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AVX Mount, right for me?

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#1 Robert T

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Posted 25 February 2014 - 09:43 PM

Hi Folks,

I'm interested in getting a mount for astrophotography with my Nikon D7000. My main interest are the brighter deep sky objects.

I'm strongly considering an Explore Scientific ED102 as an OTA (listed at 11 lbs) with a Celestron AVX mount and I have two main questions:

1) With an autoguider, is an AVX sufficient to track with this OTA for 5 mins subs? 10 mins subs? Will I be struggling to get those exposures, or will this be well within the capabilities of the mount?

2) This might be slightly off-topic for the forum, but should I be looking at a smaller/lighter OTA to get started? Mainly I'm trying to understand the tradeoffs between an 80mm f/6 vs a 102mm f/7.7 in terms of ease of getting good subs and targets (I have yet to play around in stellarium to see how this affects my desired targets).

Thanks for any help you might be able to give. Thanks!

#2 Jeff2011

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Posted 25 February 2014 - 11:04 PM

Robert,

I have been imaging with my AVX for about a year now. I have guided for 5 minute exposures and could go 10 if my light pollution permitted. A shorter focal length scope will be easier to learn with regardless the mount. I would recommend you start with an 80ED or smaller (shorter focal length).

The main issue with the AVX mount is lack of a bearing on the declination axis. This causes problems when guiding. If you were to post your question on the beginners and intermediate imaging forum the response would be to get an Orion Sirius are even better an Atlas mount. Conversely there are a lot of guys that image with an AVX and it is quite workable with a short focal length telescope. The other conventional wisdom is to get a used CG5 as the AVX is a newer version of this mount and you could save $300 or more. Personally I am not too sure I agree that they are equivalent in tracking performance but since I don't have a CG5 to compare against I can't say for sure.

Knowing what I know now I would probably go with the Sirius mount if I could start over, but the AVX has been a good learning mount for me. You should also plan ahead, so if you want to get a bigger scope one day then the Atlas would be a good choice.

#3 mmalik

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Posted 26 February 2014 - 12:56 AM

Robert, have you considered ZEQ...? Regards

#4 Robert T

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Posted 26 February 2014 - 01:57 AM

I would recommend you start with an 80ED or smaller (shorter focal length).


Thanks for the reply! Can you expand on this recommendation a bit more? I've read quite a few threads with a similar recommendation, and I'm strongly considering it. My primary concern is that the shorter focal length won't get me the smaller targets I'd like to go after. For example, with my camera and an f/6 80mm scope, I'd get about 29 pixels per arc minute, meaning that m51 would be about 321 pixels across in an image, compared to 507px with the ED102. Of course that isn't worth it if they're blurry pixels. To distill that into a question, does a mount like the AVX encounter significant problems when you jump from ~480mm to ~770mm [and the weight that goes with it]?

Conversely there are a lot of guys that image with an AVX and it is quite workable with a short focal length telescope.


I've heard the term "short focal length" used in an number of threads, what's considered a short focal length?


Knowing what I know now I would probably go with the Sirius mount if I could start over, but the AVX has been a good learning mount for me. You should also plan ahead, so if you want to get a bigger scope one day then the Atlas would be a good choice.


Yeah, this is always the difficult question. I don't at the moment crave anything bigger than the 102, *but* if the AVX doesn't cut it and something like an Atlas is the price of entry, then I'll just have to account for that (which is what I'm trying to suss out in this thread). I'm tempted to say that if I can get 5-6 years happily imaging with a 102mm+AVX then I'm happy to buy a heftier mount with my bigger OTA if that's what I want in 2020 (yikes!).

#5 Robert T

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Posted 26 February 2014 - 02:00 AM

Robert, have you considered ZEQ...? Regards


I have not, for lack of awareness. I'll read up on it, but I'd pose the same questions about its capability of handling a load from a ED102mm refractor. Thanks!

#6 Falcon-

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Posted 26 February 2014 - 02:15 AM

1) With an autoguider, is an AVX sufficient to track with this OTA for 5 mins subs? 10 mins subs? Will I be struggling to get those exposures, or will this be well within the capabilities of the mount?


Having used a CG5 and a ZEQ25 for ap I very much lean towards the ZEQ25 or Sirius rather then an AVX. In fact in my case I purchased the ZEQ25 specifically to replace the CG5 as my portable astrophotography rig.

The AVX is probably the easiest to use mount (vs ZEQ25 and Sirius) but that DEC guiding issue really rears it's head when you get to the 10-minute exposures you are contemplating - or at least I know it certainly was for me with my CG5.

Between the ZEQ25 and Sirius (or Atlas) it becomes a question of capacity. The Orion mounts are physically bigger and heavier, and as a result can take a larger scope. The Atlas could mount up an 8" newt for example. The ZEQ25 is easily portable and easy to polar align when setting up and (so far for me) has performed very well, but has a lower maximum payload. The two scopes you mention, the 80 f/6 and 102 f/7.7 should be well within the capabilities of all three of the mounts.

2) This might be slightly off-topic for the forum, but should I be looking at a smaller/lighter OTA to get started? Mainly I'm trying to understand the tradeoffs between an 80mm f/6 vs a 102mm f/7.7 in terms of ease of getting good subs and targets (I have yet to play around in stellarium to see how this affects my desired targets).


The f/6 vs f/7.7 aspect can actually make quite a bit of difference, especially for unguided imaging, as it will allow for shorter exposures.

In addition the 80mm at f/6 would have a focal length of ~480mm vs the 102mm f/7.7 would have a focal length of ~785mm. The shorter focal length will be more forgiving of any tracking errors that may exists (again, especially useful when unguided) and even be less sensitive to seeing induced blur.

So that is two points in favour of the 80mm f/6 as regards making it easier for yourself when starting AP. On the other hand it is worth checking out the properties of the two scopes. Is the 102mm optically beter then the 80 or is the 80 better then the 102? If they are both the same check out the availability (and effectiveness) of any Correctors or Reducer/correctors for both scopes. If the 102mm has a good well matched reducer/corrector you may be able to image with it at f/6 or faster and in that case, price being no object, would work out quite well.

For whatever it is worth I used this page here to calculate out the spacial resolution and FOV for each scope with your camera:
- 80 f/6 refractor: 2.054 arc-seconds per pixel, 2° 49‘ 1" by 1° 51‘ 43"
- 102 f/7.7 refractor: 1.256 arc-seconds per pixel, 1° 43‘ 23" by 1° 8‘ 20"


If I may suggest though perhaps you should look at what camera lenses you have (if any). A good 200mm or 300mm camera lens often makes an excellent "Widefield Astrograph Refractor Telescope". Wider lenses in the 100mm to 28mm range are also very useful for targeting groups of objects or milky way shots or constellation shots. I do a lot of my own imaging with such camera lenses.

#7 Robert T

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Posted 26 February 2014 - 02:39 AM

If I may suggest though perhaps you should look at what camera lenses you have (if any). A good 200mm or 300mm camera lens often makes an excellent "Widefield Astrograph Refractor Telescope". Wider lenses in the 100mm to 28mm range are also very useful for targeting groups of objects or milky way shots or constellation shots. I do a lot of my own imaging with such camera lenses.


Thanks for the reply! I do have a reasonable 300mm lens, which is part of why I'm looking at the 102mm f/7.7 because the difference in FOV is definitely pronounced enough to be worth the money.

A couple points for others in the thread:

* I do plan on auto-guiding
* Portability is a consideration since I will be imaging from a dark site that isn't at home. I think the Atlas might be pushing it for me.

#8 jakecru

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Posted 26 February 2014 - 03:25 AM

I don't know if you are interested, but OPT telescopes currently has a sale on the celestron CGEM. I called OPT and verified it is brand new, not a used or open box mount. I don't know how long the sale lasts, but here is the link if you are interested!

CGEM at OPT

#9 SpaceWatcher

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Posted 26 February 2014 - 09:27 AM

Hi all,

I too am someone considering an AVX mount and am interested in AP. I have seen a few references to DEC guiding issues caused by the AVX not having a DEC bearing. What is the nature of the difficulty this causes?

#10 Falcon-

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Posted 26 February 2014 - 01:18 PM

Hi all,

I too am someone considering an AVX mount and am interested in AP. I have seen a few references to DEC guiding issues caused by the AVX not having a DEC bearing. What is the nature of the difficulty this causes?


It is called "sticktion" - basically the metal-on-metal (with grease) bearing surfaces slightly resist movement (a relatively high static friction). In normal usage this does not matter but when autoguiding the mount is making *very* small corrective movements and those are often too small to overcome the sticktion inherent in the DEC axis. As a result error in one direction will build up and as the guider tries repeatedly to tell the mount to correct it uses longer and longer guide pulses until sticktion is suddenly overcome. At that point the mount usually overshoots (sometimes by a lot) and the mount then has to start correcting the overcorrection.

Depending on balance, polar alignment, etc, I have on my CG5 seen this be a relatively tame issue that crops up once every couple of frames and at other times seen it create a saw-tooth like DEC guide graph pattern. This means that I was very often rejecting 25% to 50% (on a bad night) of my captured frames at 912mm focal length. When I got the AT6RC with it's slightly longer focal length I could not reliably guide the 10+ minute exposures I required. This was enough of a problem for me that I basically gave up and purchased a replacement mount - first an old used CI-700 for use at home, and then eventually the ZEQ25 for portable AP.

Now if you have a *perfect* polar alignment you should not need to guide in DEC at all, but unless you have a permanent pier mount that is not reasonable to expect. There are also things you can do to mitigate the issue such as setting guiding to one direction only (depending on the direction of DEC drift that night), etc. Keeping the weight low may also help and as other people have had success at longer focal lengths some copies may have smoother DEC axis' then others.

The RA axis does not have this problem as it uses a ball bearing and so has hugely lower static friction.


The iOptron ZEQ25, Orion Sirius (also called Skywatcher HEQ5), Orion Atlas (also called Skywatcher NEQ6 or EQ6), Celestron CGEM, iOptron iEQ30 and iEQ45 all use ball bearings in both DEC and RA so do not have that sticktion issue. Each of those mounts is of course more expensive then the AVX is (or CG5 was) and each has it's own quirks, but in my opinion from my experience when astrophotography is your primary goal it is worth the extra cost.

#11 rmollise

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Posted 26 February 2014 - 01:22 PM

I guided the CG5, which is not quite the mount the VX is successfully many, many times, at focal lengths up to 1300mm and subs up to 5-minutes (my sky precluded longer). The secrets to guiding? The declination axis isn't much of a problem. Or never was for me. What made all the difference in the world? Keeping the mount east heavy (barely) in RA at all times. ;)

#12 Robert T

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Posted 26 February 2014 - 03:25 PM

So let me sum up what I'm hearing and make sure it's what you guys are saying

1) It's reasonable to expect an AVX + autoguider will track at 770mm for ~5 min subs

2) 10 min subs will be more difficult because of the aforementioned issue with dec tracking

3) Other mounts like CGEM, Sirius and Atlas and ZEQ25GT don't have the issue in #2 and will have better success, ease of getting good data.

#13 SpaceWatcher

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Posted 26 February 2014 - 04:25 PM

Thank you for the comprehensive answer, Sean! I will examine the other mounts in Robert's list in the above post. Interesting that the mfr. markets the AVX as an AP mount, yet it has this shortcoming.

#14 Whichwayisnorth

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Posted 26 February 2014 - 04:35 PM

I really loved my AVX until I bought a ZEQ25. Now the AVX is a outreach and visual scope only and my ZEQ25 is home to my 80mm refractor for widefield imaging.

#15 rmollise

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Posted 26 February 2014 - 05:37 PM

So let me sum up what I'm hearing and make sure it's what you guys are saying

1) It's reasonable to expect an AVX + autoguider will track at 770mm for ~5 min subs

2) 10 min subs will be more difficult because of the aforementioned issue with dec tracking

3) Other mounts like CGEM, Sirius and Atlas and ZEQ25GT don't have the issue in #2 and will have better success, ease of getting good data.


I wouldn't necessarily sign off on your number two. I did occasionally go for ten minutes, and found that if I could go five, I could go ten. The problem was the bright sky background. ;)

#16 SpaceWatcher

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Posted 26 February 2014 - 06:01 PM

Whichwayisnorth, what mount do you use with the 8Edge? Is the ZEQ25 sufficient for that scope? :question:

#17 tjugo

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Posted 26 February 2014 - 06:04 PM

Hi Robert,

I have practical experience with the mounts mentioned in this thread and all will fulfill your requirements.

The ZEQ25 is the lightest while the Atlas is the heaviest, the AVX is in between the ZEQ25 and the Sirius.

Cheers,

Jose

#18 Robert T

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Posted 26 February 2014 - 07:06 PM

So let me sum up what I'm hearing and make sure it's what you guys are saying

1) It's reasonable to expect an AVX + autoguider will track at 770mm for ~5 min subs

2) 10 min subs will be more difficult because of the aforementioned issue with dec tracking

3) Other mounts like CGEM, Sirius and Atlas and ZEQ25GT don't have the issue in #2 and will have better success, ease of getting good data.


I wouldn't necessarily sign off on your number two. I did occasionally go for ten minutes, and found that if I could go five, I could go ten. The problem was the bright sky background. ;)


That's a good point, 10 mins might be moot since my skies may not be dark enough for it, except for the occasional star party way out in a really dark site.

#19 Jeff2011

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Posted 26 February 2014 - 11:51 PM

Sorry that I am a bit late to respond.

Can you expand on this recommendation a bit more?


Shorter focal length scopes are more forgiving to tracking errors and are easier to learn with. It is good that you are thinking about targets, however, there is not one scope that does all well, but if you get a 600 to 900mm focal length scope, it gives you a good range of targets. The other thing to take into consideration is your field of view. The longer the focal length, the smaller the field of view will be. For example, I can fit the Andromeda galaxy in my AT72ED at 430mm but not my 6 inch newt at 610mm.

To distill that into a question, does a mount like the AVX encounter significant problems when you jump from ~480mm to ~770mm [and the weight that goes with it]?



The mount can handle 770mm, it just won't be as forgiving as 480. That means you may have to throw out a few more exposures, especially when you are starting out. Concerning the weight it is a good idea to keep the weight of your scope and accessories to 1/2 the stated payload capacity of the mount. Counterweights not included for most mounts. The AVX has a payload rating of 30lbs so 15 pounds of scope, cameras, guide scope and other optical accessories like a field flattener. An eleven or twelve pound scope would work just fine.


I've heard the term "short focal length" used in an number of threads, what's considered a short focal length?



That is a relative term and you will get different responses from different people, but I would consider around 600mm and under to be short.

#20 calypsob

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Posted 27 February 2014 - 12:38 AM

Robert, I think it is safe to say that the ZEQ would easily deal with the weight of the 102mm Posted Image
TPO 8" Ritchey–Chrétien in Carbon Fiber by LMNO Sunset Deluxe, on Flickr

As for guiding, the zeq, with its centralized fulcrum and dual ball bearing axises will easily out guide a cgem and certainly give the atlas a run for its money. What it lacks is weight performance, 28lbs max, and some of the mounts need a tune up upon arrival, encoders, and axial tensioning. I love my zeq, best investment I have made in the field of atrophotography as of yet. I owned an extremely finnicky cg5 before this and would never go back because the company will not allow user service on bad gears and ioptron will help you with just about anything that could go wrong on this mount.






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