Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Fieldrotation on Alt/Az - max achievable exposure time?


graemlourens

Recommended Posts

Hi guys.

My setup:

Nexstar Evolution 6" on standard delivered alt/az-mount (tracked)

Nikon D610

Primefocus

Location: Warsaw

After reading an interesting article about fieldrotation: http://calgary.rasc.ca/field_rotation.htm

i have started playing around with what is the max exposure time i can get on alt/az in different directions of the sky.

My targets are DSO.

Obviously imaging with alt/az towards north will give you fieldrotation even after 20 seconds.

In other directions i can go around 1 minute before i get fieldrotation.

Whats your experience with Fieldrotation, how long can you guys go until you get startrails?

Before you ask why i got a Alt/Az if i'm into Photography, i only realized that much later, that my true interest lies in photography. So i'm playing with a thought to sell the alt/az/mount and going EQ.

But i must say, that for beginners like myself Alt/AZ is MUCH easier, so maybe i should hang on to it for a while longer, until i have more experience.

I guess the question is, will alt/az put too much problems in my way when trying to get long exposures, or can i create decent images of Nebula also with Alt/Az and only switch to EQ-Mount at a later stage?

Grateful for any thoughts

Graem

Link to comment
Share on other sites

Hi Dave.

Yes, processing is not a problem, DSS handles it beautifully.

Its more the question of if i will achieve exposure times that are required for not brightest objects, or if with alt-az i am limited to for example max 1-2 minutes.

For examle your Meade have you got that on an alt-az? if yes, what exposure times are you doing without having problem with fieldrotation?

Kind regards

Graem

Link to comment
Share on other sites

My Meades on a wedge, but they used to do a derotater that compensated for the Alt Az mount, quite expensive and I don't know if anyone still does them.

Only ever imaged the Moon when I had it originally so can't be much help with exposure times.

Dave

Link to comment
Share on other sites

Looking at the last 2 images in the link it seems to depend on both where you are aiming and where on the earth you are. I was not expecting to see that effectively due south was as bad as it came out.

Another factor is you have a 6" SCT so a focal length of 1500mm the image taken were on a 270mm lens so through your scope things should show more rotation.

Throw in that your scope if f/10 and the one used is said to be f/5.6, I suspect you will have problems.

Link to comment
Share on other sites

Hey ronin.

Yes, thats true, for 'us' with telescopes Fieldrotation will be even worse.

(Btw i have the focal reducer to 6.3 so not soooo bad as f10)

But its very noticeable the difference between aiming north or southwest for example, you can quadruple your exposure times without Fieldrotation if you know that fact.

What i'm trying to find out with my question is if i should sell my (nearly new) mount/scope (and loose probably a lot of money) and go for smth else with EQ-mount, or if I will be perfectly able to achieve very good results of not too difficult nebula (Heart-N, Andromeda, Orion-N and so on)

If somebody here sais he could achieve 2-4 Minute exposures without Fieldrotation, then i know that its possible with my scope, if not, then i will be fighting forever to get enough photons :)

Regards, Graem

Link to comment
Share on other sites

Hey Merlin.

Thank you so much for that Link, that is a very good representation of the limits of alt/az.

Sadly my location is probably the worst (around 50 latitude), but based on the calculations, i should be able to get (theoretically) up to 2 minute exposures directly east or west.

My understanding is that with 2 minute exposures (and making a lot of them) i could reach some decent images?

Obviously 10 x 20 minutes is better than 100 x 2 minutes, but maybe i should just do with what i've got currently.

Nobody else had experience with AP on alt/az with long exposures?

Regards, Graem

Link to comment
Share on other sites

Nobody else had experience with AP on alt/az with long exposures?

I just use 30secs and do lots of them.  The only down-side with short exposures is read noise, so if you have a camera with very low read noise you will do better (the Nikon 610 seems to have 3e-, which isn't bad - about half my Canon 1000D).

NIgelM

Link to comment
Share on other sites

Might be worth lowering the North facing tripod leg to counteract rotation, not a mathematician so don't know if this would help, mind it doesn't fall over !!!  (the management take no responsibility for this suggestion)  :evil:

Dave

Link to comment
Share on other sites

Hi Graem,

I've been dabbling with "long" exposures on an alt-az mount operating at nearly f/12. It's hard work! Good that you have the focal reducer to make life a little easier.

I thought I had noticed some variation in the exposure length I could get away with, depending on where in the sky I was looking, but hadn't really come around to thinking about how / why that was!

30 - 50 seconds is about it for me unfortunately. Any longer and the sausages start appearing at the edges of the image and the central object starts smudging. Currently scrimping and saving and hoping to get an heq5 sometime in the new year.

It's all good experience though. Gives a good chance to learn the ropes in the meantime, but boy I do look forward to a time when I don't have to scrap 70% of my subs because of trailing! I've not seen anyone pulling 2 - 4 mins off alt-az before, but best of luck to you and look forward to seeing your images!

Cheers, Derek

Link to comment
Share on other sites

Thx Derek.

I think i'll stay on the alt/az for a few Years, as currently in any case i don't have the luxury of plenty evening 'free'

So mostly i'm bound to our backyard, and lightpollution is so heavy, that i guess 2-4 mins would in any case ruin my photographs.

So i'll focus on the easy targets where i can get decent images with 30-60 sec, and then upgrade at a later stage when it makes sense.

I think i've made all the beginners mistakes you can, but its given me a massive insight in the past months, that also lead me to buying binoculars, and boy you can see much with 12 by 70's...

Thx for all your answers! I really appreciate SGL!

Link to comment
Share on other sites

Nexstar Evolution 6" on standard delivered alt/az-mount (tracked)

Given,

that Celestron did market (maybe still do ) an eq wedge for the 6SE (models up to the 6SE, athough I think they did for the 8SE but later withrdrew it ?)

And considering,

the much advertised increased stblility of the tripods/mounts of the Evolutions over the similar SEs,

as well as the advertised improvement of the Evolution worm over the SE spur gears,

then,

you might consider a wedge mount ?

I dont think C. do one yet for the Evos but you might fabricate one whilst you wait for them.

I am told that the software already contains the equatorial star alignment routines ???

just a few thoughts for your consideration, ymmv :)

good luck.

PS the field rotation is an angle and thus  is independent the magnification of the 'scope or wide-field camera,

so said one adviser to me here on SGL,

I cant find the posts of my correspondence in that ancient thread, SGL seems to be limiting my look-back search :(

Link to comment
Share on other sites

Hi Ptarmigan.

Thx for your thoughts.

True, a wedge would be a solution, but in my research you can not reach the same amount of stability, especially when adding a heaving DSLR as with a EQ-Mount, as you don't have the counter weight balance.

And as buying a Wedge is not very cheap, i tend to the solution of getting an EQ-Mount later when i am more advanced in time.

But like i said, i guess i'll stay on ALT/AZ until i've mastered all the objects that can be caught with 30-sec exposure stacks (and thats going to take me a few years in any case :) )and then switch to EQ at a later stage.

Kind regards, Graem

Link to comment
Share on other sites

If your handy with the tools you can make a serviceable wedge out of Plywood  Ok it will have issues  but will be better than working in Alt/AZ...

I made a wooden prototype  wedge before making it in Ally - good job I did as I got the angles the wrong way round... :(  I then made an integrated pier and wedge for the obs which had +/-1 degree of preset adjustment in it  but the job only needed doing once so it kept the design simpler...

I will try and dig out the threads later today...

Link to comment
Share on other sites

Do you need the option in the handset to tell the mount it's in EQ mode ?

Dave

This from Celestron, bit misleading

Superior tracking fit for astroimaging

If you’re interested in astroimaging, the 6-inch NexStar Evolution is an excellent, affordable way to get started. It’s the only fork-mounted telescope in its price range that offers worm gears, along with improved motors. NexStar Evolution’s tracking accuracy is so impressive, you can capture images of deep-sky objects like the Orion Nebula by simply attaching your DSLR camera. 

Link to comment
Share on other sites

Here's a sample drawing for a 6SE wedge in pdf format...

https://dl.dropboxusercontent.com/u/8191904/Projects/Pier%20%26%20Wedge%20Drawings/6se%20wedge%20v3.pdf

A Picture of  the scrap ally wedge I made in the garage at home...

PICT1464.JPGPICT1462.JPG

And a picture of the test setup of my combined CPC800 pier and wedge before the obs got built around it...

Pier%26Wedge.jpg

Peter...

Link to comment
Share on other sites

Wow i like the setup billy! :) Would love to have that kind of garden & space...

Sadly i don't really have access to the machines and tools i would need to be able to build this.

But i'll keep it in mind that maybe if i stumble upon somebody that could make that for me rather simply that i would get it for some kind of humble price :)

Regards, Graem

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.