<p>The cooldown fan is purely to speed getting the primary mirror to the local ambient temperature. It is not used - and <strong>should not</strong> be left on during observation, as it'll undoubtedly be a vibration source. </p>

<p>That shouldn't be a significant factor in your final decision.  I would go with total weight, and perhaps adaptability to an equatorial mount for star-tracking.  </p>

CC BY-SA 4.0
2019-11-01T15:47:06.340Z
There's some optimum for weight which likely varies from person to person. Too light, and you'll get more jiggle. Too heavy and you won't be able to carry it. Quality of X and Y axis bearings matters a lot too.
Wayfaring Stranger
CC BY-SA 4.0
2019-10-25T14:35:17.337Z
2019-10-25T14:35:17.337Z
Carl Witthoft
<p>What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes? I'm primarily interested in comparing mirror quality and weight (price I can do myself of course). Two examples of the category I'm asking about are the Sky-watcher 300P Flextube and Meade Lightbridge.</p>

<p>One of them appears to have a fan to cool the mirror, how can I evaluate that in terms of a positive or negative towards performance?                                                                                                                                                                                                                                     </p>

CC BY-SA 4.0
2019-10-25T03:38:19.337Z
Questions about specific brands of telescopes are [off topic](https://astronomy.stackexchange.com/help/on-topic).
Mike G
CC BY-SA 4.0
2019-10-25T04:48:48.703Z
@MikeG I've edited the question accordingly. Very often new users are unfamiliar with the site and SE doesn't require any reading of the help section before asking, so for a new user's first question, a helpful edit s probably more instructive than the insta-close.
uhoh
CC BY-SA 4.0
2019-10-25T07:40:31.973Z
There are many different aspects to compare for 12" truss Dobs and I think you should start with finding some reviews of the ones you are interested in.  If then you have specific questions, or points you want clarified, then edit your question,
Dr Chuck
CC BY-SA 4.0
2019-10-25T14:21:53.163Z
Dobson would be appalled :-), as his whole point was to build a quick, cheap, powerful telescope.  He used to tell his students to stop over-figuring &polishing their primary.
Carl Witthoft
CC BY-SA 4.0
2019-10-26T02:01:37.003Z
@DrChuck is right. You can also look for amateur astronomy clubs in your area. They may have both observing parties where you can go and look through telescopes, and meetings indoors where you can meet other amateur astronomers and discuss telescopes. Before buying one it's a really good idea to spend a lot of time reading and discussing with others. You might decide that your interests are best severed with a different kind of telescope than you originally anticipated! You might also find a used telescope that is either cheaper, or better for your needs than a new one.
uhoh
CC BY-SA 4.0
2019-10-28T21:50:50.870Z
The positives  and negatives of various Dobsonian scopes depends very much on what you plan to do with it.  Some are easier to tear down and re-assemble for travel.  Some have better focusers and other hardware.  Some are made of lighter materials.  Just what kind of observing are you planning to do?
Dan Hanson
CC BY-SA 4.0
2019-10-30T01:13:46.690Z
Most of the time I will observe in my yard but sometimes I will go to darker places near my town. I don't care about the weight but I want a telescope that I could easily change the mount, I would like to make an equatorial mount so I could do some planetary astrophotography.
Tornado 77
<telescope><amateur-observing><dobsonian-telescope>
What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes?
Tornado 77
# What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes?

(Asked by: username_0 on 2019-10-25)

What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes? I'm primarily interested in comparing mirror quality and weight (price I can do myself of course). Two examples of the category I'm asking about are the Sky-watcher 300P Flextube and Meade Lightbridge.

One of them appears to have a fan to cool the mirror, how can I evaluate that in terms of a positive or negative towards performance?

**Tags:** telescope, amateur-observing, dobsonian-telescope

> **Comments:**
> **username_1**: Questions about specific brands of telescopes are [off topic](https://astronomy.stackexchange.com/help/on-topic).  
> **username_2**: @username_1 I've edited the question accordingly. Very often new users are unfamiliar with the site and SE doesn't require any reading of the help section before asking, so for a new user's first question, a helpful edit s probably more instructive than the insta-close.  
> **username_3**: There are many different aspects to compare for 12" truss Dobs and I think you should start with finding some reviews of the ones you are interested in.  If then you have specific questions, or points you want clarified, then edit your question,  
> **username_4**: Dobson would be appalled :-), as his whole point was to build a quick, cheap, powerful telescope.  He used to tell his students to stop over-figuring &polishing their primary.  
> **username_2**: @username_3 is right. You can also look for amateur astronomy clubs in your area. They may have both observing parties where you can go and look through telescopes, and meetings indoors where you can meet other amateur astronomers and discuss telescopes. Before buying one it's a really good idea to spend a lot of time reading and discussing with others. You might decide that your interests are best severed with a different kind of telescope than you originally anticipated! You might also find a used telescope that is either cheaper, or better for your needs than a new one.  
> **username_5**: The positives  and negatives of various Dobsonian scopes depends very much on what you plan to do with it.  Some are easier to tear down and re-assemble for travel.  Some have better focusers and other hardware.  Some are made of lighter materials.  Just what kind of observing are you planning to do?  
> **username_0**: Most of the time I will observe in my yard but sometimes I will go to darker places near my town. I don't care about the weight but I want a telescope that I could easily change the mount, I would like to make an equatorial mount so I could do some planetary astrophotography.  

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## 1 answers

### Answer by username_4 on 2019-10-25. Score: 2

The cooldown fan is purely to speed getting the primary mirror to the local ambient temperature. It is not used - and **should not** be left on during observation, as it'll undoubtedly be a vibration source.

That shouldn't be a significant factor in your final decision. I would go with total weight, and perhaps adaptability to an equatorial mount for star-tracking.

> **Comments:**
> **username_6**: There's some optimum for weight which likely varies from person to person. Too light, and you'll get more jiggle. Too heavy and you won't be able to carry it. Quality of X and Y axis bearings matters a lot too.  

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