Need Advice for Cassegrain 7" "project"

[Note:  I posted this thread in the "beginning astronomy" topic forum, but am re-posting here because, although I am just starting out, my questions are more about the equipment and equipment modifications for building a telescope.  So, I hope this forum subtopic is better suited to my questions.]

I am completely new to astronomy and telescopes, but I have a partial setup that I think is very high end, and I would like to see if I can get it to create an image.  I have a Questar 7" telescope that came from an old, but very high-end atmospheric science instrument that I saved from the trash.  When I looked up the 7" Questar, it looks like a fully functional telescope from this manufacturer goes for around $10,000.  From what have researched, I have a 178 mm diameter Maksutov Cassegrain with an effective focal length of 2400mm.  This gives a focal ratio of 13.48.  It has the focusing knob that moves the primary mirror, and that does seem to change the focus when just looking through the axial hole (see picture attached).  The mirrors that I can see appear flawless.  Please try to answer ANY of the questions I pose below, or just comment on where I have misunderstood my research on this whole topic.  My total experience is reading on the internet for the past few days, and I have no practical experience at all with telescopes or optics, but have a solid engineering background, and I am willing to learn.

I am not sure I will stick with this hobby (just joined this forum yesterday), but I thought it might be a good place to get some used parts to complete this and to see if it creates a nice image.  I checked on the web, and found that the telescope would normally come with a "swivel" and then a "2" diagonal", into which I can put an eyepiece.  To get an image, I don't need the diagonal (which seems to be a simple 45 deg turn mirror, and only needed for convenient viewing).  I just need to place an appropriate eyepiece at the correct distance beyond the axial hole in my telescope.  Note that I have female threads, and my research indicates that this is a VERY non-standard, 1.64" thread.  I measure the minor diameter at 1.6" with a caliper, and close to 32 threads per inch.  The website wants a couple hundred dollars just for the coupling piece that provides male threads to male threads, and I think the "swivel" then attaches to that was another couple of hundred.  I am not spending that kind of money to see if I might have a working telescope.  I do have a 3D printer, and one thought was that I would make a custom press fit black plastic tube that inserts into the axial hole, rather than deal with the threads at all. 

First question:  Is this a bad idea?  Do I risk damaging or mis-aligning the secondary by attaching directly to the tube inside the axial hole?  Clearly, this is not the way it is intended, but it seems to be made of sturdy, thick anodized aluminum, and it seems to be the easiest way to get me to a single piece that I need to design with the proper focal length, and insert a good eyepiece to better understand what I have and what is functional, and if it is properly aligned (primary to secondary mirrors inside the telescope tube).  There are also 4 bolts that I can use to make a custom adapter plate avoiding the threaded axial hole and the tube inside that holds the secondary, that can be a light-tight box printed in black plastic, that holds my eyepiece at the correct focal distance.  This seems slightly safer in terms of not disturbing the alignment between the primary and secondary, and might be a more stable structure that would not flex with the weight of the eyepiece.  Would it be better if the box had a tube inside to keep out stray light, or just a big box with the eyepiece at the far end is better?  I think a box that holds the structure and bolts on, with a tube inside would be best.  Then I only need to seal for light leaks around where my custom adapter plate attaches to the base of the telescope.

2nd Question:  Regarding the eyepiece.  I do understand that I really cannot evaluate my optics without a good eyepiece, and I am willing to spend a couple hundred on a good part that would complete the telescope (at least initial build for evaluation). For my atmospheric viewing conditions in the suburbs, it seems that much greater than 200x magnification is limited by atmosphere.  Given that, and my 2400mm focal length, I could consider
200x with a 12mm eyepiece
150x with a 16mm eyepiece
120x with a 20mm eyepiece
Also, I wear glasses and have astigmatism, so the eye relief spec is important for the eyepiece.
From reading these forums, it seems that a used 17 mm T4 Nagler would give a nice magnification for likely crappy atmospheric conditions, an 18 mm eye relief (for my glasses), and boasts an 82 deg FOV (which I understand is great, but really don't have a clue how this compares).  Another calculation that I made was the "exit pupil" which I understand should fall between 0.7mm and 5mm for the human eye to be able to make an image, with 2mm as an optimum.  A 17 mm eyepiece divided by 13.48 Focal ratio gives a 1.26mm exit pupil, which sounds small.  But if I approach 2 mm, I will lose a great deal of magnification, needing a much longer focal length eyepiece.  So, the question is, is this a good choice of eyepiece for my specific setup?  Will this give me a good idea of what I can expect with the primary and secondary optics that I have?

Regards,
Roman

Attached Image:

Good find Roman !!!

Yes, you have a high end Questar Mak that is worth lots of money if the optics are good.

Anything you do to rig up something so an eyepiece can be put at the focal point is worth trying IF you adopt the Medical Montra "First, Do No Harm"

A 24mm eyepiece can give you 100x, which is where I would start. Later, If the 100x power looks good, then venture to higher powers.

Note that the diagonal does "turn" the light cone thru 90 Deg for convenient viewing, but it does something else: It makes the light path longer too. This might affect your experiment with eyepieces in that without a diagonal, your eyepiece might have to be positioned further from the back of the scope than you might think. Although the Mak design usually has LOTS of ADJUSTMENT RANGE because the primary mirror movement is AMPLIFIED by the power of the convex SECONDARY MIRROR, so 1mm of primary mirror travel results in several mm of focal position change.

Ed (aka eblanken)


REPOSTED MY REPLY HERE ALSO, ED

Hi again Roman & All,

Yes, this forum seems like a better place for your post. Perhaps an even better place would be on the Cloudy Nights Forum which has been much more active these days. I am eblanken in both places. I am excited about your acquisition. I hope to be helpful to you.

As far as eye relief, I'd recommend 20mm or more of eye relief. As an eyeglass wearer myself, I find that 18mm is just not quite enough for me. As far as Aparent Field of View (AFOV), 82 Deg would be wonderful, but this "slow" scope (at f/10 or more) will be very forgiving of eyepieces, so you might start with a 25mm Televue Plossl or if you want to spend some more money, a 24mm Televue Panoptic. The 25mm would perform fine in this scope and would be much less expensive as a "starter" eyepiece. The down side would be the narrower AFOV of about 50 Deg compared to the 68 Deg Panoptic with the wider field. 

There are two major sizes of eyepiece barrels: 2 inch and 1.25 inch. The 1.25 inch eyepieces max out at about 32mm or 35mm in True Field of View (TFOV) whereas the 2 inch eyepieces can give a wider TFOV, but would ultimately be limited by your scopes rear port size, so the 2 inch eyepieces (which are generally much more expensive) would be a future consideration.

Another question: How will you point your telescope ? This is usually done with a telescope mount. A tracking mount allows one to look longer at an object before the Earth's rotation moves the object out of the field of view. Your 7 inch Mak needs a sturdy mount to perform well.

Some thoughts,

Ed (aka eblanken)

Hello Ed,

Thank you for the advice on eyepieces.  

Ed,
The attached image shows what I have currently for a make-shift mount.  It is heavy, but sturdy. At some point I can buy or make something that tracks earth rotation, but currently, my biggest concern is whether everything is in good shape, with a collimated primary and secondary, and that I can get a nice image.  I saw some instructions for checking collimation using printed out set of concentric circles with a point source of  light in the center.  I may try that, or if there is some practical way of looking at a star, what would I expect to see if they my mirrors needed to be aligned?
Also, I have added an aluminum plate with a precise 2" hole centered over my axial port, so that I do not need to deal with the custom threads from Questar.  I think (hope, really), that a standard 2" "slip fit" diagonal can attach directly.  I may need to adapt it slightly to make sure it does not fall out, but this at least gets me away from having to purchase expensive accessories just to attach to the back of the telescope, and does not risk my jamming something into the axial hole itself which could risk some telescope damage.  Then, the eyepiece will work already with the diagonal, so I really only need to worry about this first mechanical interface between the custom 1.64" female threads in the axial hole, and diagonal.  I purchased the diagonal from the Astromart classifieds, and it is in the mail!  So, I should be able to make progress soon.  

Regards,
Roman

Attached Image:

Roman,

Looks like good progress to me !!! Can you show a photo of the back of the scope with your plate ?

If you want to check collimation using your "stand", you should consider using Polaris as a target Star. Assuming you are in the northern hemisphere, Polaris moves very little in the night sky because it is near the North Celestial Pole (NCP) and is very convenient as a star for checking your collimation with a mount that does not track.

Ed (aka eblanken)

Ed,
Yes, sure.  Below is the plate, with a cutout for both the axial hole and the focus adjustment.  By the way, your explanation of the how the focus has a significant adjustment due to the magnification of the secondary was very helpful, and makes sense.  If I need to "shim" the distance between the 2" diagonal in order to make the path length a bit longer (if my focus is still out of range due to my non-standard interface to the telescope), I can 3-D print a section with the hole in the same spot, perhaps more of a press fit into the plastic, rather than a slip fit into the aluminum plate (to help keep the diagonal in place), and held by the same 4 mounting bolts that currently hold the plate, with appropriately longer screws.  I think this should work, and get me to a proper distance from the back of the telescope.  Once I have the eyepiece, I can see if I will need the 3-D printed section as a spacer.  One step at a time.  I can't believe that folks were going to trash this telescope.  As I said, the mirrors look perfect, so I have high hopes of making this into a nice instrument, eventually adding a way to add an SLR camera as well, and even a tracking mount.  

The telescope was previously used for atmospheric observations of the boundary layer using a laser as a source, and all of that hardware has been stripped from it.  I did try just looking through the axial hole with my eye at a terrestrial object, and I was able to bring the object into focus with the knob.  That image looked beautiful.  It did not have huge magnification, but I understand that comes mainly when I add the eyepiece.  Lots of this is new to me, so every small thing I learn is fun.  Truth be told, the reason that I keep asking about alignment is that I accidentally loosened too many screws on the back when dis-assembling the old instrument, and realized some of the were "set screws", and it quickly dawned on me that those were for aligning the primary to the secondary.  I loosened all the primary holding screws, but only three of the set screws before I figured it out, but then gently put the set screws back till "snug", and tightened it down again.  Worst case, I will need to find a way to align it.  Your suggestion of looking at the Polaris star seems good.  I am in Maryland (northern hemisphere), but I am lacking the knowledge of what I would see or not see if I was not aligned.  Would the star be blurry, or would my field of view be reduced due to the misalignment.  Some trial and error is likely in my future.

Roman

Attached Image:

Hi Roman,

Thanks for posting the photo of the back with the plate installed. Interesting to read about how you acquired the Questar and what you have done to it so far.

Oh and by the way, welcome to Astromart. You may or may not get much response here, but I can help you so far as what is in your immediate future.

Getting a diagonal is a next logical step and you have already got one "on-the-way" so that is good. There are ways to check for gross-mis-alignment that are pretty easy. Then there is a much more precise alignment that will be more challenging.

As you can imagine, the entire optical path must be aligned for optimal performance. However, for your purposes, checking for gross-mis-alignment is your next task after you figure out how to "position" the diagonal and the eyepiece in the "best" positions in that optical path. Good external focusers are available to supplement the internal  MIRROR Moving Mechanism (which has it's limitations), but that's where you are starting. 

Do you have any eyepieces at all ? If no, then getting even a cheap one would be a step in the right direction. So far you have seemed to be somewhat cost concious, which is understandable. Perhaps I can help you buy or borrow an eyepiece or two or three ?

Oops, gotta check on something here at work.

Later,

Ed (aka eblanken)

Thanks Ed,
Many thanks for your feedback.
I do not have any eye pieces, but I now have enough information to get something that will work well with the telescope, and viewed with glasses.  I don't have an issue on spending for components that are universal and can be easily resold, like a diagonal and eyepiece.  I was trying to avoid spending a lot on the adapting hardware that would be unique to my particular telescope tube.  Particularly if I find that there is some damage to my telescope that is not obvious.

One thing you mentioned about a focusing mechanism caught my attention.  I have the focus knob on the back of the scope that moves the primary.  From your comment, I inferred that this should be used only for coarse focusing, and that it will be difficult to use that for general focusing unless I have another fine focus in line with the eye piece.  Did I interpret that correctly?  This is likely since moving the mirror is amplified, as you mentioned.
Roman

Hi Roman,

Sorry to be absent: Worked 12 hours each day in the last three. Some parts of my job allow time for AstroMart, some parts not.

So your Mak (Maksutov) is a Cassegrain often referred to as a MCT. I have 8 inch Schmidt Cassegrains (SCTs for short) and (smaller) MCTs as well. It is standard to have a single speed focuser. I have upgraded the focus knob/mechanism on my each of my three SCTs to a Feathertouch(R) branded dual speed focuser which has both a course and a fine focus (with a gear ratios of 10:1 or 11:1, I just can't remember). Others put an external focuser on the back of their MCT or SCT especially if their SCT has what is called "focus shift" or "Mirror flop" limiting their ability to do a proper fine collimation. Your Questar is probably better quality and precision that many of the SCTs out there so may be just fine with only a single speed focus knob, but may also suffer from some lesser level of this "Mirror Flop" as well. I don't know. Some prefer using an external focuser, especially for imaging to avoid the problem. They only use the internal focus knob for "course" focus like you infered. The problem you have is to be able to locate an eyepiece or diagonal/eyepiece precisely enough onto the back of your Questar. That will take some clever engineering on your part.

One thing you should know is that as you move the focus knob which moves the primary mirror, you change the spacing between the primary mirror and the secondary mirror, which changes the focal ratio, so stating the ratio to 4 significant figures (f/13.48) is a bit of being over-precise. I'd round that to f/13.5 and know that as you run the focus knob from one extreme to the other you are going to change the value slightly. Some others know how much, I just round. Your value of 2400mm will also change by a related amount as well, but don't get hung up on that.

Whatever you do to try and locate the diagonal/eyepiece or camera needs to have adjustments for up/down, left/right and tilt but also needs to have provisions to lock those down after you find the precise location after you find it.

Ed (aka eblanken)