Exposure times for a GoTo but Unguided mount

[timfuller]

Just wondering how long I may be able to push my exposure times with a GoTo upgrade.

I currently have a Bresser NT203/1000 reflector on a Mon2 Manual mount and using a Canon EOS1000D.

Using the "500" rule as a guide (500/(1000*1.5)) I got to 0.33sec. This was about right in my testing as anything longer started to stretch the stars.

Just wondering how much I may be able to push this with the GoTo upgrade that I have just ordered from Bresser ? I appreciate this will depend very much on good alignment (2 or 3 star)

Autoguiding will come at some point down the line but this is the first step.

Thanks

PS - Apologies for the cloudy skies due to my recent spending on Astro gear :-)

[MARS1960]

I just posted my first image unguided, i did a quick polar align, fairly accurately and a one start align using my GT5 mount.

I used 60sec exposures on my 600mm frac at prime with my 70D, as you can see the stars are just starting to get a little misshapen.

[dph1nm]

Well, with an HEQ5 and the same camera and similar focal length I get about 30secs. The goto-alignment is irrelevant. The killer is periodic error in the mount, and maybe polar alignment (but you can always improve that).

NigelM

[Stub Mandrel]

I am up to 120s unguided using my EQ3-2 mount on an EQ5 tripod. Light pollution, good balance and effort put into polar alignment are my limiting factors so far, not trailing.

 

[wornish]

I have a Skywatcher Star Adventurer mount on a good tripod. I find spending time on getting a really good polar alignment allows me to go unguided up to 40secs using my M4/3rds DSLR with a 300mm equivalent lens and the stars stay round.  

I am going to try and push it to 60secs but I think guidance will be my next step.

[MARS1960]

5 minutes ago, wornish said:

I have a Skywatcher Star Adventurer mount on a good tripod. I find spending time on getting a really good polar alignment allows me to go unguided up to 40secs using my M4/3rds DSLR with a 300mm equivalent lens and the stars stay round.  

I am going to try and push it to 60secs but I think guidance will be my next step.

I did a lot of research on those as i nearly bought one, from what i gathered i would think 60s at 300mm should be achievable unguided.

[timfuller]

Thanks everyone for your feedback.

I will start by concentrating on getting a good polar alignment.

I plan on testing my way up the exposure scale and see where I get to.

[Pompey Monkey]

9 minutes ago, timfuller said:

Thanks everyone for your feedback.

I will start by concentrating on getting a good polar alignment.

I plan on testing my way up the exposure scale and see where I get to.

Bravo, sir. You will learn and achieve much with this attitude.  

[alacant]

Hi 

I have a Bresser exos2 goto. They seem to come in a sorry state from China. It needed to be dismantled and cleaned before it would do an honest 30s. There was hardly any lubrication, the backlash adjustment was way out and the grub screw on the drive gear was seized. Oh, and the goto really thrashes the tiny little motors; you have to get the balance just right. I get the feeling that it's best use the motors just for tracking. To their credit, Bresser are great in getting it working properly by sending replacement bits and a replacement RA motor when I sent them the symptoms. Good luck and clear skies.

[ollypenrice]

The question you ask cannot be answered other than, as Paul said, by experiment.

Firstly the deviation of mount relative to sky depends on where in the sky you are looking.

Secondly every mount varies in accuracy.

Thirdly pixel scale determines whether or not a given tracking error will remain sub pixel or not. (Focal length is only a shorthand for pixel scale here.)

Fourthly you may be satisfied with round stars. However, round stars are not a test of accuracy, despite it being widely assumed that they are. Random errors in both axes produce round stars and also a loss of resolution which can be considerable.

Just experiment and see what gives you what you will accept.

Olly

[Stub Mandrel]

Keep in mind Olly's warnings, but this is a single 120 second sub at 400mm with my (unguided) setup of the Crab nebula/Zeta Tauri area. All I've done is convert a RAW to JPg and correct colour and exposure a bit.

I won't pretend it's perfect, but its defects may help you manage your expectations.

I'm looking at all the subs very critically, and I reckon about 50% of them are OK and about 50% have slightly extended or triangular stars.

[Juel]

With my 600mm scope I currently get 60s exposures mounted on HEQ5 Synscan, unguided. As my polaralignment gets better, I've improved from 40s exposures, and I'm confident I will be able to get a good deal more.

[timfuller]

On 2/19/2016 at 19:03, ollypenrice said:

The question you ask cannot be answered other than, as Paul said, by experiment.

Firstly the deviation of mount relative to sky depends on where in the sky you are looking. Understand this

Secondly every mount varies in accuracy. Understood and realise that my mount can only go so far

Thirdly pixel scale determines whether or not a given tracking error will remain sub pixel or not. (Focal length is only a shorthand for pixel scale here.) Excuse the beginner in me but this is all very new to me. Do you mean that the size/scale of the target (star) relative to the pixel size will determine whether it moves out of one pixel and into another ?

Fourthly you may be satisfied with round stars. However, round stars are not a test of accuracy, despite it being widely assumed that they are. Random errors in both axes produce round stars and also a loss of resolution which can be considerable. Assumption on my part and we all know the saying about assumption..... :-) If so then what is the best way to determine accuracy or is this down to subjective and individual acceptance of the images you are producing ?

Just experiment and see what gives you what you will accept.

Olly

Olly - Thanks for you response. I understand that I have a massive amount to learn but it is places like this forum that make a big difference... :-)

On 2/19/2016 at 19:49, Stub Mandrel said:

Keep in mind Olly's warnings, but this is a single 120 second sub at 400mm with my (unguided) setup of the Crab nebula/Zeta Tauri area. All I've done is convert a RAW to JPg and correct colour and exposure a bit.

I won't pretend it's perfect, but its defects may help you manage your expectations.

I'm looking at all the subs very critically, and I reckon about 50% of them are OK and about 50% have slightly extended or triangular stars.

Thanks Neil - This is a great guide and certainly something to aim for.. :-)

[russp]

This was the final image of my first deep sky image and was a combination of 120sec unguided exposures (AZEQ6-GT) so depending on your mount it is possible to get a 'reasonable' longer exposure unguided 

 

[timfuller]

Thanks Russ - I think it is definitely going to be a process of trial and error... Prob more error than anything else to start with.... :-)

[russp]

1 minute ago, timfuller said:

Thanks Russ - I think it is definitely going to be a process of trial and error... Prob more error than anything else to start with.... :-)

I have lots of errors to be sure - including almost whole nights imaging the wrong bit of sky!!!! It's just perseverance and learning from mistakes

 

 

[dph1nm]

Please remember the original poster has a setup which gives 1"/pixel. That is pretty challenging unguided for sub 2k mounts (and maybe for some which cost more that that!).

NigelM