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Hello from western WA, USA, and a question


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I woke up today with a general interest in astronomy, and a vague notion of "I should get a telescope someday" well down the priority list.  This afternoon, I'm the sudden owner of a 8" homemade Dobson telescope, which was donated to a neighborhood Goodwill store.  We were there dropping off some used household things, and it was sitting out in their donation area.  Since it was homemade they couldn't sell it, so they were going to compact it into trash.  I couldn't let that happen.  So it's now sitting in my backyard, and two eyepieces are sitting in their original shipping boxes on my desk.  I guess that makes me a telescope owner.


The attendants at Goodwill, who didn't want to trash it either, were very happy to send it home with someone who appreciated telescopes.  They said the person who dropped it off just didn't have room for it anymore, and reported that it still worked just fine.  That's all the information they had on it.  I've been reading about Dobsons all afternoon and trying to decide what to do first.  The primary mirror is very dusty, and looks like it has some kind of spot or stain on it.  I'm debating whether to remove it and how best to clean it.  I would deeply appreciate any suggestions.  Thanks all!

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I think Dob's were all supposed to be homemade, that was the idea. Well before mass produced ones came out of China. I don't know much about Newtonians to be honest. Firstly I'd check the primary mirror, and then the secondary. These need to be collimated with each other. 

 

nm9RnT3l.jpg

 

I just use a Cheshire type collimator for this. It's fairly straightforward to use and most come with instructions. Some people use laser collimators or particle accelerators or something. Although if you ask me that's just overkill. This does all depend on whether you can actually move/re-position the mirrors in your homemade scope. Then I'd check the focuser. Make sure it winds in and out without any problems. A shaky or problematic focuser will severely hamper any attempts to set-up or collimate the scope itself. 

 

8tF2sXe.jpg

 

If you look at the back of my 150mm GSO Newtonian above you can see the knobs used to collimate the primary mirror.

 

hpP1v11.jpg

 

The secondary mirror, held by the spider vanes, can also be adjusted (ignore the red arrow in the picture lol). And don't mention the metal silhouette cats either ROTFL! I wouldn't worry too much about spots or marks on the primary, these are often pollen and accrue over time. Even mirrors that appear dirty can work pretty well. What's it like to look through in daytime?  

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Thanks for all the information.  I did sight it in that first night, and was able to pick up a very dull, faint Saturn but I couldn't see rings.  Decided to pull the primary mirror and clean it.  I've attached pictures of the mirror before and after cleaning.  Yikes!  Wonder how long it had been gathering dust?!?

 

Once I got it reinstalled, which went pretty well, I noticed that the alignment was WAY off.  So I started reading about collimation.  The mirror doesn't have a center dot on it, so I bought in a template to apply a center dot, along with both a Cheshire collimator and a laser unit.  I'm seeing multiple different methods for doing the alignment and I figured hey, I got the telescope for free, I may as well make the most of it by aligning it well.  Those parts are due in within the next few days.  Missed Jupiter's closest pass in 59 years last night (bummer) but should have it realigned in time to still see Jupiter and Saturn within the next few weeks.  Just getting that primary cleaned up with hopefully make a big difference.

PrimaryMirrorBefore.jpg

PrimaryMirrorAfter.jpg

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Update: since I can't look through my telescope until I get the collimation supplies, I decided tonight to look more carefully at my telescope, and I found something concerning.  The secondary mirror is in really good shape, clean and no scratches, but the plastic housing for it has numerous cracks around the base.  The whole thing looks very aged, maybe sun-damaged, and should probably be replaced.  Additionally, one of the spider's four attachment points has a sheared-off set of threads such that it can only pass through the OTA, with no threads on the outside of the tube to fasten it.  So I'm probably going to need a new housing for the secondary mirror, and maybe a new spider as well.  Any suggestions for where to get those parts?

Secondary1.jpg

Secondary 2.jpg

Secondary Closeup.jpg

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Today was a big day.  I cleaned up the two mirrors last weekend and decided to go ahead and keep the spider and secondary mirror assembly for the time being, and put everything back together.  Then, I went about figuring out how to collimate everything.  Following one set of instructions I wasn't getting very far, switched over to another set of instructions and it went really quickly.  So tonight I set up the telescope outside well before sunset so it could acclimate to the temps, and just before sunset I focused on the half-moon with the Barlow and actually saw nice crisp clean crater walls.  Then around 10:30 I went out to find Jupiter then Saturn.  I was actually able to see the bands of Jupiter, and the rings of Saturn.  The Barlow definitely brought things in closer but the support for the OTA is a little too wobbly.  It was tough to focus in on the planets without them bouncing around a lot.  I'm going to have to either stabilize the frame I have, or get a new one where I can pivot to a place and lock it down long enough to focus without losing my target.  But still, WOW.  I'm definitely hooked.  Going to have to study up to figure out what to go look at next.

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Mars is 12.1 arc seconds in diameter at the moment (88.7% illuminated phase). It's a logical next target. The southern polar cap is visible, as are other dark and light albedo features. I'm still pretty sure I can see northern polar hood clouds. Some visible dust storms have been reported in the Valles Marineris region. Yellow and yellow-green filters are useful for Mars as they attenuate the blue end of the spectrum. Light blue filters OTOH help with bright white features, particularly the polar caps. 

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