Newbie with a dob

[lividturkey]

Good afternoon All,

I'll qualify the following post with this, I'm entirely new to telescopes, but not to photography. My intention is get delve into the world of astrophotography. Based on the research I've done, few would argue that nothing can beat a newt dob in terms of aperture for price. My research has also suggested that the mount is the most important piece in the AP equation. Apparently dob mounts are not suitable for AP due to them not articulating on the equatorial plane. Will a dob with a large enough aperture, and a go-to tracking system like the Orion XX G series, or XT G series be able to produce quality images at a low ISO, and a modest shutter speed without suffering from image rotation? Furthermore, would the combination of image rotation, and shutter speed experienced with a larger aperture go-to dob, allow enough time for bracketing in white balance, and shutter speed before too much image rotation occurs in order to produce overlays in Photoshop? Sorry for the run on sentence. Your input is appreciated. Thanks!

James

[John]

Hi James and welcome to the forum !

I think you need to do quite a bit more research into suitable equipment. A dobsonian is a visual telescope and not generally suitable for imaging. The mounts just don't track accurately enough.

I would strongly recommend that you get the book "Making Every Photon Count" by Steve Richards who is a member of this forum. Here is a link to it:

http://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

Astrophotography has many aspects but the most important component is the mount rather than the telescope as you have found it. The book will explain why

Most deep sky imagers start out with a small scope on a large equatorial mount or sometimes just the camera on the mount.

 

 

 

[lividturkey]

Hi John,

Thank you for the reply. I've got a list of amateur astronomy/astrophotography books in the queue on amazon. I'll certainly add Mr. Richard's book to it. Forgive me, but  I was hoping for a quick answer to what I'm able to deduce why Dobs aren't suitable for AP. My first post was a little convoluted with crap so let me simplify the question. Would a dob, with a go-to mount, and at least 12" aperture be suitable for longer exposures? If so, is it suitable for only single exposures, or does it provide enough time to bracket white balance for multiple exposure overlays? I have some degree of experience with glass, so I'd like to start with something modestly capable. Again thanks for the reply.

James

[John]

I can understand why you might think that large aperture can overcome the problem of the alt-azimuth drives not being accurate enough to allow long exposures but my (limited) understanding of deep sky imaging is that aperture is just not important.

The accuracy of the drive systems seems to be the highest priority and many very fine images are created on here using very small aperture scopes (70-100mm) being very accurately driven on carefully setup and callibrated equatorial mounts.

The reality seems to be that the sort of scopes that deliver good views of deep sky objects (ie: larger aperture ones with basic or no drive systems) are really the opposite of what tends to be used to embark on deep sky imaging.

I'll stop there as I've exhausted my limited knowledge of imaging but I've been on this forum for a decade now so I've picked up a few bits and pieces along the way and have watched imagers at work at star parties. It's a very different aspect of the hobby of astronomy to the visual observing that I do

 

[Moonshane]

What are you looking to image? The requirements for solar system objects and e.g. nebulae are completely different.

The size of the objects also matters as does the focal ratio of the scope. Assuming deep space / faint objects you are likely to be using very long exposures (several minutes each) with high iso to allow adequate data to be captured.  then the processing starts.  Heed the advice to read much more before you waste your money and time with the wrong gear.  Not being negative,  just want to try and encourage you to make an informed decision.

 

 

 

 

 

 

 

[Moonshane]

P.s.

I am visual only too but a very keen photographer.  I quickly realised that astro imaging is not for me as I don't have the time to get the quality of result I'd demand of myself.

 

[lividturkey]

John, and Moonshane,

Thank you for your input. To answer your question Moonshane I hope to capture mostly colorful nebulae, super nova, and galaxies. Im sure Ill go for our bodies in our own system but the colors if distant bodies interests me most.

 

[Moonshane]

In that case a small good quality refractor maybe 80mm with a large equatorial mount is the received wisdom.

[PeterCPC]

Hello and welcome. Good advice given above. If you are looking to image DSOs then a short focal length refractor/reflector on an EQ mount is what you need. The Dob will be no good for DSOs.

Peter

[gnomus]

10 hours ago, lividturkey said:

Good afternoon All,

I'll qualify the following post with this, I'm entirely new to telescopes, but not to photography. My intention is get delve into the world of astrophotography. Based on the research I've done, few would argue that nothing can beat a newt dob in terms of aperture for price. My research has also suggested that the mount is the most important piece in the AP equation. Apparently dob mounts are not suitable for AP due to them not articulating on the equatorial plane. Will a dob with a large enough aperture, and a go-to tracking system like the Orion XX G series, or XT G series be able to produce quality images at a low ISO, and a modest shutter speed without suffering from image rotation? Furthermore, would the combination of image rotation, and shutter speed experienced with a larger aperture go-to dob, allow enough time for bracketing in white balance, and shutter speed before too much image rotation occurs in order to produce overlays in Photoshop? Sorry for the run on sentence. Your input is appreciated. Thanks!

James

I had a fair bit of experience of photography before getting into this AP malarkey.  If anything, I think it made it more difficult for me because my 'assumptions' were well ingrained and difficult to shake off.  I'm not sure how you feel that White Balance bracketing will help.  As to shutter speed, this is an entirely unprocessed 600 second image taken through a moderately fast f/4.9 telescope - a small refractor with a 71mm aperture:

The signal we are looking for is buried deep in the dark stuff.  As soon as we try to drag it out of there we are going to start running into problems with noise.  Therefore, we take multiple exposures to try to improve our signal to noise ratio so that when we do drag it out of there it looks 'acceptable'.  A number of the photos you see above combined to give me (after some hours of processing) - https://stargazerslounge.com/topic/266430-rosette-rework/?do=findComment&comment=2917344

I make no great claims for the image, I'm simply trying to make a point about one difference between AP and regular photography - that is in regular photography teh signal is 'there' and just needs some tweaking whereas in AP one has to go mining deeply for it.

Take a look at some of the real top quality imagers on here - people like Olly Penrice, Sara Wager, Barry Wilson (and apologies to anyone I've missed out).  Go onto Astrobin and look up guys like Roberto Colombari.  Or take a look at Fabian Neyer's stuff http://www.starpointing.com/  Look at the equipment they are using.  None of these chaps (or chapesses) are trying to do DSO imaging with a Dobsonian.   

I would second the suggestion already made to get hold of 'Making Every Photon Count' before embarking on a spending spree - if you do get into AP it will be a 'spree' and then some.....

Good luck

[ollypenrice]

Why not a tracking Dob for imaging?

1) Alt Az introduces field rotation. You cannot match 10 x 100 seconds with 1000 x 1 second. You need long subs to image deep faint targets. If someone says this is not true I would ask to see their images. You need to generate a smaller number of instances of read noise and you need to get the signal above the noise floor. Faint signal takes time to get above it.

2) The tracking needs to be accurate. How accurate depends on the number of arcseconds per pixel you intend to capture, but if you go for larger aperture you inventably get longer focal length. Longer focal length means fewer arcseconds per pixel which means more accurate tracking needed. It creates a vicious circle. In reality the way to break that circle is to go the opposite way. Use a shorter focal length and the fastest F ratio you can. If you do that you don't need such accurate tracking. With inadequate tracking a short fast camera lens beats a big telescope. (By a mile).

2b) A moderate Dob (say 10 inches and F4) with a typical DSLR might be imaging at 1.2 arcsecs PP. It's widely felt that the average tracking error needs to ba about half that, so 0.6 arcseconds. That is what you can expect to achieve on an autoguided equatorial mount. Without autoguiding or encoder guiding a premium (6K dollar plus) EQ mount might run at 4x that error. A more budget EQ mount might well default to 60x that error until guided. A tracking Dob will be way, way worse than that. And there would be no proprietory way to autguide it. Make that a 20 inch Dob at F4 and you simply make the problem worse, not better.

3) In the camera world many of your assumptions will be formed by the fact that F ratio is varied by changing aperture without affecting focal length. In telescopic photography F ratio is controlled by using reducers to shorten FL or by going for more aperture - but again this influences FL.

If there were a simple way to beat the need for autguided EQ mounts we would all be doing it. If all we needed was to swallow the Starizona website hook, line and sinker, we'd all be using F2 Hyperstars. But we know it's not that simple. We don't spend thousands on premium mounts because we like spending money. Well, I certainly don't!! (It gets me into trouble...)

Olly

[happy-kat]

Dob as a term refers to the mount a dobsonian. On that mount is often a reflector aka newtonian telescope but it could also be a refractor or a maksotuv. 

[wuthton]

If a Dob is what you've got then you may as well use it. 

The great nebula in Orion is still quite high and a is a good starting point. You should be able to get something that'll impress your friends and family. Try 5-20sec exposures, iso800-1600 and see what works best. 

It's often seen that having a good knowledge of daytime photography can be a hindrance to astrophotography.