This can happen if:

- you have had your gain set on higher when taking darks, or maybe messed with offset (I don't believe it is offset because it can't make such a difference)

- you have a light leak that was worse when doing darks - you had some additional ambient light when doing darks or it was not fully dark yet

- subs were altered in some way

4. Flat and flat dark look ok as far as can tell, but I can't apply them to see if they work properly - dark calibration (first step in calibration) fails because darks are not suitable because of higher mean value (point 3).

5. Don't do sharpcap auto stacking of anything - it simply produces very different results:

How can you stack subs that have average value of ~5400 and get average of 1014?

If one stacks 30 subs - each being 16bit, you want result to be 32bit floating point - don't use sharpcap auto stacking as it produces 16bit result (and you have no idea what it did with the data)

6. It would be good if you could do following:

- if you are using sharpcap to capture your subs - use ASCOM driver instead of native driver (in case you are not already using it) - but do consider changing capture software as Sharpcap is better suited for planetary and fast video applications than it is to long exposure imaging

- try using files directly from capture software and not loading them in another application (pixinsight) and then saving them from there

- try doing darks with camera removed from scope, covered with plastic cap and set "face down" on thick wooden table (or plank or whatever you have), just to eliminate any possible light leak and see if you get proper darks. Just a few test darks will be enough.

18 minutes ago, Lockie said:

I've never tried it with a scope as I picked it up cheap and it's just been stored since, so it's a gamble, but I could pick up a 6" f8 Newt from FLO for £185 to cover the lunar planetary side of things?

That could well be the solution to planetary and lunar needs for both visual and imaging.

Why don't you give it a test run? No need to put scope on it - just plug it in and see if it moves. If it moves, it is probably good enough for both visual and planetary AP. For planetary AP you don't really need any sort of precision in your mount. As long as it keeps planet somewhere in FOV of camera - you are good.

I imaged planets with Eq2 with simple DC motor that had potentiometer to regulate tracking speed - it worked well.

If you do that, you still have something like £500 of your budget to put towards AP, and in that case, it's worth checking out AzGti with EQ accessories (counterweight and wedge, although you can use photo head for wedge - it won't have precise polar alignment and it needs to be rather heavy duty to support both mount and scope / lens - but maybe you have one lying around) and maybe a small scope - like 130pds or small refractor?

4 minutes ago, Chris49 said:

The guide scope is the standard Celestron 50mm finder #51611 and holder that comes with the EdgeHD8 - not very satisfactory as it is not rigid.  Guide cam is SX Ultrastar.  I have since moved it to rings mounted on a Vixen dovetail on top of the main tube.  This is much more rigid and is symmetrical for balance so that should help.

That setup has nowhere near needed precision to guide your setup for close in shots.

First "upgrade" would be switching to OAG.

4 minutes ago, Chris49 said:

Mount is iOptron GEM45 - just getting used to it, but I am impressed so far, way better than my A-VX which had so much Dec backlash that it never came back!

This mount should provide you with enough precision for high resolution work, provided you don't overload it an or do something else that could hamper its performance.

There are a few options out there for a scope that is capable of doing both planetary and DSO imaging well.

Even modest 6" F/8 newtonian is going to be rather good at both, provided that person operating it knows what they are doing. If I was asked to choose a scope that does it both well, then this would be my choice:

https://www.teleskop-express.de/shop/product_info.php/info/p10753_TS-Optics-8--f-12-Cassegrain-telescope-203-2436-mm-OTA.html

However these sort of scopes and required knowledge and skill to produce good results with them are beyond beginner level, so that would not be my recommendation for OP.

My recommendation for good photographic kit that can do both DSO imaging and planetary imaging with such a limited budget would be EQ5 mount and 6" newtonian with suitable coma corrector and barlow. Something like 150PDS.

If shopping second hand then even Heq5 might not be out of reach?

2 minutes ago, Chris49 said:

A lot to think about here.  Clearly I will try to find a place to put the filter and concentrate on improving the guiding.  Also maybe I should include the #94242 0.7x reducer - I was trying without because it is the tiny galaxies that I'm really after.

Thank you for your advice.

Chris

You have hit major obstacle (and hit it rather hard) - there is limit to how "close" in we can get. After you cross that line - you just end up with zoomed in image - but blurry and without detail. Bloated stars are obvious consequence of that.

If you are after maximum magnification and as close in as you can get for galaxies or other small object (like planetaries), you want to do 3 things.

Well - you want to do two things, and wait for third - as there is noting you can do about it, except moving to Atacama or similar.

Do whatever it takes to get your guiding sorted and that means guide RMS at 0.5" or below. Make your sampling rate somewhere between 1 and 1.2"/px and of course - wait for night of good seeing. Actually that last one can be helped - avoid anything that can mess up seeing locally (read on how to optimize planetary viewing) and work on targets when they are highest in the sky and / or best positioned.

In order to get to certain resolution with EdgeHd8" and that color camera, you need to do two things - add reducer and debayer using super pixel mode. That will put you in 1.32"/px and that is quite ok to start with for high resolution work.

Just now, Star101 said:

Stars do look more bloated on reflectors.

I don't think this is necessarily true, it is down to matching pixel scale, guide performance and seeing conditions. In fact due to aperture size, I constantly find that smaller refractors have larger stars (when resolution and rest is matched).

SCTs have some issues, and I personally don't like how stars look because of that - regular SCTs have a bit of spherical aberration that depends on focus position. Because focusing is done by moving mirrors - this introduces a bit of spherical aberration because mirrors are not separated with optimal distance.

I'm not sure if Edge models suffer the same - they should be better corrected.

You are sampling at 0.46"/px and you have guide RMS error of about 1".

These are main reasons why your stars appear large. They are not in fact large, they are "average sized" stars.  Most people will get such stars in their images. Problem in your case is that you have huge pixel scale and inappropriate guiding for such pixel scale.

While these stars look big:

Once you do proper matching of resolution to setup (guiding performance and the rest), you'll get much better looking stars

As you can see - now stars look as they should.

It it is scope side and it is first place where light falls - it is probably field stop of the eyepiece, and no, you won't get more light if you remove it (even if it is not field stop).

4 minutes ago, Lockie said:

You're right. It's certainly equally about how fine the camera pixels are as well as the focal length. The combination of which giving the sample rate. My Fuji mirrorless has a weak IR filter so I'll probably use that over my Pany GH3. The Fuji also has the larger APS-C sensor at 24MP, so these are pretty fine pixels. I'm not sure if I can bin 2x2 etc with a Bayer sensor? I know you can do this with Mono astro cams. 

Certainly a good thing to point out to me though, and I agree you can do basic DSO imaging with a basic but sturdy mount as long as you're aware of how to get round periodic error. 

I don't know. I get paid tomorrow. I didn't want to stretch my budget as there are other things to buy, but if an extra 200 quid made a difference I can't over look how little this is in the scheme of things. Our replacement front door cost nearly 1800!

I think it is good info regardless of what your final choice may be.

Bayer matrix is a bit different, but can also be binned in very similar way to mono CMOS data. First thing to realize is that OSC sensors have lower sampling rate by factor of x2 in comparison to mono sensors of same pixel size (this is because pixels are spaced twice the distance if you look at individual colors - where green is thought as green1 and green2, regardless if they have same filter and end up in same channel). Once you debayer your image like that - by splitting it into channels, you can then further bin each channel by certain factor as if it was mono image (because it is really at that point). This will of course give you twice lower sampling rate than you calculated in the first place - and twice lower pixel count.

In fact color sensors have "full resolution" because algorithms make up missing pixels anyway (interpolate in certain way). You can do the same if you want with your final image - just enlarge it x2 (but you won't get any more detail by doing so either way - by debayering with interpolation or with resizing final result).

1 hour ago, Lockie said:

 

Hi both, with regards to the DSO imaging side of things there are two sides to me:

1) the sugar coated side which thinks I would regularly get a big enough gap in the weather and my schedule to setup a mobile goto rig with guiding, laptop, plus the fine tweaks of PA needed for longer focal lengths etc.

Then there's person 2) Slightly depressingly realistic individual that thinks using my 200mm lens camera combo on a motor driven mount is the most I'd realistically get do both regularly and with any great success. 

Person 1 would with my tight budget probably buy FLO's 130pds EQ3 Pro combo for £628. Or try and scrape more money together for an EQ5 Pro thus waiting until early next year. 

Person 2 would probably buy an EQ5 with motor drive, and use it for DSO imaging with a camera and lens. Then a separate OTA for visual, planetary, and lunar such as a 150pds, or even a 200p or 127Mak or whatever. 

 

 

Maybe it would help to stop thinking in terms of focal length and start thinking in sampling rate when you want to think about periodic error for example or polar alignment error and exposure length.

Let's suppose that you have drift due to poor PA or RA periodic error of something like 0.1"/s. How much can you expose before this trailing becomes 1 pixel long? Well - that depends on how "big" your pixels are in relation to focal length (and not focal length alone).

Suppose that with 200mm lens you sample at something like 4.5"/px. In this case you can have exposure length of 45 seconds and still keep trailing less than one pixels - virtually undetectable. With 750mm focal length scope you will sample at 1.18"/px, and in this case exposure length will be something like 11.8 seconds.

But what happens if you bin data from 750mm scope? If you bin x2 - then you are no longer at 1.18" but rather at 2.36"/px, and exposure length now can go as much as 23.6s. If you bin x3 then you have 3.54"/px and possible exposure goes to 35.4s. Bin by x4 and you will be matching your 200mm lens (with much larger aperture) at 4.73"/px and 47.3s exposure without any visible trailing.

Only drawback is smaller FOV, but if you want wider field - you can switch to lens or even if you don't have lens - you can still use scope and do mosaics. That will be almost as equally fast as using F/5 lens (provided that scope is 150PDS - which is F/5) with small focal length.

I've written this to say that Person 2 can use larger OTA for DSO imaging as well if there is solid understanding of how things work, what are limitations and how to process your data to get there. I would however keep things simple on EQ5 class mount and go with 6" aperture max (8" is just too much). Maybe even consider F/6 newtonian if coma corrector is too much for the budget.