more magnification with newton + dslr?

[Gerhard]

Hi all!

I was just wondering about something, and would much appreciate to hear some opinions /suggestions...

Premise: I'm just starting, very green, and very curious... 

With my f5 Newton and DSLR I have been getting some very satisfying results so far, apart from the expected coma and tracking issues.

I was wondering if it was at all possible to have more magnification with such a setup, to be able to catch also the smaller DSOs...

I have been told that those smaller objects are difficult in that one needs longer focals, and thus longer exposures, and thus better guiding.

So, just slapping on a barlow between the tube and the dslr would only frustrate things, apart from focussing issues... Can anyone confirm this?

This is obviously a long term issue for me, as at the moment, I'm quite happy with the setup I have, and the lack of funds for now puts any ideas I might have on hold. 

But for the future it would be very interesting to know what options there are for really far DSOs...

I would definitely first of all start with setting up auto guide (not just for far DSO), so guide scope, guide cam and PC + connections and software. That's a big bite.

Afterwards it would probably be: buying a purpose scope for deep DSOs, long focal, big aperture. SC? RC? A good APO?

Opins?

thanks all! 

Gerhard.

[Leveye]

Get your polar alignment as perfect as you can. See what your absolute limit of exposure time is before trailing starts. Take exposures just before that limit and stack them. Crop off the edges afterwards in post. Worked for me in the early days. Most important thing?...Have fun!

[Tinker1947]

For imagine faint DSO's the first thing you will need is a mount that will track for may be 30 minutes something like one in this link.....

one like this will do the job.... http://www.mesu-optics.nl/mesu200_en.html

[Gerhard]

For imagine faint DSO's the first thing you will need is a mount that will track for may be 30 minutes something like one in this link.....

one like this will do the job.... http://www.mesu-optics.nl/mesu200_en.html

uhmmm... are you saying those faint DSOs are only possible using 4000€ mounts?

I know from the members of my club that the HEQ5 I use can do 10, 15 minute exposures, guided. I don't know if they do more...

[ollypenrice]

Hi all!

I was just wondering about something, and would much appreciate to hear some opinions /suggestions...

Premise: I'm just starting, very green, and very curious... 

With my f5 Newton and DSLR I have been getting some very satisfying results so far, apart from the expected coma and tracking issues.

I was wondering if it was at all possible to have more magnification with such a setup, to be able to catch also the smaller DSOs...

I have been told that those smaller objects are difficult in that one needs longer focals, and thus longer exposures, and thus better guiding.

Afterwards it would probably be: buying a purpose scope for deep DSOs, long focal, big aperture. SC? RC? A good APO?

Opins?

thanks all! 

Gerhard.

Magnification is not a term used in imaging. The useful term is image scale. To make a given object larger on the chip you simply need a longer focal length. Longer focal lengths do not need longer exposures if the F ratio is the same in both cases. The exposure length depends on the focal ratio. Longer focal lengths only need longer exposures if the F ratio is slower (higher number.) Exposure time goes as the square of the F ratio so, unfortunately, if you put in a 2x Barlow you will need 4x the exposure. That's why it is almost never done in DS imaging.

Longer focal lengths also need more accurate tracking so when you add a Barlow you need 4x longer exposure and 2x better tracking at the same time. Not an appealing prospect!

Olly

Edit Tinker1947, on 22 Apr 2015 - 5:11 PM, said:

For imagine faint DSO's the first thing you will need is a mount that will track for may be 30 minutes something like one in this link.....

one like this will do the job.... http://www.mesu-opti...mesu200_en.html

uhmmm... are you saying those faint DSOs are only possible using 4000€ mounts?

I know from the members of my club that the HEQ5 I use can do 10, 15 minute exposures, guided. I don't know if they do more...

 

How long a mount can guide for successfully depends on the focal length it is carrying (or more accurately the image scale of the system in arcseconds per pixel.) At short focal lengths budget mounts guide fine for 30 minutes. With good polar alignment if you can guide for the 8 minutes of one turn of the worm wheel then you can guide for pretty well as long as you like.

 

However, if you move to a scope with a long focal length (say 2 metres or more) then if the scope has a fast F ratio it will be big and need a big mount. And if you want to get reliable tracking it will have to be accurate. Some clever folks do manage to do this on the EQ6 but it will never be easy. I do it on a Mesu as in the link and it is very easy indeed. So easy that this mount has never made a significant guiding error in over three years.

[Gerhard]

Magnification is not a term used in imaging. The useful term is image scale. To make a given object larger on the chip you simply need a longer focal length. Longer focal lengths do not need longer exposures if the F ratio is the same in both cases. The exposure length depends on the focal ratio. Longer focal lengths only need longer exposures if the F ratio is slower (higher number.) Exposure time goes as the square of the F ratio so, unfortunately, if you put in a 2x Barlow you will need 4x the exposure. That's why it is almost never done in DS imaging.

Longer focal lengths also need more accurate tracking so when you add a Barlow you need 4x longer exposure and 2x better tracking at the same time. Not an appealing prospect!

Olly

This is exactly what they told me...

So, hypothetically if I acquired a 1000/200 tube, instead of my 750/150, I could obtain bigger images without changing the exposures? 

Just a theoretical exercise... :-)

"With good polar alignment if you can guide for the 8 minutes of one turn of the worm wheel then you can guide for pretty well as long as you like."

not sure what you mean with this... (green, as stated... :-) )

I assume you're referring to the gears in the mount, but not quite following how this translates in practice...

[ollypenrice]

This is exactly what they told me...

So, hypothetically if I acquired a 1000/200 tube, instead of my 750/150, I could obtain bigger images without changing the exposures? 

Just a theoretical exercise... :-)

"With good polar alignment if you can guide for the 8 minutes of one turn of the worm wheel then you can guide for pretty well as long as you like."

not sure what you mean with this... (green, as stated... :-) )

I assume you're referring to the gears in the mount, but not quite following how this translates in practice...

1000/200 and 750/150 are both F5 so they have equivalent photographic speed, yes.

A driven mount usually has a worm gear driving a wheel. In the case of the Skywatcher mounts I believe one revolution of the wheel takes 8 minutes. Most mounts are similar in this respect. Because of machining errors there will be similar cyclical errors which repeat themselves during each revolution so they are called 'periodic errors' because during the period of one revolution they will be fairly consistent with the errors of the next revolution, etc. This means that if you can guide those errors out on the first 8 min. revolution you should be able to do so on the second and third, etc. However, if you are not well polar aligned the image will slowly rotate so this places an upper limit on exposure time.

Many mounts have 'periodic error correction' which allows you to record their errors during one revolution and have the mount computer do its best to anticipate and correct them during each revolution. It can be quite helpful but some manufacturers imply that it is a complete fix and this is very misleading. It isn't. It's a help.

One other thing about long focal lengths and any one shot colour camera: you are stuck with the pixel size that is native to the hardware. At long FLs modern DSLR pixels are too small to work well. They don't get enough light and they try to resolve details which are smaller than the atmosphere will allow, so they are inefficient in that they don't resolve more detail and they don't get enough light. A monochrome camera working behind filters not attached to the chip can be set to read in 2x2 or 3x3 binning. Here 2x2 pixels (or 3x3) are read as one larger pixel. They don't lose real resolution when, if read singly, the atmosphere would blur their data but they do gain in sensitivity to light. Those imaging at long focal lengths either use cameras with big pixels or they bin their small pixels 2x2.

Counting and worshipping magapixels is a game for the daytime. In astronomy you are often better off with pixels like house bricks. I just posted this today from a FL of 980mm and huge 9 micron pixels which would be totally obsolete on a DSLR but are just the ticket for DS at mid to long FL. http://stargazerslounge.com/topic/242489-triplet-tail-and-fuzzifest/

Olly

[Gerhard]

Thanks a lot for that, Olly! 

Correct me if I'm wrong, but using a guide scope should avoid those problems, no? Or are there other things, besides PA, that I can do to minimize these errors, and thus try for longer exposures?

[ollypenrice]

Thanks a lot for that, Olly! 

Correct me if I'm wrong, but using a guide scope should avoid those problems, no? Or are there other things, besides PA, that I can do to minimize these errors, and thus try for longer exposures?

A guidescope will correct periodic errors and, to some extent, polar misalignment but eventually, over long exposures, field rotation will show if polar misalignment is bad. Be aware, though, that autoguiders don't 'just work.' The longer the focal length the more accurately the guided mount has to work. Budget mounts start to struggle at longer focal lengths, even guided. If they didn't nobody would buy expensive ones!

Olly

[Gerhard]

A guidescope will correct periodic errors and, to some extent, polar misalignment but eventually, over long exposures, field rotation will show if polar misalignment is bad. Be aware, though, that autoguiders don't 'just work.' The longer the focal length the more accurately the guided mount has to work. Budget mounts start to struggle at longer focal lengths, even guided. If they didn't nobody would buy expensive ones!

Olly

I concede my HEQ5 is not top of the range, far from it, but I would want to hope that for my current newton it would suffice with autoguiding... Even though it's not new...

I don't see myself buying a €4000 mount... not now, and not in the future... I mean, I like astrophotography, and am very keen to get better, but there's a limit... and it's called "my budget"... :-)

I will complete the autoguide setup in time, so laptop, guide scope, guide cam, and various connections and softwares.

Then maybe a better scope, but as long as the HEQ5 does not fail completely, it will have to do.... That's also one of the reasons I bought that, instead of a lesser mount. So I would be "covered", mount-wise, for, if not always, a considerable amount of years...

If I ever wanted to have real upscale photos, I'd rent one of those setups in Spain... 

http://www.astrocamp.es/