Beginner looking for some help (DSO photography)

[ThriftyAnvil]

Hi all,

New to astrophotography and have recently started imaging the Orion nebula (see attached photo)

My setup is Cannon 450D and a Celestron C8 super with byers drive for tracking.

The main issue I'm facing is "vertical drift" in my images/tracking which limits my exposure times to 30s at the very most and i cant for the life of me figure out which part of my alignment is causing this.

Any help or advice would be much appreciated

[PeterCPC]

As I understand it the byers drive will severely limit your exposure time. The main issue in your image to me is that it is out of focus.

[ThriftyAnvil]

12 minutes ago, PeterCPC said:

As I understand it the byers drive will severely limit your exposure time. The main issue in your image to me is that it is out of focus.

I have been struggling a bit with the focusing, theres a lot of movement when I touch the dial, and the cameras LCD screen doesnt give me the best view

I will keep practicing when the weather permits

I appreciate the reply

[vlaiv]

Hi and welcome to SGL.

I don't think that focus is the issue here.

Image looks just right for the equipment you are using. Long focal length means that you'll be over sampled if you don't bin your data and use of regular SCT telescope is bound to impart some spherical aberration on the stars when imaging (it is very hard to hit exact design distance between two mirrors and SCTs introduce spherical aberration even if perfectly figured when you change distance between mirrors - which you do when focusing). Ideally, you'd want to put your sensor where the eyepiece is when observing for best performance - but no one does it as it would mean adding several extension tubes (think diagonal + EP away from OTA) - and even then you are not 100% sure you are spherical aberration free.

1 hour ago, ThriftyAnvil said:

The main issue I'm facing is "vertical drift" in my images/tracking which limits my exposure times to 30s at the very most and i cant for the life of me figure out which part of my alignment is causing this.

When you say "vertical" drift - what exactly do you mean? Vertical with respect to RA/DEC coordinate system or vertical with respect to the image above?

DEC drift (one in direction of DEC) - happens due to poor polar alignment.

RA drift happens due to periodic error of the drive - or in another words, because of gears in mount drive being slightly out of shape (a bit eggy and not perfectly round - this is because of manufacturing tolerances) - sometimes drive lags behind earth's rotation and sometimes it leads. This creates star elongation or perceived drift.

There could be other reasons - like improper tracking rate or similar - but if it's related to mount's tracking - it is RA, if its due to polar alignment - then it is in DEC.

In above image RA direction is left-right and DEC direction is "up/down". So if you have vertical drift with respect to the image - it is due to polar alignment.

 

[ThriftyAnvil]

9 hours ago, vlaiv said:

....When you say "vertical" drift - what exactly do you mean? Vertical with respect to RA/DEC coordinate system or vertical with respect to the image above?

DEC drift (one in direction of DEC) - happens due to poor polar alignment....

Thank you for the welcome, and the detailed reply.

Based on your description I am having DEC drift, the target is moving from top to bottom along the center of the frame.

My tripod is level and my wedge is set at 46~ degrees which i believe is correct for my location, so am I right in thinking that it is my North South alignment that is wrong?

[vlaiv]

1 hour ago, ThriftyAnvil said:

My tripod is level and my wedge is set at 46~ degrees which i believe is correct for my location, so am I right in thinking that it is my North South alignment that is wrong?

You need to perform proper polar alignment. Not sure how it's done on southern hemisphere, but even slight deviation from SCP (south celestial pole) can have significant impact on drift.

It does not matter if you missed polar alignment in altitude or azimuth - both result in DEC drift

You can use this calculator to calculate drift rate based on your error:

http://celestialwonders.com/tools/driftRateCalc.html

You'll need to estimate your error - but if you manually set just altitude and eyeballed south pole - then you can be even a degree or two off.

For such error - you can drift up to 15 arc seconds in half a minute at declination 0 (higher declination will have lower drift)

That is significant drift over the course of half a minute and can cause significant trailing.

Even PA error of half a degree will have 4 arc seconds of drift in half a minute.

If you are not guiding - you want to be as accurate with your polar alignment as possible. Most people that guide are happy if they get within 10 arc minutes of polar alignment error - then drift is small enough that its easily corrected by guiding. You should aim to get polar alignment error at least as low as that if at all possible.

[vlaiv]

Ok, so I looked it up - and there is handy star that will put you in vicinity of SCP - close enough for good tracking.

Here is quote from wiki article:

Quote

An even closer star BQ Octantis of magnitude +6.9 lies 10' from the South Pole as of 2016. Although not visible to the naked eye, it is easily visible in most polar 'scopes. (It will lie its closest to the South Pole, namely 9', in the year 2027.

Not sure what your mount is like - but there are two things that you need to do to get good enough polar alignment.

First is to make sure your polar scope is properly aligned (once you get polar scope onto any star - best use one close to pole as those don't move much - then if you rotate mount in RA manually - star should stay in center of the polar scope - if it's not align polar scope first - look up polar scope alignment / adjustment).

Then simply put cross of polar scope on BO Octantis and you are done (using alt / az controls at base of your EQ mount).

[ThriftyAnvil]

16 minutes ago, vlaiv said:

Ok, so I looked it up - and there is handy star that will put you in vicinity of SCP - close enough for good tracking.

Here is quote from wiki article:

Not sure what your mount is like - but there are two things that you need to do to get good enough polar alignment.

First is to make sure your polar scope is properly aligned (once you get polar scope onto any star - best use one close to pole as those don't move much - then if you rotate mount in RA manually - star should stay in center of the polar scope - if it's not align polar scope first - look up polar scope alignment / adjustment).

Then simply put cross of polar scope on BO Octantis and you are done (using alt / az controls at base of your EQ mount).

Excellent! A lot of good info there for me to try out some different things!

Now I just need to hope for some clear skies!

[rickwayne]

You can check your alignment with nothing more than your camera and the ability to slew slowly back and forth in RA. Basically you start an exposure, deliberately make a star trail by slewing during the exposure, and then slew back. If your PA is perfect the "out" and "back" trails coincide and you see only a single line. If not, then some DEC drift occurs -- remember this happens at right angles to the RA axis -- so the lines form a V, with the point of the V where you reversed direction.

Here's a detailed recipe, see Drift Alignment by Robert Vice (aka DARV).

[ThriftyAnvil]

5 hours ago, rickwayne said:

You can check your alignment with nothing more than your camera and the ability to slew slowly back and forth in RA. Basically you start an exposure, deliberately make a star trail by slewing during the exposure, and then slew back. If your PA is perfect the "out" and "back" trails coincide and you see only a single line. If not, then some DEC drift occurs -- remember this happens at right angles to the RA axis -- so the lines form a V, with the point of the V where you reversed direction.

Here's a detailed recipe, see Drift Alignment by Robert Vice (aka DARV).

Oh yeah, that seems "easy" enough

Ill probably spend the next clear night playing with alignment more than any imaging

Thanks for the tip

[michael8554]

Hi Thrifty

That's a great image given the unguided long focal length setup.

No coma, and just a little elongation due to PAE.

Without a PC screen to look at an enlarged LiveView display, focusing will be difficult.  

A f/6.3 Focal Reducer from Meade, Celestron, Antares etc will help, spaced at 105mm Back Focus from the camera chip

This adapter gives the correct FR Back Focus with a Canon DSLR:

https://www.firstlightoptics.com/adapters/celestron-sct-t-adapter.html

For the focal length "sweetspot" vlaiv refers to, there is probably a figure published somewhere.

Otherwise, if you can be bothered, adjust until you get a 2000mm Focal Length when the image is Plate Solved.

Michael

 

 

 

[ThriftyAnvil]

5 hours ago, michael8554 said:

....This adapter gives the correct FR Back Focus with a Canon DSLR:

https://www.firstlightoptics.com/adapters/celestron-sct-t-adapter.html

Thanks for the support Michael,

That adapter is the same one I am using currently, I have had a brief look into field reducers but haven't gotten to the point that I'm 100% sure which one I need etc

I get a bit impatient and want to take the photos more than do the research, but i guess learning is half the fun!