Best Ways to Increase FOV on a 10" Dob??

[JonCarleton]

I have a 10" Dobsonain that I am very happy with, especially observing, where I can use a 2" 34mm lens and get a good look a a large chunk of the sky.  Being a newbie, this is very helpful.

When I start imaging, I'm back down to a camera that has a very tight FOV, similar or slightly less than my 1.25" 9mm Plossl.  So, to increase the FOV, I'm thinking .5X reducer seems reasonable.  Then I got that nagging, "am I missing something" thing going on in my head.  Maybe a different camera?  Maybe a DSLR?  That brings me to the question of DSLR vs. dedicated astronomy camera.  What are the advantages and disadvantages really?

This, then, is a multi-part question.  Is a DSLR better/worse and why?  Will one camera or camera style give me significantly more FOV than another? But finally, what is the "best practices" way to increase FOV with a camera?  (and let's avoid the obvious option of a different scope for that use, my wife hasn't threatened to kill me yet and it is probably best to stay on this side of the line for the present).

[ollypenrice]

The problem is that a Dobsonian is not an imaging mount. Although you have GoTo and tracking, you are tracking the sky right-left and up-down, while an object takes a circular path across the sky. Think of Orion who rises leaning to the left and sets leaning to the right. Only an equatorial mount can follow that curving path while keeping the object in the same orientation on the camera. Also the drives on your Dob will be accurate enough for visual observing but imaging requires an almost insanely precise motion.

There are other problems, too. A larger chip will indeed cover more sky but how large a chip will your optics cover without distortion? Without a coma corrector it won't be very much.

Since a Dob isn't naturally an imaging scope your best bet would be to try what you have rather than buy anything specifically for it. Maybe you have or could borrow a DSLR? A dedicated camera would be an expensive disappointment. The widely accepted order of priority in astrophotography is mount, camera, optics. If your mount is not pointing in precisely the right direction throughout the exposure it won't let the rest do their job. You won't get a good picture from a Hasselblad if someone is nudging your elbow.

I'd rather spell this out at the beginning. Astrophotography is quite unlike daytime photography and doesn't just involve putting a camera in a scope and taking a picture. It really is nothing like that. For a great over-view this book is excellent: https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html  I know this may seem counter intuitive and hard to accept but we've all been through this!

Olly

 

[johninderby]

Dobs are really for visual only and AP wise only suitable for shots of the moom and planets. I have taken some nice moon shots using a smartphone where I can get away with single shot images. Adding an EQ platform can allow EQ tracking for a limited time with a dob.

https://www.teleskop-express.de/shop/index.php/cat/c20_Mounts-----Equatorial.html

For proper AP you need an EQ mount big and strong enough to support you choice if scope. A small ED doublet refractor might be a better starting point as the mount won’t need to be too expensive. A lot of thinking and research needed and plenty of advice on this forum to help.

Few simple smartphonr images with a dob. 

Edited April 24, 2020 by johninderby

[miguel87]

I can only speak from the DSLR side of things as I dont have experience with other cameras. With a DSLR in 'prime focus' (no lenses involved or barlows or anything) with a telescope, there is a set FOV which doesnt change and depends only on your camera and the focal length of your telescope (think of the dslr as one eyepiece).

As above, many images will talk about expensive mounts, star trackers and the level.of precision required but it is not really true, depending on the quality of photo you want.

People take great pictures with eq 2 or 3 mounts, dob mounts, non tracking mounts etc etc.

What camera are you using to image now? We might be able to work out how it compares to an average dslr in terms of FOV.

These pictures is not great, but, I love them. It's a photo of a glowing nebula which is light years away and I took it, on my own, from my back garden! 😁 that just blows my mind.

All were taken with my old cheap pentax dslr and a 200p on an unguided equatorial mount, just 1 exposure of 30 seconds. No processing on a computer, no stacking, they just came out of the camera as they are.

Good luck with the imaging.

[Ships and Stars]

After spending almost two weeks imaging with a 20" SW dob with goto  it's hit and miss and you will be taking thousands of exposures and deleting a lot of them at times. With a 10" dob (still a fairly large scope) you'll need two to three times more exposures to capture the same amount of light and/or detail presumably. 

I'm using a full frame camera but the image circle only really covers about half the total sensor area. I shoot in 5:4 mode and crop heavily, maximum 8" subs at ISO 400 or 800. At least 75 really to get enough decent data for an image, and upwards of 200 is not uncommon, plus darks/bias and flats (which I've yet to do).

Sometimes the mount tracks nicely for short exposure photos, sometimes it's jerky and all my subs get deleted. Not designed for imaging as Olly says, but possible if you are determined. It's fine for visual, which of course a dob excels at. The only thing going for my setup is sheer aperture, I can get in 30 minutes integration what would take a refractor or small reflector 2-4 hours I'm guessing, but there can't be any wind and the mount has to perform near perfectly. Plus taking subs near zenith can be impossible. It's a fun experiment and if you have a go-to dobsonian and a DSLR laying around with a remote release or intervalometer or interval setting, go for it, but you'll have to be patient! Otherwise look at a proper imaging rig. Hope that helps.

PS still working on getting the colours right, so sticking with B&W. Images below are lights only, between ten and twenty minutes total integration I think, no darks, dark flats, bias or flats. 

Edited April 24, 2020 by Ships and Stars

[ollypenrice]

2 hours ago, Ships and Stars said:

After spending almost two weeks imaging with a 20" SW dob with goto  it's hit and miss and you will be taking thousands of exposures and deleting a lot of them at times. With a 10" dob (still a fairly large scope) you'll need two to three times more exposures to capture the same amount of light and/or detail presumably. 

I'm using a full frame camera but the image circle only really covers about half the total sensor area. I shoot in 5:4 mode and crop heavily, maximum 8" subs at ISO 400 or 800. At least 75 really to get enough decent data for an image, and upwards of 200 is not uncommon, plus darks/bias and flats (which I've yet to do).

Sometimes the mount tracks nicely for short exposure photos, sometimes it's jerky and all my subs get deleted. Not designed for imaging as Olly says, but possible if you are determined. It's fine for visual, which of course a dob excels at. The only thing going for my setup is sheer aperture, I can get in 30 minutes integration what would take a refractor or small reflector 2-4 hours I'm guessing, but there can't be any wind and the mount has to perform near perfectly. Plus taking subs near zenith can be impossible. It's a fun experiment and if you have a go-to dobsonian and a DSLR laying around with a remote release or intervalometer or interval setting, go for it, but you'll have to be patient! Otherwise look at a proper imaging rig. Hope that helps.

PS still working on getting the colours right, so sticking with B&W. Images below are lights only, between ten and twenty minutes total integration I think, no darks, dark flats, bias or flats. 

Your pictures really are very good for the equipment used. Hats off. I don't agree with your view of aperture/ light grasp. It is not a simple matter of how much light you grab, it is a matter of how much light per pixel you grab. There are informative discussions on the forum including this one: 

 

When you say, 'The only thing going for my setup is sheer aperture, I can get in 30 minutes integration what would take a refractor or small reflector 2-4 hours I'm guessing,' I'm going to disagree with you. What matters is not the total light grasp of your scope but the amount of light per pixel it puts on your chip. (Talking of chips, if you have a million fine British chipshop chips and 10 people to eat them they will die from obesity. If you have a million chips and 10 million consumers they will die of hunger. It isn't how many chips you have, it's how many chips per person. Same in AP. It's not total light, it's light per pixel.) At 3.5 arcsecs per pixel I promise you my 106mm Tak is pretty darned quick. Below is a Heart Nebula image form this rig with just two hours' exposure. 20 mins per colour and 2x30 minutes in Ha. 4 inch refractor. A 20 inch can only beat this in the time (and of course it can) if you have big enough pixels. Big 'if,' however.

Olly

 

Edited April 24, 2020 by ollypenrice

[Ships and Stars]

1 hour ago, ollypenrice said:

Your pictures really are very good for the equipment used. Hats off. I don't agree with your view of aperture/ light grasp. It is not a simple matter of how much light you grab, it is a matter of how much light per pixel you grab. There are informative discussions on the forum including this one: 

 

What matters is not the total light grasp of your scope but the amount of light per pixel it puts on your chip. 

Thanks for clearing that up Olly, I assumed more aperture would simply create a quicker image with less exposure time. I certainly will go with your opinion! That's good news actually, if I do take up imaging I won't be obsessed with aperture like I am in visual astronomy.

I've not read up much on astrophotography, other than a few basics, so lots to learn. I may buy Make Every Photon Count, lots of praise out there for that book.

Thanks also for sharing the article on small galaxies, that's just the sort of thing I was attempting last night. Hercules galaxy cluster, mixed results!

All the best and thanks for the information.

 

[JonCarleton]

I've had pretty good luck with my camera (SVBony 305) and the 10" Dob with respect to getting enough lights to stack and get reasonable images.  The big problem is FOV.  I also have an Olympus DSLR that I haven't tried yet, but I expect that the difference in FOV would be minimal without something else going on.

I do understand the differences with Alt/Az versus EQ mounts, and have found that I can get by reasonably well by using short exposures.  This, presumably, because I collect more light pressure per second with the large aperture.  The problem I am trying to solve is really FOV, although, I asked the question in a way that I hoped would bring to light some other surrounding issues.  It seems to have done so.

Thank you all for your input!!!!!

[Ships and Stars]

22 minutes ago, JonCarleton said:

I've had pretty good luck with my camera (SVBony 305) and the 10" Dob with respect to getting enough lights to stack and get reasonable images.  The big problem is FOV.  I also have an Olympus DSLR that I haven't tried yet, but I expect that the difference in FOV would be minimal without something else going on.

I do understand the differences with Alt/Az versus EQ mounts, and have found that I can get by reasonably well by using short exposures.  This, presumably, because I collect more light pressure per second with the large aperture.  The problem I am trying to solve is really FOV, although, I asked the question in a way that I hoped would bring to light some other surrounding issues.  It seems to have done so.

Thank you all for your input!!!!!

What if you did multiple panels and stitched them together to get a wider FOV? You'd have more detail as well presumably.

I just tried this the other night on some obscure galaxies in Hercules simply for a record shot, it's not a great image but kind of fun to mess with. I combined then using Microsoft Image Composite Editor.

One way to get more FOV so to speak without a focal reducer (never used one, but would be interested to know if it works).

Just a thought!? 👍

[saac]

6 hours ago, Ships and Stars said:

After spending almost two weeks imaging with a 20" SW dob with goto  it's hit and miss and you will be taking thousands of exposures and deleting a lot of them at times. With a 10" dob (still a fairly large scope) you'll need two to three times more exposures to capture the same amount of light and/or detail presumably. 

I'm using a full frame camera but the image circle only really covers about half the total sensor area. I shoot in 5:4 mode and crop heavily, maximum 8" subs at ISO 400 or 800. At least 75 really to get enough decent data for an image, and upwards of 200 is not uncommon, plus darks/bias and flats (which I've yet to do).

Sometimes the mount tracks nicely for short exposure photos, sometimes it's jerky and all my subs get deleted. Not designed for imaging as Olly says, but possible if you are determined. It's fine for visual, which of course a dob excels at. The only thing going for my setup is sheer aperture, I can get in 30 minutes integration what would take a refractor or small reflector 2-4 hours I'm guessing, but there can't be any wind and the mount has to perform near perfectly. Plus taking subs near zenith can be impossible. It's a fun experiment and if you have a go-to dobsonian and a DSLR laying around with a remote release or intervalometer or interval setting, go for it, but you'll have to be patient! Otherwise look at a proper imaging rig. Hope that helps.

PS still working on getting the colours right, so sticking with B&W. Images below are lights only, between ten and twenty minutes total integration I think, no darks, dark flats, bias or flats. 

I love those images; never thought you would have managed that with a Dob, well done. Any plans to experiment with DIY equatorial tracking base, I think they can give 30 minutes worth of useful eq tracking.  That said your images are already impressive -  you have given me some food for thought with our 16 inch dob.  

Jim

Edited April 24, 2020 by saac

[Ships and Stars]

9 minutes ago, saac said:

I love those images; never thought you would have managed that with a Dob, well done. Any plans to experiment with DIY equitorial tracking base, I think they can give 30 minutes worth of useful eq tracking.  That said your images are already impressive -  you have given me some food for thought with our 16 inch dob.  

Jim

Thanks Jim! I'd love to double the number of subs for each of these but the weather has gone quite cloudy for the duration. I think if I add darks flats etc they will be decent, and they are still quite high resolution images, even after a heavy crop. 

I looked at an EQ base, but I think at my/our latitude the whole thing would fall over! I'd have to pour some kind of concrete pier and bolt the base to it or something, but will revisit that idea again after the past few weeks. 

If you have a 16" with tracking or a EQ platform etc and a DSLR, I'd say give it a go. Focus is very important of course, and stability, etc etc. A little practice saw a lot of improvement in my images in a short span of time. 

Although I want an EQ mount an refractor/astro cam now, it's fun to see what a dob can do.

Edited April 24, 2020 by Ships and Stars

[JonCarleton]

1 hour ago, Ships and Stars said:

What if you did multiple panels and stitched them together to get a wider FOV? You'd have more detail as well presumably.

I just tried this the other night on some obscure galaxies in Hercules simply for a record shot, it's not a great image but kind of fun to mess with. I combined then using Microsoft Image Composite Editor.

One way to get more FOV so to speak without a focal reducer (never used one, but would be interested to know if it works).

Just a thought!? 👍

Hmmm.  Stiching might be a solution.  Interesting results you got there!

[saac]

1 hour ago, Ships and Stars said:

Thanks Jim! I'd love to double the number of subs for each of these but the weather has gone quite cloudy for the duration. I think if I add darks flats etc they will be decent, and they are still quite high resolution images, even after a heavy crop. 

I looked at an EQ base, but I think at my/our latitude the whole thing would fall over! I'd have to pour some kind of concrete pier and bolt the base to it or something, but will revisit that idea again after the past few weeks. 

If you have a 16" with tracking or a EQ platform etc and a DSLR, I'd say give it a go. Focus is very important of course, and stability, etc etc. A little practice saw a lot of improvement in my images in a short span of time. 

Although I want an EQ mount an refractor/astro cam now, it's fun to see what a dob can do.

This is our 16 inch Dob,  built as school telescope with a mirror gifted by Andrew S (forum member).  It's a bit heavier than perhaps it needs to be and at F5 it calls for a stepladder but the views are fantastic. It's a sight to behold when I have it in the classroom the kids are mesmerised by it. My long term aim is to try to get some spectra with it to maybe confirm the blue shift  of Andromeda as this would link in really well to our Higher physics curriculum. I don't know how practical that may be but your photographs with your 20 inch Dob have inspired me.  The lock down will give me some time to experiment. 

 

Edited April 24, 2020 by saac

[Ships and Stars]

10 hours ago, JonCarleton said:

Hmmm.  Stiching might be a solution.  Interesting results you got there!

This was sort of an accident, I was trying to image three separate sets of small (tiny) galaxy clusters in Hercules centred around NGC 6173 and NGC 6160 and NGC 6145 (I think). Had I thought of this before hand, I would take 50 subs, slew over about 50-66% of the frame, repeat, and so on. If you keep everything the same, i.e. same sub exposures, ISO and developing settings, they should stitch together in ICE in a snap, takes my 11 year old desktop about 30 seconds to stitch them. I'll try this again with more thought next time 👍

10 hours ago, saac said:

This is our 16 inch Dob,  built as school telescope with a mirror gifted by Andrew S (forum member).  It's a bit heavier than perhaps it needs to be and at F5 it calls for a stepladder but the views are fantastic. It's a sight to behold when I have it in the classroom the kids are mesmerised by it. My long term aim is to try to get some spectra with it to maybe confirm the blue shift  of Andromeda as this would link in really well to our Higher physics curriculum. I don't know how practical that may be but your photographs with your 20 inch Dob have inspired me.  The lock down will give me some time to experiment. 

 

That's a beautiful looking dob! F5 is good, less coma and less fussy on expensive eyepieces and more forgiving on focus. There is a big jump in coma between f5 and f4 from my experience and focus at f4 has to be just spot on for sharp images. 

A stepladder only adds to the drama of the moment - the process of climbing up to have a look lets you know you are using a BIG telescope  

[JonCarleton]

Ships & Stars::  Off topic...to your avatar.  I am a gyroplane CFI and a good friend of mine flew the gyroplane in that Mad Max movie.  Funny story..he sold gyroplanes in Australia and was approached by the film company to purchase one.  They arrived a few days later with a stuntman and asked him to "tell the stuntman how to fly it."  He replied, "OK...it'll take about 6 weeks to explain properly."  They hired on the spot to fly the shots, because they planned to start shooting that afternoon.

My apology for misdirecting the thread...but then, it is my thread start

 

[Ships and Stars]

Ah the avatar is quite small but it's Donald Sutherland in Kelly's Heroes, however it does look a lot like the gyroplane fellow from Mad Max, another one of my favourite films! I always wanted one of the big air-cooled Kawasakis after watching the original. And the police interceptor and the gyroplane after watching the second one The gyroplane pilot that's a friend of yours was in the right place/right time! Very cool. I'll have to watch that again. Out of curiosity I looked at gyroplanes years ago, they seem fairly safe in regards to other ultralight aircraft and lots of fun. I'd love to fly one along the coast here or up around the hills a bit. 

Edited April 25, 2020 by Ships and Stars

[Ships and Stars]

An interesting link to that!

https://madmax.fandom.com/wiki/Gyrocopter

Gerry Goodwin!

Edited April 25, 2020 by Ships and Stars

[Macavity]

💡Always thought it might be interesting to try a really LARGE Dobsonian
with a (real time) integrating Video Camera... Watec 910HX etc. You may
not like the idea of using a screen, but iit's multi-user - No need for ladders? 🥳

You could also split the Video stream... to write data to disk for subsequent
stacking - while you are observing & nudging... whatever it is Dob users do! 😛

P.S. I sense the above EEVA will be better on smaller / brighter (surface!)
targets in specific parts of the sky (less motion blurring). It's always good
to image *different* (less popular?) targets... GC's, OCs, PNs... maybe? 😎

Edited April 25, 2020 by Macavity

[saac]

4 hours ago, Macavity said:

💡Always thought it might be interesting to try a really LARGE Dobsonian
with a (real time) integrating Video Camera... Watec 910HX etc. You may
not like the idea of using a screen, but iit's multi-user - No need for ladders? 🥳

You could also split the Video stream... to write data to disk for subsequent
stacking - while you are observing & nudging... whatever it is Dob users do! 😛

P.S. I sense the above EEVA will be better on smaller / brighter (surface!)
targets in specific parts of the sky (less motion blurring). It's always good
to image *different* (less popular?) targets... GC's, OCs, PNs... maybe? 😎

That's a good point Chris and something that Andrew had also suggested, in fact he also donated a video camera and monitor as part of the package.  It's  definitely on the list of next steps and it would be well matched to  small group use viewing. 

Jim 

[andrew s]

19 hours ago, saac said:

This is our 16 inch Dob,  built as school telescope with a mirror gifted by Andrew S (forum member).  It's a bit heavier than perhaps it needs to be and at F5 it calls for a stepladder but the views are fantastic. It's a sight to behold when I have it in the classroom the kids are mesmerised by it. My long term aim is to try to get some spectra with it to maybe confirm the blue shift  of Andromeda as this would link in really well to our Higher physics curriculum. I don't know how practical that may be but your photographs with your 20 inch Dob have inspired me.  The lock down will give me some time to experiment. 

Hi @saac your scope is looking good. I  donhave a spare  Thorlabs 50x25mm echelle grating you can have if your current spectrograph is not up to the job. Mind you you will need to build the rest of it. 😛

Regards Andrew 

[saac]

6 hours ago, JonCarleton said:

Ships & Stars::  Off topic...to your avatar.  I am a gyroplane CFI and a good friend of mine flew the gyroplane in that Mad Max movie.  Funny story..he sold gyroplanes in Australia and was approached by the film company to purchase one.  They arrived a few days later with a stuntman and asked him to "tell the stuntman how to fly it."  He replied, "OK...it'll take about 6 weeks to explain properly."  They hired on the spot to fly the shots, because they planned to start shooting that afternoon.

My apology for misdirecting the thread...but then, it is my thread start

 

Jon, back to part of your original question regarding which camera.  I started with a DSLR (Conon 500D in my case). Yes DSLRs have limitations but, and this was important to me, you can use them for general daytime photography. I simply found it hard to justify spending a large sum of money when I was starting out on this on a camera I could only use for a limited application. Given our weather of late that case is still as strong as ever. There are some  fantastic astro software applications available for DSLR use and with Cannon  "Backyard EOS" is amongst the best.  I would say go with DSLR first, and if you get the bug you can work up to a dedicated astro camera while honing your processing skills with the DSLR. 

Jim

[saac]

3 minutes ago, andrew s said:

Hi @saac your scope is looking good. I  donhave a spare  Thorlabs 50x25mm echelle grating you can have if your current spectrograph is not up to the job. Mind you you will need to build the rest of it. 😛

Regards Andrew 

Andrew you do realise I would need to make it out of plywood and it would be oversized and overweight by a factor of 10 at least.    I'm really hopeful for the spectroscope coming together , Paul Gerlach's Low Spec  design is brilliant.  I know you work a lot on that area and I really find it fascinating; it offers a potential to come close to some real science in our astronomy.  I did some earlier work with the Star Analyser grating about 2 years ago (DSLR video of Vega using the grating) .  I used RSpec and managed to generate the curve but stopped short at properly calibrating the thing.  I desperately need to retire so that I can spend more time on stuff that really matters  

Jim 

[JonCarleton]

8 hours ago, Ships and Stars said:

An interesting link to that!

https://madmax.fandom.com/wiki/Gyrocopter

Gerry Goodwin!

Yep...that's him.  And yes, I flew fixed-wings for 40 years, but almost exclusively gyros now.  They really aren't Ultralights...by US standards, anyway, and the most popular machines are usually 2 place these days.  They are as safe as any aircraft and safer than some.  A lot of people died in the 1970s-1990s trying to self-train and modifying designs making them unstable.  That gave them a bad reputation.  Most of that has gone away over the past 20 years.

Edited April 25, 2020 by JonCarleton

[JonCarleton]

3 hours ago, saac said:

Jon, back to part of your original question regarding which camera.  I started with a DSLR (Conon 500D in my case). Yes DSLRs have limitations but, and this was important to me, you can use them for general daytime photography. I simply found it hard to justify spending a large sum of money when I was starting out on this on a camera I could only use for a limited application. Given our weather of late that case is still as strong as ever. There are some  fantastic astro software applications available for DSLR use and with Cannon  "Backyard EOS" is amongst the best.  I would say go with DSLR first, and if you get the bug you can work up to a dedicated astro camera while honing your processing skills with the DSLR. 

Jim

Thanks, Jim.  I have a couple of dedicated cameras I have been using and was wondering if it was worth dusting off the Olympus for FOV reasons.

[Ships and Stars]

10 hours ago, JonCarleton said:

Yep...that's him.  And yes, I flew fixed-wings for 40 years, but almost exclusively gyros now.  They really aren't Ultralights...by US standards, anyway, and the most popular machines are usually 2 place these days.  They are as safe as any aircraft and safer than some.  A lot of people died in the 1970s-1990s trying to self-train and modifying designs making them unstable.  That gave them a bad reputation.  Most of that has gone away over the past 20 years.

I flew a few times in a powered parachute a neighbour had (rotax motor) - it was interesting in a breeze! I liked it on a calm day. One of his flying friends who lived down the road from me - and who I knew through work (mechanic) - apparently decided to use nyloc nuts instead of wired aviation spec fasteners, and he took the plunge one day from a high altitude when a brace or something folded. I never got the full details. That would have been around 1996 or 1997 in Southern Indiana. 

I still harboured thoughts about flying, but looked at gyrocopters after that.

Good luck with your camera set-up. Let us know if you find a good solution to increasing FOV!