On an iOptron GEM28, for use with planetary/lunar photography what would be the largest scope you would put on it? - Forums

Topic starter

Sep 5, 2024 · Edited: Sep 5, 2024

I have an iOptron GEM28 which I use with a small 5" newtonian and imaging train (ASI533 camera, filter wheel, OAG, guide camera) and this is working fine for astrophotography. I can guide around 0.50" RMS constantly and 600-second exposures are no issue on nights where there's no wind. It took a while to tune it so that it could perform like this, but I'm satisified with that particular imaging scale and I don't see a need to upgrade to a better mount at this moment for this particular imaging type.

I'm also doing some lunar photography with that same equipment (same scope and mount), only with an additional barlow and a planetary camera, and I'd also like to take a go on the planets from time to time but have no experience in that so far. At this point, I'm 'pleased' with my lunar images but more aperture is what would make things better.

My question is: what is the largest aperture scope this mount can handle for lunar/planetary photography? I assume not only weight matters, but also focal length and physical dimensions. A scope within weight limits might be working fine but the mount might not be accurate enough at high focal lengths to keep objects within the field of view due to periodic error and such. A scope too heavy might tip the mount (I have the default tripod and while it's sturdy I have a fear that things might fall over with a gusty wind).

For what it's worth: I prefer newtonians or classical cassegrains - other ideas are welcome (eg: schmidt-cassegrain).

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andrea tasselli

Frequent contributor

Sep 5, 2024

You can put a lightweight 10" newton at no more than f/5 on that mount and still track reasonably well for planetary imaging (scale less than 0.2"/px).

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Tony Gondola

Frequent contributor

Sep 5, 2024

I've had a C-11 on an old Meade LXD-75 mount, it worked well enough for lunar/planetary. As long as you have a good polar alignment so there's not a lot of drift you're ok. Exposures are so short that really, all the mount has to do track and not break!

Not that I recommend putting a C-11 on a LXD-75 ;)

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Kristof Vandebeek

Topic starter

Sep 5, 2024

andrea tasselli:
You can put a lightweight 10" newton at no more than f/5 on that mount and still track reasonably well for planetary imaging (scale less than 0.2"/px).

A 10” f/5 newton with a 2x ES tele-extender and an ASI678mm camera would have an image scale of 0.21”/px. I can’t imagine the GEM28 will be able to carry that, but it certainly does sound interesting with regards to resolution.

It won’t break the bank, but will it break the mount?

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andrea tasselli

Frequent contributor

Sep 5, 2024

Kristof Vandebeek:
A 10” f/5 newton with a 2x ES tele-extender and an ASI678mm camera would have an image scale of 0.21”/px. I can’t imagine the GEM28 will be able to carry that, but it certainly does sound interesting with regards to resolution.

It won’t break the bank, but will it break the mount?

I have put heavier/longer scopes on similar or flimsier mounts and nothing was ever broken. Not very stable but workable, the critical bit is keep the setup as light as you can and balance the scope in all axes as well as it can be.

GTom

Sep 15, 2024

Kristof Vandebeek:
andrea tasselli:
You can put a lightweight 10" newton at no more than f/5 on that mount and still track reasonably well for planetary imaging (scale less than 0.2"/px).

A 10” f/5 newton with a 2x ES tele-extender and an ASI678mm camera would have an image scale of 0.21”/px. I can’t imagine the GEM28 will be able to carry that, but it certainly does sound interesting with regards to resolution.

It won’t break the bank, but will it break the mount?

It might, if you are not very careful with balancing. Balanced load should be ok up to the spec visual payload limit.

Tony Gondola

Frequent contributor

Sep 16, 2024

You can be sure that a decently balanced load somewhat beyond the weight specification for the mount will not break it. Also, the tracking doesn't have to be that accurate at all. The exposure times are so short that it comes down more to just keeping your area of interest from drifting out of the frame and SharpCap has the tools to make sure that doesn't happen.

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SemiPro

IOTD Staff

Sep 16, 2024

Don't forget a lot of payload ratings are dependent on the diameter of the tube. That 28 lbs of carrying capacity is rated for no larger than a 6" diameter telescope. Of course that won't stop people, but its something to keep in mind. There are a few people out there who slap EdgeHD 8's on that thing.

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Kristof Vandebeek

Topic starter

Sep 16, 2024

Don't forget a lot of payload ratings are dependent on the diameter of the tube. That 28 lbs of carrying capacity is rated for no larger than a 6" diameter telescope. Of course that won't stop people, but its something to keep in mind. There are a few people out there who slap EdgeHD 8's on that thing.

I’ve read that before but never understood. Why is the diameter a factor to consider? Hypothetically: if you have a very low weight OTA with a very large diameter, is this statement then still valid?

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GTom

Sep 16, 2024

Kristof Vandebeek:
Don't forget a lot of payload ratings are dependent on the diameter of the tube. That 28 lbs of carrying capacity is rated for no larger than a 6" diameter telescope. Of course that won't stop people, but its something to keep in mind. There are a few people out there who slap EdgeHD 8's on that thing.

I’ve read that before but never understood. Why is the diameter a factor to consider? Hypothetically: if you have a very low weight OTA with a very large diameter, is this statement then still valid?

You have a much bigger moment arm: try to lift a 10kg weight with your arm bent vs fully extended. Just as a comparison: I've assembled my tracker mount this afternoon, a Lightrack II. When set up with a ball head, it doesn't need a counterweight even under my 3kg's heavy 2.8/300 nikon telephoto+camera! Simply because the kit has near zero moment arm in RA, balanced right above the rotation axis.
However, when I put the precious kit on my stronger camera fixing, a pentax WR giro mini alt/az mount, that adds a huge moment arm, placing the center of payload gravity approx 10cm away from the rotation axis. Result: I need to add almost 2kg counterweight on my makeshift 30cm long(!) counterweight bar to avoid slippage!
Eventually the counterweight version makes the entire setup heavier, loads of momentum in every little move.

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