I have an OTA for a 90mm refractor with a focal length of 910mm. The air-spaced achromatic objective lens assembly is broken. I'm thinking of replacing it with a 90mm objective with a focal length of 1000mm. I'm wondering whether that will give me enough in-focus for a binoviewer. I've read a recommendation for 4 inches of in-focus, but I'm wondering whether 90mm would be enough. If it is not enough, is it very difficult to cut the tube down a bit more, and end up with a straight cut? Any tips on how to make the cut would be very appreciated. Thanks!
Assembling Refractor
Started by stuper1, 03/12/2008 04:05AM
Posted 03/12/2008 04:05AM
Opening Post
Posted 03/12/2008 04:46PM
| Edited 03/12/2008 04:49PM
#1
Typically there would have been some in focus for the original configuration to allow for the use of erecting prisms and barlows. This allowance may have been quite small but then again you may pick up 25-30mm or more here. This would give you about 115-120mm extra path length to use with a bino viewer. It may be enough for your bino viewer. Don't forget to include a diagonal in your path length calculations.
You can use a hack saw if necessary to cut the tube. Aluminum is not difficult to work with. Put a ring of masking tape around the tube to mark the end postion for the cut. Adjust it until it is nice and straight. (You can measure with a ruler from the existing edge to the masking tape.) Then put several layers of paper on the tube to protect the finish. Don't bother to protect the part you are going to cut off. Stuff something (an old rag or tissue paper will work) into the tube to keep metal filings and bits from getting into the tube. They are a pain to get out. Assemble some sort of a vee block to hold the tube and cut away. I have found it helpful to make a cut all the way around with going all the way through the metal. This provides a good guide for the final cut. Don't allow yourself to get too close to the edge of the tape. Cleanthe cut edge right up to the tape with a file and you are done. You can use a square to check the edge to see how accurate it is or you can simply stand the tube up on a flat surface and gently rotate it. You can tell by the way the tube leans where you need to file a little more off.
Note that if you cut the tube you probably won't be able to use a barlow with your binoviewer.
good luck with your project,
dan
You can use a hack saw if necessary to cut the tube. Aluminum is not difficult to work with. Put a ring of masking tape around the tube to mark the end postion for the cut. Adjust it until it is nice and straight. (You can measure with a ruler from the existing edge to the masking tape.) Then put several layers of paper on the tube to protect the finish. Don't bother to protect the part you are going to cut off. Stuff something (an old rag or tissue paper will work) into the tube to keep metal filings and bits from getting into the tube. They are a pain to get out. Assemble some sort of a vee block to hold the tube and cut away. I have found it helpful to make a cut all the way around with going all the way through the metal. This provides a good guide for the final cut. Don't allow yourself to get too close to the edge of the tape. Cleanthe cut edge right up to the tape with a file and you are done. You can use a square to check the edge to see how accurate it is or you can simply stand the tube up on a flat surface and gently rotate it. You can tell by the way the tube leans where you need to file a little more off.
Note that if you cut the tube you probably won't be able to use a barlow with your binoviewer.
good luck with your project,
dan
Funding Member
