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Alt Az imaging?

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#1 JimP

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Posted 12 June 2018 - 07:57 AM

Do some of you use an all azimuth mount to take short images of the deep sky without an equatorial Mount with motor drive?
I have just purchased a Takahashi FSQ106N and intend to use it on an AP Mark 1 Mount but, there are times when I could use the scope at a darker site if I did not have so much equipment to carry. Using a good alt azimuth mount with no motor drive would decrease the burden greatly.

I am brand new to this, only being used to doing high-resolution images of the moon and planets and any thoughts would be appreciated. I know I’m going to have to download deep sky stacker and I have purchased a light pollution filter. I will have to figure out dark frames, bias frames, etc. and how they get added to the batch before I can get started.

Thanks for any help.

#2 bunyon

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Posted 12 June 2018 - 08:07 AM

There are two separate technical issues. 

 

An alt-az mount can track but, in doing so, the field rotates around the centerpoint. Your idea for using very short exposures, so long as the field of view is not large, can work if you align and stack the subs in software that accounts for field rotation.  It's not great, but it can be done. Exposures here would be on the order of tens of seconds or less.

 

The second issue is the lack of tracking. Without tracking, at the focal length you're contemplating, you will be limited to extremely short exposures - a second, give or take a second. In practice, this simply can't work. 

 

You have found the big problem with moving from visual observing to imaging. There is a LOT more equipment needed and each little bit and bob is extremely important. 


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#3 JimP

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Posted 12 June 2018 - 09:02 AM

Thank you. I was hoping I would not need to track if exposures were ~30 seconds. Then I could stack those images realizing the East and West edges would have to be cropped because of the earth’s rotation. An alt-ax Mount with motor drive was never a consideration.
I have an AP Mach1 in an Observatory where I can do equatorially guided images. But, there is no way for me to carry the Mach1 plus tripod plus counterweights plus battery plus telescope on foot several hundred yards to a darker location. Looks like I will just have to pass on the second location.
I do understand the need for special equipment as I have been imaging the moon and planets for almost 20 years. Wide field imaging is a new adventure and the learning curve is a part of the challenge.

Edited by JimP, 12 June 2018 - 09:16 AM.


#4 Alen K

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Posted 13 June 2018 - 08:27 AM

This may be a radical suggestion but a Pentax DSLR just might allow you to do what you want. With a stationary camera and lens the Pentax Astrotracer feature can compensate for the earth's rotation. It works best with lenses of moderate focal length but a few people have tried it at 500mm and beyond; see here and here, for example. That said, the best results I have seen are with a 300mm telephoto. The longer the focal length the shorter the exposure, of course, but 30 seconds may be doable at 530mm (your FL). I hear tell that the Astrotracer feature can be finicky to calibrate but that persistence is rewarded. 

 

Full disclosure, I do have a K-3II (APSC) but haven't had the opportunity yet to test out Astrotracer. So I am not speaking from experience, only based on the research I did before buying it. BTW, also going by the research the H-alpha sensitivity of recent Pentax cameras is pretty decent, so you may not need to modify it (personal call).



#5 t_image

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Posted 14 June 2018 - 05:35 AM

 An alt-ax Mount with motor drive was never a consideration.
Using a good alt azimuth mount with no motor drive would decrease the burden greatly.

FWIW,

When people mention "Deep Space,"

the concept of "not tracking" is contradictory, unless you presume to use manual dials to track with.

To say "good alt azimuth mount" and at the same time "no motor drive" also seems a little contradictory for your stated purpose.

You presume it will lighten the load, but it won't help any burden of taken good DSO images.......

 

I use an alt-az mount all the time,

and it is very light-weight,

and it has a motor drive that tracks,

and I use a small blackweb power brick to power it.

I find it also helps when I use my modified camera with an h-alpha filter at higher ISOs to get short exposures if I'm going for Nebula.

I have most success with 200mm f/2.8, 85mm f/1.4 since they give me subs very fast....

 

Picture a Venn Diagram of ranges of benefit given your particular camera, scope, intended target, sky brightness, weather conditions, area of sky, ability to track and desired image quality which will limit:

your result.

cost of equipment,

weight of gear and portability.

 

Consider the trade-off triangle of:

  • time/effort/portability
  • money
  • quality result

You can't have all three, most often not even two.

What is interesting is that there are "hacks" that allow one to improve a single condition by offsetting another.

Spend more money, spend more time or effort, etc.

I've found the most success by experimenting and looking at the result from fellow club members on location.

 

To your particular question Jim,

I've found the great benefit of a darker sky is that it allows you to go deeper with longer exposures,

which is contradictory of trying to keep things short.......

That's when the value of an EQ mount is best to get tracking as long as possible for exposures.......

 

 

Cheers!



#6 JimP

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Posted 14 June 2018 - 08:11 AM

Thank you very much!

#7 Slabs1960

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Posted 25 June 2018 - 11:08 AM

Can be done. But it takes some effort. You need to take a LOT of short exposures (100 +). Depending on the FOV, 5 - 20 sec. The closer your target is to the zenith, the longer your exposures can be. Darker skies preferred. Be prepared to on stack about 20% of images taken, hence the lot of exposures. Lots of people have made a success of it.


Edited by Slabs1960, 25 June 2018 - 11:10 AM.


#8 ToxMan

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Posted 25 June 2018 - 06:31 PM

Plan for the burdens and don't cut yourself short. You are trading off for frustrations.




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