Using a camera on a telescope with eyepieces or reducers?

[sploo]

As a reasonably competent "technical" photographer, but with no telescope experience, I'm trying to get started with a telescope and astrophotography. Since purchasing my first telescope there (of course) has been a healthy mixture of cloud, rain, rain, snow, cloud, gales, frogs, locust, and a bit more rain; so if any of the following questions seem dumb it's because I'm working from zero practical experience.

My scope is a Skywatcher 300P (1500mm focal length, f/5). I understand that when used for prime focus; I get just that - a 1500mm f/5 lens (though I haven't yet got my head around the size of the image circle that would be projected, or what field of view that would give when a full frame DSLR is mounted to the focuser).

Experimenting with https://astronomy.tools/calculators/field_of_view/ I see that M42 would be too "large" for this configuration, but good with a 0.5x reducer (yellow and green on the attached image).

I have a 1.25" adaptor to attach a Canon (EF mount) camera to the focuser, but there doesn't appear to be any way you'd mount an eye piece or reducer in the pipeline. How is this done, and what reducers would be recommended?

I see that for visual work, a long eye piece can result in an exit pupil (EP) that's too large for comfortable use (e.g. 40mm on an f/5 gives an 8mm EP). If using an eye piece for imaging, how do you calculate what would be seen by the sensor? E.g. with a full frame (36x24mm) sensor, and a 25mm eyepiece with a 40 degree field of view (5mm EP and 0.67 degree TFOV on my scope) what would I actually see? Or is the answer "whatever the astronomy.tools field of view calculator shows you"?

[happy-kat]

You'd probably love to read the book 'making every photon count' as it details all that sort of stuff. Your camera's sensor is behind what you'd get when using a camera.

[vlaiv]

That is really complicated topic.

Let's start by saying this:

if you are interested in proper astrophotography - what sort of mount are you using with 12" F/5 scope? That is a lot of telescope and you need really big mount for that.

This is actually not what I wanted to say - I wanted to say - use prime focus and forget about focal reducers. Forget about eyepiece projections and all of that.

Let's say that you want to use x0.5 focal reducer. It will try to take field that is twice as large as sensor and put it onto sensor. DSLR sensor is probably APS-C type sensor (worse if you actually have full frame sensor). That is about 27mm diagonal sensor. You want to put twice as much of field onto sensor so you need 54mm of clear aperture. 2" Accessories have 48mm of clear aperture at maximum (usually 47mm).

Your telescope needs to have flat and corrected field that is 54mm wide for that to work, and reducer needs to be matched to the telescope optics. This simply won't happen in real life. Telescope that you are using has Coma, and a lot of it. So you'll need coma corrector. Best you can get is x0.95 coma corrector in terms of field reduction. You can spend a lot of money and get x0.73 Coma corrector like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p9779_TS-Optics-NEWTON-Coma-Corrector-0-73x-Reducer---2--Connection.html

or this one:

https://www.teleskop-express.de/shop/product_info.php/info/p4685_ASA-2-inch-Newton-Coma-Corrector-and-0-73x-Reducer-for-Astrophotography.html

But pay attention to specs of such coma correctors:

They correct and illuminate less than APS-C sized sensor - up to about 11mm of radius or about 22mm diagonal. Last few mm will be vignetted and stars bloated.

If you want to record more of the sky at prime focus - do mosaic, that is best way to go about it with such scope.

Did I ask about the mount? Such tube is 27Kg in weight, so you need something like EQ8 class mount to hold it. With such scope, weight and focal length, I think you have other things to worry more than FOV of sensor at prime focus.

[sploo]

48 minutes ago, happy-kat said:

You'd probably love to read the book 'making every photon count' as it details all that sort of stuff. Your camera's sensor is behind what you'd get when using a camera.

Another plus for that book then (I've seen it recommended a few times). I'll have to get hold of a copy - thanks.

[happy-kat]

I'm also a fan of making the most of what you already have hence why I dabble with an none EQ mount

[dan_adi]

If you are just begging it would be recommended to do some reading about mounts, telescopes, focal reducers etc, before you spend money on expensive astro gear. Astrophotography has a steep learning curve. It is very rewarding and awesome when you learn how everything works .
For astrophoto the Precision of the mount is perhaps more important than the telescope. You will have faster success with shorter focal length telescopes because they demand less on the mount and autoguiding. Image scale is also very important for success. I think the recommended scale is between 1 and 2 arsec/ pixel. Below 1 arcsec/pixel you must have very clear skies and very good mount.

So first:

1. make sure your mount capacity is ok for your telescope

2. get an apropriate coma corrector/reducer

3. check your spacing requirements. All correctors have a required backfocus, so you will need additional spacings

4. get an appropriate camera as to get 1-2 arcsec per pixel scale

5. Off axis guiding is reccomended for long focal lengths. Another option is separate guidescope. A third option is on axis guiding but I’m not sure a newtonian has the 60 ish mm backfocus requirement. You have to know your telescope backfocus.

6.software. Some are free some are not. I like Prism because it is all in one solution.

7. When in doubt ask on the forum, there are a lot of helpful people here

[sploo]

37 minutes ago, vlaiv said:

That is really complicated topic.

Let's start by saying this:

if you are interested in proper astrophotography - what sort of mount are you using with 12" F/5 scope? That is a lot of telescope and you need really big mount for that.

This is actually not what I wanted to say - I wanted to say - use prime focus and forget about focal reducers. Forget about eyepiece projections and all of that.

Let's say that you want to use x0.5 focal reducer. It will try to take field that is twice as large as sensor and put it onto sensor. DSLR sensor is probably APS-C type sensor (worse if you actually have full frame sensor). That is about 27mm diagonal sensor. You want to put twice as much of field onto sensor so you need 54mm of clear aperture. 2" Accessories have 48mm of clear aperture at maximum (usually 47mm).

Your telescope needs to have flat and corrected field that is 54mm wide for that to work, and reducer needs to be matched to the telescope optics. This simply won't happen in real life. Telescope that you are using has Coma, and a lot of it. So you'll need coma corrector. Best you can get is x0.95 coma corrector in terms of field reduction. You can spend a lot of money and get x0.73 Coma corrector like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p9779_TS-Optics-NEWTON-Coma-Corrector-0-73x-Reducer---2--Connection.html

or this one:

https://www.teleskop-express.de/shop/product_info.php/info/p4685_ASA-2-inch-Newton-Coma-Corrector-and-0-73x-Reducer-for-Astrophotography.html

But pay attention to specs of such coma correctors:

They correct and illuminate less than APS-C sized sensor - up to about 11mm of radius or about 22mm diagonal. Last few mm will be vignetted and stars bloated.

If you want to record more of the sky at prime focus - do mosaic, that is best way to go about it with such scope.

Did I ask about the mount? Such tube is 27Kg in weight, so you need something like EQ8 class mount to hold it. With such scope, weight and focal length, I think you have other things to worry more than FOV of sensor at prime focus.

Lots more useful info - thanks.

To answer the mount question: you've already helped me with that in another thread (I'm looking at making a DIY, probably fork, mount). I have some ideas under way for making a friction drive for the required reduction.

I currently have both full frame and APS-C DLSR cameras; though I have concerns about hanging a large DSLR off the telescope focuser. I've also got some smaller sensor boards (e.g. Raspberry Pi camera), but I'd be open to buying one of the many astro camera modules such as ZWO models. I think I understand what you're saying re clear aperture and being able to project a field large enough to cover the sensor.

Ignoring the question of coma, is it possible then to take a long focal length telescope (e.g. this 1500mm scope) and project a wide field of view onto a relatively small area? E.g. project about 2 degrees (I think) for M42, but inside an image circle of about 15mm diameter; such that you could capture it with a ~11mmx11mm sensor like the ZWO ASI533MC?

[sploo]

13 minutes ago, happy-kat said:

I'm also a fan of making the most of what you already have hence why I dabble with an none EQ mount

Indeed. I found a thread (on this forum I think) of people doing DSO imaging using Alt-Az mounts with no tracking. Good fun to see that it's possible. I will experiment - as soon as it stops raining where I am!

[dan_adi]

And the 8. Big targets like nebulas require short focal length, small targets require long focal lengh like planets and galaxies. The thing is you will, with time, hopefully have 2 telescopes  for double fun. And if you get bored with imaging do not forget about spectroscopy and photometry - especially exoplanet transits. A lot to learn but you will enjoy the hobby for maaany years

[sploo]

6 minutes ago, dan_adi said:

If you are just begging it would be recommended to do some reading about mounts, telescopes, focal reducers etc, before you spend money on expensive astro gear. Astrophotography has a steep learning curve. It is very rewarding and awesome when you learn how everything works .
For astrophoto the Precision of the mount is perhaps more important than the telescope. You will have faster success with shorter focal length telescopes because they demand less on the mount and autoguiding. Image scale is also very important for success. I think the recommended scale is between 1 and 2 arsec/ pixel. Below 1 arcsec/pixel you must have very clear skies and very good mount.

So first:

1. make sure your mount capacity is ok for your telescope

2. get an apropriate coma corrector/reducer

3. check your spacing requirements. All correctors have a required backfocus, so you will need additional spacings

4. get an appropriate camera as to get 1-2 arcsec per pixel scale

5. Off axis guiding is reccomended for long focal lengths. Another option is separate guidescope. A third option is on axis guiding but I’m not sure a newtonian has the 60 ish mm backfocus requirement. You have to know your telescope backfocus.

6.software. Some are free some are not. I like Prism because it is all in one solution.

7. When in doubt ask on the forum, there are a lot of helpful people here

Thanks. I've got plenty of photography experience (including long exposure), and some astro (using a DSLR + camera lens + Star Adventurer). The 300P came up used for a very good price, so whilst I understand it's probably not the best first telescope for a noob, I wanted something significantly "better" than the lenses I have - as I can cook up anything from a 400mm f/5.6 to an 1100mm f/16 with my current lenses. I'll clearly need to look further info coma correctors.

[sploo]

6 minutes ago, dan_adi said:

And the 8. Big targets like nebulas require short focal length

Yea, dumb *ss here knows that now 😉

The stupid thing is that I've shot M42 with shorter focal length camera lenses, so there's no reason why I couldn't have worked out that 1500mm was a bit long beforehand! Still, I do hope to be able to see some of the planets at some point, and the one few brief moments (since buying the scope) that the moon appeared out of a cloud showed me that it can produce really impressive results (at least for visual work).

[dan_adi]

7 minutes ago, sploo said:

Indeed. I found a thread (on this forum I think) of people doing DSO imaging using Alt-Az mounts with no tracking. Good fun to see that it's possible. I will experiment - as soon as it stops raining where I am!

I think you mean with no autoguiding. Every mount has to track the object beeing photographed. Indeed there are alt az mounts that have direct drives and absolute encoders. Those are very expensive and hard to make as diy. Another thing is an alt az mount need derotator or an equatorial wedge for photography.

[alacant]

2 hours ago, sploo said:

My scope is a Skywatcher 300P

Hi

Phew. To get started in astro-photography with that telescope and stand a chance of getting results, I think you'd need to imitate one of these. OTOH, I'm almost certain @laser_jock99 may be able to help with alternative ideas.

Good to have others on board with big reflectors:)

Cheers and good luck.

Edited February 28, 2020 by alacant

[dan_adi]

Do not get frustrated about 1500 mm focal length as a noob. I started with a sct with 2438 focal length :))) a really bad idea, so I got a 980 mm focal length refractor and I was able to learn everything faster. Now I’m confident enough in my skills to put a 0.62x reducer on the SCT , an external focuser, lock the mirror and test that bad boy under the stars. Remember to have patience, it takes time to master all this stuff. I congratulate you for the willingness to make your mount as a diy project. If for some reason you change your mind, in my opinion, the best bang for buck is the Mesu 200 mount. It has a huge carrying capacity, friction drive and guides very well. 

[vlaiv]

28 minutes ago, sploo said:

Ignoring the question of coma, is it possible then to take a long focal length telescope (e.g. this 1500mm scope) and project a wide field of view onto a relatively small area? E.g. project about 2 degrees (I think) for M42, but inside an image circle of about 15mm diameter; such that you could capture it with a ~11mmx11mm sensor like the ZWO ASI533MC?

I remember discussion about making mount for this scope and my recommendation to go for equatorial mount.

Now if you want to project certain surface angle onto certain sensor size - you are limited by focal length.

Either that or really doing a mosaic. Thing with mosaics is that you won't waste time (people often thing it is slower to do mosaic because you need to shoot multiple panels) - if you bin your image accordingly.

Let's say that you want to use 533 sensor and put 2 degrees on it. 533 sensor has 3000 pixels and 2 degrees has 60*60*2 = 7200 arc seconds. We end up with 7200 / 3000 = 2.4"/px. Pixel size is 3.75um and required focal length in that case is about 322mm. This is about x4 or so times less than 1500mm, so you would need something like 0.25 reducer - again not going to work.

But you can use APS-C sensor and do mosaic. Do 3x3 mosaic and you will have something like 2.5 degrees x 1.5 degrees of FOV with your scope and APS-C sensor.

[happy-kat]

34 minutes ago, dan_adi said:

Another thing is an alt az mount need derotator or an equatorial wedge for photography.

An altaz tracking mount can be used to image with staying within the capabilities of the mount without the use of a wedge or derotator.

[dan_adi]

1 minute ago, happy-kat said:

An altaz tracking mount can be used to image with staying within the capabilities of the mount without the use of a wedge or derotator.

Thanks for the correction. How long can you expose without a wedge or a derotator? Always good to learn something new

[barkis]

You could do some nice wide field Astro photography with your dslr and various lenses. 
Work towards a acquiring a manageable fast Newtonian or an Apo. chromatic  refractor if you can raise the 
the money.  A good equatorial mount can be bought second hand too, not everything needs to be new.
Getting what we desire  is not easy, but a lot of satisfaction can be got doing it all slowly, and with forethought 
This AP can murder your Bank Account, so proceed with caution🙂.
Ron.

[happy-kat]

2 hours ago, dan_adi said:

Thanks for the correction. How long can you expose without a wedge or a derotator? Always good to learn something new

This a very long thread link here using such a mount, there are limitations to work within, and these can be exasperated depending on where you point in the sky and focal length.  It is also possible to work out how long by aperture/altitude/latitude and direction or just try and see, can be anything from 10 to 45 seconds or so. But not ideal and I would not say a deliberate direction to take unless the gear is already owned so nothing lost just time.

Edited February 28, 2020 by happy-kat

[sploo]

On 28/02/2020 at 16:45, dan_adi said:

I think you mean with no autoguiding. Every mount has to track the object beeing photographed. Indeed there are alt az mounts that have direct drives and absolute encoders. Those are very expensive and hard to make as diy. Another thing is an alt az mount need derotator or an equatorial wedge for photography.

I'm pretty certain the thread was Alt-Az mounts with no tracking or guiding at all (not even goto allowed). Lots of examples of multiple 1s exposures! Surprisingly good results TBH.

[sploo]

On 28/02/2020 at 16:59, alacant said:

Hi

Phew. To get started in astro-photography with that telescope and stand a chance of getting results, I think you'd need to imitate one of these. OTOH, I'm almost certain @laser_jock99 may be able to help with alternative ideas.

Good to have others on board with big reflectors:)

Cheers and good luck.

Cheap at 1/10th of the price 😉

I've just scored a 100:1 harmonic drive from a seller I've used before (already got one of the same type, but it's currently running my CNC machine's 4th axis). I've got some stock to have a go at creating a friction drive for the final stage (10:1) to give me 1000:1 for building a mount for use with a stepper. The torque specs of the mechanical parts look to be more than sufficient for the telescope mass; I guess the challenge is going to be getting enough stiffness in a DIY mount.

[sploo]

On 28/02/2020 at 17:26, dan_adi said:

Thanks for the correction. How long can you expose without a wedge or a derotator? Always good to learn something new

In 35mm (full frame) DSLR photography terms, the "500 rule" is often used; that is, an exposure time no longer than 500s divided by your focal length. I.e. for very wideangle shots (16mm lens) you can expose for 500/16=31 seconds before star trailing becomes an issue. A 1500mm telescope used for prime focus would, I assume, only allow 500/1500=1/3s exposures.

If using a Canon APS-C body then it's 1.6x shorter (as you get a field of view on the crop sensor that's approximately the same as a lens with a 1.6x longer focal length).

[sploo]

On 28/02/2020 at 17:49, barkis said:

You could do some nice wide field Astro photography with your dslr and various lenses. 
Work towards a acquiring a manageable fast Newtonian or an Apo. chromatic  refractor if you can raise the 
the money.  A good equatorial mount can be bought second hand too, not everything needs to be new.
Getting what we desire  is not easy, but a lot of satisfaction can be got doing it all slowly, and with forethought 
This AP can murder your Bank Account, so proceed with caution🙂.
Ron.

Indeed (bank account killing).

I've taken a few shots using lenses and a Star Adventurer mount (anything from about 16mm up to 400mm). Obviously the 1500mm telescope is a different ball game.

[vlaiv]

4 minutes ago, sploo said:

In 35mm (full frame) DSLR photography terms, the "500 rule" is often used; that is, an exposure time no longer than 500s divided by your focal length. I.e. for very wideangle shots (16mm lens) you can expose for 500/16=31 seconds before star trailing becomes an issue. A 1500mm telescope used for prime focus would, I assume, only allow 500/1500=1/3s exposures.

If using a Canon APS-C body then it's 1.6x shorter (as you get a field of view on the crop sensor that's approximately the same as a lens with a 1.6x longer focal length).

No, that 500 rule is gross approximation. If you want to do proper calculation for this case - use sampling rate and sidereal rate and see what you get from the two.

Let's say that you are using modern DSLR sensor that has pixel size of about 4um or so. You are using 1500mm focal length. This gives you 0.55"/px, or each pixel is 0.55 arc seconds "long". Sidereal rate is about 15"/s. This means that in a single second - star will streak across 30 or so pixels. If you have FWHM of about 3" - that is 6 pixels, and you can start to see star elongation at 20% larger major radius. This means that elongation can be at most about 1.2px (6 pixels * 20% = 1.2px). That is 0.55 * 1.2 = 0.66 arc seconds.

With sidereal rate of 15"/s - this will give you 44ms exposure not to exceed 0.66 arc seconds elongation.

As you see - this figure is about x7.5 less than you estimated using 500 rule.

[sploo]

1 hour ago, vlaiv said:

No, that 500 rule is gross approximation. If you want to do proper calculation for this case - use sampling rate and sidereal rate and see what you get from the two.

Let's say that you are using modern DSLR sensor that has pixel size of about 4um or so. You are using 1500mm focal length. This gives you 0.55"/px, or each pixel is 0.55 arc seconds "long". Sidereal rate is about 15"/s. This means that in a single second - star will streak across 30 or so pixels. If you have FWHM of about 3" - that is 6 pixels, and you can start to see star elongation at 20% larger major radius. This means that elongation can be at most about 1.2px (6 pixels * 20% = 1.2px). That is 0.55 * 1.2 = 0.66 arc seconds.

With sidereal rate of 15"/s - this will give you 44ms exposure not to exceed 0.66 arc seconds elongation.

As you see - this figure is about x7.5 less than you estimated using 500 rule.

Absolutely; I suspect it's an old rule of thumb that dates from the film days. Certainly even with a wide angle lens I wouldn't be exposing as long as 30s without tracking. I've found that 10s at 16mm focal length is just about acceptable from the point of view of a wide milky way image, but you can still see trailing if zooming into the image.

(I probably should have made that clearer in my earlier post - there is the "500 rule", but I personally wouldn't stretch exposures that long with a modern high res sensor)

Edited March 2, 2020 by sploo