Tracking With 400mm Lens

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hatflyer
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Tracking With 400mm Lens

#1

Post by hatflyer »


I have the Sky Watcher AZ-GTi mount in EQ mode. Can it track with a 400mm lens on a crop sensor without auto-guiding? I've had success at 300mm.

Thanks.
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Richard South Africa
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Re: Tracking With 400mm Lens

#2

Post by Richard »


No the mount is alt/Az mount so unless you have a wedge on the mount will default to Alt/Az thats what mine did and then it looks like its working but you get max 1min exposure
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hatflyer
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Re: Tracking With 400mm Lens

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Post by hatflyer »


Richard wrote: Sun Sep 04, 2022 3:35 pm No the mount is alt/Az mount so unless you have a wedge on the mount will default to Alt/Az thats what mine did and then it looks like its working but you get max 1min exposure
As stated, the mount is in EQ mode. I'd be happy with 60 sec exposures at 400mm.
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JayTee United States of America
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Re: Tracking With 400mm Lens

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Post by JayTee »


EQ mode is not enough, the mount itself also needs to be polar aligned. If the right ascension (RA)axis is not parallel with the Earth's axis, accurate tracking becomes very difficult.

Back to your original question, in most instances like this where you ask for an opinion on whether a certain piece of equipment will do what you want it to do, typically the best approach is to experiment with it yourself and if you can't find a solution then come to us and see if we can think of something else.

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∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
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hatflyer
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Re: Tracking With 400mm Lens

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Post by hatflyer »


JayTee wrote: Sun Sep 04, 2022 7:26 pm EQ mode is not enough, the mount itself also needs to be polar aligned. If the right ascension (RA)axis is not parallel with the Earth's axis, accurate tracking becomes very difficult.

Back to your original question, in most instances like this where you ask for an opinion on whether a certain piece of equipment will do what you want it to do, typically the best approach is to experiment with it yourself and if you can't find a solution then come to us and see if we can think of something else.

Cheers
Yes, of course it would be polar aligned. I am asking as I have not bought the 400mm lens yet. I'd like to know what to expect.
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JayTee United States of America
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Re: Tracking With 400mm Lens

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Post by JayTee »


If you had a high success rate with the images you obtained with the 300 mm and it did not give you too much difficulty, then I would consider the 400 mm. On the other hand, if you threw out most of your images from the 300 mm for one problem or another, then I would give serious thought to not getting the 400 mm. That's the best advice I can give you not knowing the accuracy of your mount or how well you are polar aligned. Hopefully, someone who owns that mount will come along to give you a better answer

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∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
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∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
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sdbodin United States of America
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Re: Tracking With 400mm Lens

#7

Post by sdbodin »


Maybe the real question is, does 400mm really make a difference in the final image vs. 300mm? Personally, I use lens/scope combinations that approximately double one to another. My combinations are 28mm, 50mm, 135mm, 250mm, 480mm, 1000mm, 2500mm, 4000mm.

Of course, maybe the reason is one of superior quality, faster optics, or the like, and that is a valid reason too, but that small a gain in focal length would result in a minor difference in image scale.

As for your mount, if 300 works, so will 400. Unless the weight is vastly different and over taxes the mount,

Just thoughts, good luck,
Steve
Scopes; Meade 16 LX200, AT80LE, plus bunch just sitting around gathering dust
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OzEclipse Australia
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Re: Tracking With 400mm Lens

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Post by OzEclipse »


I agree with Steve.
I use the best of my lens collection, those that give the very best pinpoint stars. I don't worry too much about focal length. A superb quality or very well guided 300mm lens can be cropped to the field of a 400mm lens.

14mm f2.8 [Samyang ED]
50mm f1.7. [Pentax M]
135mm f2. [Rokinon ED]
300mm f4 [Pentax EDIF]
600mm f7.5 [ED80]
1280mm f6.4 [Vixen VC200L]

Unguided tracked exposures

Now you are in my wheelhouse. All my work is unguided.
You will need to polar align within 1-2 mins of the pole so that you'l get minimal dec drift during a few minutes exposure.
The periodic error for small mounts like this is around 40-50 arc seconds. If you want single pixel perfect tracking during the full periodic error oscillation eg a 5-10min exposure, then you need to stay below a focal length of about 30mm.

However, you should experiment with shorter exposures. As the length of your exposure gets shorter, you sample less of the variation.
Periodic-error-lucky-imaging.jpg
.

Each sub won't have as much trail and you can even discard the worst ones where the periodic error was imparting the greatest error
You can also shrink your image, nobody needs to view at 100%. None of us can tell you the precise answer to your question. Trial and (periodic) error is the way forward.

Set up
Don't overload the mount. Keep as much of the weight as close to the polar axis as possible. The picture below shows my Star Adventurer carrying a 2kg load and the resulting picture. I don't have a collar for this lens so I have the 2kg load centred exactly on the polar axis and I don't have it hanging out one side with a counterweight hanging out the other.
.
SWSA-1.jpg
.
SW-SA-test run-M8-M20 copy.jpg
.


The same night, I took a second photo taken with the same camera and lens but with the load slightly off balance. Big trails! Unfortunately, I don't have the picture to display. I deleted it.

Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
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Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
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