Long focal length imaging? Options + sense check?!

[rnobleeddy]

I've been imaging using a 130PDS on an old but passable EQ6 for the last 9 months or so. Most recently with a QHY163M + FW, but I've tried a few cameras over the months. Mainly DSOs but might focus on lunar work in twilight over the lighter months.

Someone nearby was selling a 250PDS so I thought I'd get it to give it a go, mainly as I was always a bit annoyed that some of my favorite images (e.g. the dumbell nebula, globular clusters) were so small. I guess I did this without really thinking it through.

I got the 250PDS out last weekend, and had the usual first usage issues - focusing with no idea of where to start was hard, and my attempt to barlow a cheap refractor to make a guidescope ended up in failure, and so I ended up guiding with a 180mm guidescope in the finder shoe. My EQ6 struggled a bit under the increased load, and I don't have a flat panel that's anywhere near large enough for the 250PDS.  But most of all, it's impossible for to safely get it out of the house/in to the garden without help, and that help usually goes to bed before midnight. So perhaps this isn't long for my collection.

I've resolved to have another go before I sell it, but I also started to think about what I was hoping to achieve. The 130PDS + QHY163M end up at a resolution of 1.2" per pixel, which as I understand, is going to roughly the same as a good nights seeing? The 250PDS ends up at 0.65" per pixel, which is (according a google search) the best seeing you'd ever get on Earth and I don't think they mean my garden!

So I may be doing this somewhat simplistically, but I'd guess this would mean the 250PDS is collecting ~4x as much light as the 130PDS I'd expect to collect more signal more quickly. Based on seeing, I'm not going to get a better resolution? So does it make sense to use the 250PDS? 

 

Hoping someone can sense check my ramblings above - which if true, mean that the 250PDS isn't going to "zoom" in on objects at all, so I've just got to weigh up if the size/weight is worth the extra light?

Given that the 250PDS is impractically large for someone who doesn't have an obsy, are there other scopes that people would recommend for this kind of work? Or should I just stick with the trusty 130PDS?

 

 

Edited March 23, 2021 by rnobleeddy

[michael8554]

Your own figures say it all.

130PDS ....... 1.2" per pixel

250PDS.......  0.65" per pixel

So you have nearly doubled your "magnification".

1.2arcsecs/pixel is an okay image scale for even UK conditions.

180mm guidescope with a 3.75um pixel guidecam, properly mounted, will work.

But the 1200mm FL is crying out for an Off Axis Guider with a Binned mono guidecam.

I don't know if that lot is too much for an EQ6 or not.

Michael

 

 

 

 

 

[JSeaman]

I can only add from my own experience - I am running a 300PDS on an NEQ6 Pro (1,500mm rather than the 1,200mm of the 250 PDS).

It weighs an awful lot more but I manage it just about on the limits of the mount with all my other equpiment on there. It does work but I wouldn't dream of setting it up each night, I would definitely damage something.

The magnification is superb, I was actually too close for imaging most things so I changed my camera to an ASI 1600mm and the FOV is now much better. Changing the focal length (or sensor size) changes the field of view so you can get up close and personal on those targets.

In terms of guiding, I use an ED80 with an ASI 120mm which to try and get less than 0.88" accuracy which was just about doable. I have recently switched to 2x2 binning and that makes things a lot more palatable though.

[Paul M]

I've had a 250PDS for nearly 8 years and started trying to image with it a year ago.

It's great with a DSLR but I bought a poor choice of imaging camera. The ASI178MC, small sensor with small pixels. It's a struggle to get tight stars. Image size is only 22x15 arc mins.

I did the belt mod on my NEQ6 Pro but it went pear shaped. Tracking was bad and guiding even worse. I put it down to the torque involved in moving the scope. I took the belts off, tuned up the gearing and it's as sweet as sweet can be

The whole lot is a big job to set up every session and it seems that there is an equipment issue every time too.

Not a route I'd recommend to someone who's primary interest is imaging. But it's what I have. Maybe I'll buy a more suitable camera later in the year. Until then I'll carry on down the road to frustration!

[vlaiv]

As far as choosing imaging resolution goes, here is the quick break down:

1. Seeing FWHM somewhat relates to imaging resolution but it is not straight 1-1 correspondence

2. Aperture size and your guiding / tracking play a part too

3. Seeing + aperture + guiding will give you some final FWHM of stars in your image. This is usually larger than what would seeing alone produce. Having stars of 2" FWHM or less in your image is rather difficult to achieve although seeing of 2" FWHM is common

4. There is simple relationship between sampling rate and FWHM - which is star FWHM / 1.6 = sampling rate.

In my view 1.2"/px is oversampling on 5" aperture in regular seeing and tracking conditions. Say you guide at 0.8" RMS and your seeing is 2" FWHM - what would be good sampling rate?

You can expect star FWHM to be 2.88" and corresponding sampling rate around 1.8"/px

Examine some of your subs that you've taken with 130pds to get the idea of what FWHM you can expect. By the way - above is for perfect optics. For example, newtonian with simple 2 element coma corrector will bloat stars somewhat due to introduced spherical aberration and you can easily have stars with FWHM 3" or 3.5" in your subs because of that.

250PDS will have large aperture and this means a bit tighter stars. In same conditions as above - 2" FWHM seeing and 0.8" RMS guiding, you'll get 2.78" star FWHM instead of 2.88" star FWHM - so there is a bit of improvement. In fact - better the seeing and your guiding - more improvement you'll see over smaller aperture.

However, even scope that large requires good seeing and good guiding to hit 1.2"/px resolution. In 1.5" FWHM seeing and 0.5" RMS guiding, ideal 10 scope will produce stars with FWHM 1.96" and corresponding sampling rate will be 1.22"/px

In any case - not all is lost - you can bin your subs in software after calibration to get to good sampling rate.

Bottom line - increased focal length may give you larger objects - but not more resolution. You'll get same effect if you simply enlarge your images in the end - larger object without additional detail. In order to really get detail - you need sharp largish optics (say 8"-10"), exceptional mount that tracks/guides smoothly (rms in range of 0.2-0.3) and night of very good seeing (1.2-1.5" FWHM range).

 

[pete_l]

Two points/

First, you can always alter your arc-sec per pixel value by adding a focal reducer. That will give you the benefit of the larger aperture.

Second, you say the setup is too unwieldy for one person to manage. That would seem to shut down any further discussions about using it.

[AbsolutelyN]

I'm currently using a 250PDS as my main imaging scope. It's on a AZEQ6 and I've added losmandy mounting bar and a vixen bar across the tube rings to try to strengthen it. It's pushing the mount capability somewhat but you can absolutely can get very good images with it as long as there is very little wind as it's like a sail. I've got a skywatcher coma corrector attached which brings the focal length down a tad. It's semi-permanent but personally I'd be ok setting it up each night, though it is big and heavy.  So it's certainly possible and it's a very good scope, I've been pleasantly surprised with images I've got with it so far. Not at all saying its ideal but at the moment it's what I have to work with so intend to make the most of it. 

Edited March 23, 2021 by AbsolutelyN

[rnobleeddy]

8 hours ago, michael8554 said:

Your own figures say it all.

130PDS ....... 1.2" per pixel

250PDS.......  0.65" per pixel

So you have nearly doubled your "magnification".

1.2arcsecs/pixel is an okay image scale for even UK conditions.

180mm guidescope with a 3.75um pixel guidecam, properly mounted, will work.

But the 1200mm FL is crying out for an Off Axis Guider with a Binned mono guidecam.

I don't know if that lot is too much for an EQ6 or not.

Michael

 

 

 

 

 

It's within the spec of an EQ6 but mine is a little stiff so I think it's hard to balance. I'm planning to belt mod it once the nights get lighter, so will have a go at sorting it out then.

The calc I found based on the pixel scale of the guide cam + imaging cam etc came out at about 240mm, so I guessed 180mm would probably suffice in a pinch.

From what I've read 1200mm is about the limit after which an off-axis guider is essential - do you expect that the performance is likely to be limited by the guiding at that focal length?

[rnobleeddy]

7 hours ago, vlaiv said:

As far as choosing imaging resolution goes, here is the quick break down:

1. Seeing FWHM somewhat relates to imaging resolution but it is not straight 1-1 correspondence

2. Aperture size and your guiding / tracking play a part too

3. Seeing + aperture + guiding will give you some final FWHM of stars in your image. This is usually larger than what would seeing alone produce. Having stars of 2" FWHM or less in your image is rather difficult to achieve although seeing of 2" FWHM is common

4. There is simple relationship between sampling rate and FWHM - which is star FWHM / 1.6 = sampling rate.

In my view 1.2"/px is oversampling on 5" aperture in regular seeing and tracking conditions. Say you guide at 0.8" RMS and your seeing is 2" FWHM - what would be good sampling rate?

You can expect star FWHM to be 2.88" and corresponding sampling rate around 1.8"/px

Examine some of your subs that you've taken with 130pds to get the idea of what FWHM you can expect. By the way - above is for perfect optics. For example, newtonian with simple 2 element coma corrector will bloat stars somewhat due to introduced spherical aberration and you can easily have stars with FWHM 3" or 3.5" in your subs because of that.

250PDS will have large aperture and this means a bit tighter stars. In same conditions as above - 2" FWHM seeing and 0.8" RMS guiding, you'll get 2.78" star FWHM instead of 2.88" star FWHM - so there is a bit of improvement. In fact - better the seeing and your guiding - more improvement you'll see over smaller aperture.

However, even scope that large requires good seeing and good guiding to hit 1.2"/px resolution. In 1.5" FWHM seeing and 0.5" RMS guiding, ideal 10 scope will produce stars with FWHM 1.96" and corresponding sampling rate will be 1.22"/px

In any case - not all is lost - you can bin your subs in software after calibration to get to good sampling rate.

Bottom line - increased focal length may give you larger objects - but not more resolution. You'll get same effect if you simply enlarge your images in the end - larger object without additional detail. In order to really get detail - you need sharp largish optics (say 8"-10"), exceptional mount that tracks/guides smoothly (rms in range of 0.2-0.3) and night of very good seeing (1.2-1.5" FWHM range).

 

Thanks - really helpful explanation. 

Guiding with the 130PDS currently ranges between 0.7" and 1" depending on conditions/balance/wind/target position. Usually about 0.8".  I guess I'll need another night to work out how much that degrades with the 250PDS.

I'm guessing it is fair to assume most images will have some stars in them that are small, so it won't be dependent on where in the sky I was targeting?

If all else was equal, is it fair to assume the 250PDS has 4x as much light hitting each pixel as the surface area of the aperture/mirror is 4x, or is that maths more complex?

 

 

[rnobleeddy]

6 hours ago, pete_l said:

Two points/

First, you can always alter your arc-sec per pixel value by adding a focal reducer. That will give you the benefit of the larger aperture.

Thanks. Are there general purpose reducers for Newtonians? I've seen some coma correctors that change the focal length a little.

 

Quote

 Second, you say the setup is too unwieldy for one person to manage. That would seem to shut down any further discussions about using it.

I guess I should probably have said it's not ideal but not impossible. I can't seem to work out a way to screw the dovetail on to the mount on my own, as the mirror is so heavy and the scope too large to reach around. But I can almost always get help setting up, and I did manage to take it off the mount on my own. Longer term I'd probably just get a cover that I can use until the morning, but I've got the 130PDS routine down to less than 5 minutes at either end of the night, so it maybe just feels like a lot more hassle!

 

[vlaiv]

10 minutes ago, rnobleeddy said:

I'm guessing it is fair to assume most images will have some stars in them that are small, so it won't be dependent on where in the sky I was targeting?

Not sure what you mean by this - but even if stars seem small or large in the image (bright or faint) - they will have fairly similar FWHM value. That is why FWHM is used as a measure of things.

FWHM will depend on where in the sky your scope is pointing. Out of 3 things that impact resulting FWHM the most - two depend on where the scope is pointing.

Seeing is usually the best in direction of zenith and degrades as you move down towards horizon. This is because light has to travel thru thicker piece of atmosphere so it has more chance to get distorted along the way.

Guiding performance also depends on where the scope is pointing. RA error is the worse at equator - but it gets smaller as you increase declination and move towards NCP. If you think about it - Polaris takes 24h to make very small circle on the sky - you can fit that circle on your sensor with ease. Path that Polaris takes is something like maybe few thousand pixels - and it takes 24h for it to make it. Close to north celestial pole - large movement in RA results in very small movement in pixels on sensor - this also means that any error in RA will be very small when translated into pixels. Star won't be smeared as much due to RA error close to NCP. At 60° declination RA error is effectively halved because of this effect and continues to drop (times cos declination).

17 minutes ago, rnobleeddy said:

If all else was equal, is it fair to assume the 250PDS has 4x as much light hitting each pixel as the surface area of the aperture/mirror is 4x, or is that maths more complex?

Math is more complex than that. In fact - with same sensor both scopes are F/5 so both scopes will be equally fast.

This is because of fixed pixel size.

Although aperture is half in diameter - so is focal length and that means that sampling rate is half of larger scope.

Light gathering is for smaller scope is 1/4 of larger one but effective pixel size (how much of the sky pixel covers) is 1/4 with larger scope - so each pixel in principle gets the same amount of light.

250PDS indeed gathers more light, but you need to pair it with matching sensor (both sensor size and pixel size) to take advantage of that. Or - you need to bin your data (which is in effect the same as having larger pixels) to match the resolution between the two scopes.

If you match the resolution / sampling rate / effective pixel size - how ever you want to call it - thing that is expressed in arc seconds per pixel for both scopes - then you have the math right - x4 larger aperture will be x4 faster.

[JSeaman]

"From what I've read 1200mm is about the limit after which an off-axis guider is essential"

I'm running 1,500mm without OAG for info

[rnobleeddy]

1 hour ago, vlaiv said:

Not sure what you mean by this - but even if stars seem small or large in the image (bright or faint) - they will have fairly similar FWHM value. That is why FWHM is used as a measure of things.

FWHM will depend on where in the sky your scope is pointing. Out of 3 things that impact resulting FWHM the most - two depend on where the scope is pointing.

Seeing is usually the best in direction of zenith and degrades as you move down towards horizon. This is because light has to travel thru thicker piece of atmosphere so it has more chance to get distorted along the way.

Guiding performance also depends on where the scope is pointing. RA error is the worse at equator - but it gets smaller as you increase declination and move towards NCP. If you think about it - Polaris takes 24h to make very small circle on the sky - you can fit that circle on your sensor with ease. Path that Polaris takes is something like maybe few thousand pixels - and it takes 24h for it to make it. Close to north celestial pole - large movement in RA results in very small movement in pixels on sensor - this also means that any error in RA will be very small when translated into pixels. Star won't be smeared as much due to RA error close to NCP. At 60° declination RA error is effectively halved because of this effect and continues to drop (times cos declination).

 

Sorry - I wrote that without thinking about it. Unfortunately whilst I did study physics, I've since forgotten everything I ever knew . But I can learn again!  

DSS reports average FWHM ranging from low 3's to mid 4's across a few different stacks. As best I can tell, these are average values in pixels, so that's roughly 3.6" to 5.5". Just trying to work out if there's any decent way using free software to get a better idea of the spread of the FWHM.

 

Edit - I'm using the relatively cheap Baader MPCC, which I also imagine isn't helping.

Edited March 23, 2021 by rnobleeddy

[vlaiv]

42 minutes ago, rnobleeddy said:

DSS reports average FWHM ranging from low 3's to mid 4's across a few different stacks. As best I can tell, these are average values in pixels, so that's roughly 3.6" to 5.5". Just trying to work out if there's any decent way using free software to get a better idea of the spread of the FWHM.

You can get AstroImageJ to accurately measure FWHM but I'm not sure it will do batch measurement (like multiple stars across multiple subs).

I do know that you can CTRL+click or something like that to get FWHM profile on any star. I find AstroImageJ more accurate than other software.

1 hour ago, JSeaman said:

"From what I've read 1200mm is about the limit after which an off-axis guider is essential"

I'm running 1,500mm without OAG for info

Yeah, I think that rule of thumb is from old days when things were measured in focal length

Nowadays it is measured in resolution / sampling rate irrespective of focal length used.

Back then - if you were using short FL that meant you were working on low resolution and if you were using long FL - that meant that you were working at high resolution. Now we have both small and large pixels and we know how to bin - that means you can use 650mm FL and still work at very high resolution of 0.76"/px for example.

In my view, OAG is better way to guide than guide scope for number of reasons. People are sort of afraid of using OAG because of "difficulty of setup" and "trouble with finding stars". I did not experience any of those with my OAG setup - it is as easy to use as guide scope and more effective.

In order to achieve good guiding results that your mount is capable of - you need to provide it with good resolution of star position from guide system. Short FL guide scopes paired with larger pixels don't really offer good resolution for that. Even smaller pixels struggle.

Say you want to guide with F/4 50mm guide scope and ASI120 - that is 200mm of FL and 3.75µm pixel size. Single pixel covers 3.87" by 3.87" and you want your mount to guide at say 0.5-0.6" RMS. You want average measured error to be something like 8 times less than single pixel size.

Although centroid algorithms can measure star position down to say 1/16 of a pixel, in this case ~0.241875" - can you really reliably measure error in mount position down to 0.5" or 0.6" of average error if your best position estimate is limited to every 0.25"?

Using OAG let's you use much longer FL to guide on - and that simply means better precision in determining error / correction and average error, resulting in tighter stars.